SoilImprovement and Foundation Remediation · 2009. 12. 3. · T. Tumay, Director. This supportis gratefully acknowledged. We especially appreciate the encouragement and ideas provided

Soil Improvement andFoundation Remediation

with Emphasis on Seismic Hazards

by Steven L. Kramerand

Robert D. Holtz

A report of a workshop

sponsored by theNational Science Foundation

and held at theUniversity of Washington

Department of Civil EngineeringSeattle, WashingtonAugust 19-21, 1991

I

I - 0---

Note

This report presents a compilation of the discussions of the participants in a National ScienceFoundation workshop, along with supporting background information. As such, it reflects the views ofthe authors, which are not necessarily those of the National Science Foundation or the University ofWashington. No endorsements of any product or process described in the report is intended.

Front cover: Slope failure along Union Pacific railroad right-of-way in 1949 Olympia, Washingtonearthquake (photo by G.W. Thorsen).

I i

TABLE OF CONTENTS

Summary _ _ _ _................................................ 1

Acknowledgments _ _ __ __ __. 3

1 Introduction...................................................................................................................................... 5

1.1 General............................................................................................................................. 51.2 Background...................................................................................................................... 51.3 Objectives 61.4 Workshop Organization................................................................................................ 71.5 Organization of Report.......................................................... 8

2 Overview of Seismic Hazards 9

2.1 Introduction 92.2 Ground Shaking 9

2.2.1 Examples............................................................................................................ 102.2.2 Causes................................................................................................................. 102.2.3 Mitigation.......................................................................................................... 132.2.4 Discussion 13

2.3 Liquefaction 132.3.1 Examples............................................................................................................ 142.3.2 Causes................................................................................................................. 152.3.3 Mitigation.......................................................................................................... 162.3.4 Discussion 16

2.4 Foundation Failure 172.4.1 Examples............................................................................................................ 212.4.2 Causes................................................................................................................. 222.4.3 Mitigation.......................................................................................................... 242.4.4 Discussion 24

2.5 Slopes and Retaining Structures 242.5.1 Examples............................................................................................................ 252.5.2 Causes................................................................................................................. 272.5.3 Mitigation.......................................................................................................... 282.5.4 Discussion.......................................................................................................... 28

2.6 Summary and Condusions........................................................................................... 28

3 Soil Improvement and Foundation Remediation Techniques 31

3.1 Introduction 313.2 Densification Techniques 31

3.2.1 Description of Densification Techniques...................................................... 313.2.2 The Practice of Soil Improvement by Densification 34

3.2.2.1 Historical Use of Densification Techniques................................ 353.2.2.2 Case Histories................................................................................. 363.2.2.3 Observed Effectiveness of Densification Techniques............... 38

3.2.3 Current Issues in Application of Densification Techniques...................... 393.2.4 Summary 39

National Science Foundation iii

TABLE OF CONTENTS (Continued)

3.3 Drainage Techniques 393.3.1 Description of Drainage Techniques.............................................................. 393.3.2 The Practice of Soil Improvement by Drainage............................................ 46

3.3.2.1 Historical Use of Drainage Techniques 463.3.2.2 Case Histories 463.3.2.3 Observed Effectiveness of Drainage Techniques 47

3.3.3 Current Issues in Application of Drainage Techniques 483.3.4 Summary 48

3.4 Physical and Chemical Modification Techniques 483.4.1 Description of Physical and Chemical Modification Techniques 493.4.2 The Practice of Soil Improvement by Physical and Chemical

Modification 523.4.2.1 Historical Use of Physical and Chemical Modification

Techniques 523.4.2.2 Case Histories.................................................................................. 543.4.2.3 Observed Effectiveness of Physical and Chemical

Modification Techniques 553.4.2.4 Potential Applications to Seismic Hazards................................ 55

3.4.3 Current Issues in Application of Physical and Chemical ModificationTechniques 56

3.4.4 Summary 563.5 Inclusions Techniques 56

3.5.1 Description of Inclusions Techniques........................................................... 573.5.2 The Practice of Soil Improvement Using Inclusions.................................... 61

3.5.2.1 Historical Use of Inclusions.......................................................... 613.5.2.2 Case Histories 613.5.2.3 Observed Effectiveness 63

3.5.3 Issues in Use of Inclusions Techniques.......................................................... 633.5.4 Summary 64

3.6 Foundation Remediation Techniques 643.6.1 Description of Foundation Remediation Techniques................................. 643.6.2 The Practice of Foundation Remediation 65

3.6.2.1 Historical Use of Foundation Remediation Techniques........... 653.6.2.2 Case Histories 663.6.2.3 Observed Effectiveness of Foundation Remediation

Techniques 673.6.4 Issues in the Use of Foundation Remediation Techniques......................... 673.6.5 Summary 67

4 Verification of Soil Improvement and Foundation Remediation •••_.__..... .....__._....._ 69

4.1 Introduction..................................................................................................................... 694.2 laboratory Testing Techniques.................................................................................... 69

4.2.1 Advantages 694.2.2 Disadvantages 70

4.3 In Situ Testing Techniques........................................................................................... 714.4 Geophysical Testing Techniques................................................................................. 72

4.4.1 Seismic Techniques........................................................................................... 724.4.2 Ground Probing Radar.................................................................................... 724.4.3 Resistivity/Conductivity 72

,

iv Soil Improvement and Foundation Remediation

TABLE OF CONTENTS (Continued)

5 Research Needs _ _ _ __ _ _..__ _..... 73

5.1 Introduction 735.2 Research Needs by Technique Group.......................................................................... 73

5.2.1 Densification Techniques................................................................................ 745.2.2 Drainage Techniques....................................................................................... 755.2.3 Physical and Chemical Modification Techniques........................................ 765.2.4 Inclusions Techniques...................................................................................... 775.2.5 Foundation Remediation Techniques........................................................... 78

5.3 Prioritization of Research Needs 795.3.1 ffigh Priority Research Needs 795.3.2 Medium Priority Research Needs 805.3.3 Low Priority Research Needs 81

6 Future Directions for Research on Soil Improvement and Foundation Remediation ...._ 83

6.1 Introduction 836.2 Small-scale Testing......................................................................................................... 836.3 Large-scale Testing 846.4 University-Industry Cooperation 846.5 National Test Sites Program......................................................................................... 856.6 Summary 87

7 Summary and Conclusions ..••.- _ _...•._ _.__ _ _.................. 89

7.1 Summary 897.2 Conclusions...................................................................................................................... 90

References __ __..-. __ _ __ __ __ __ __ 91

Appendix A. Workshop Roster •..._ _ _ _ ..__.._ _..... 103

National Science Foundation V

LIST OF FIGURES

2.1 Variation of ground shaking characteristics at different soil profiles in 1957San Francisco earthquake 11

2.2 Comparison of response spectra for rock and soft clay site in Mexico City 122.3 Accelerograms for Yerba Buena Island (rock outcrop) and adjacent Treasure Island

(soft soils) in 1989 Loma Prieta Earthquake................................................................... 122.4 Tilting of apartment building in Niigata, Japan.................................................................... 142.5 The Turnagain Heights landslide near Anchorage, Alaska................................................ 152.6 Mobilization of resistance to vertical, lateral, and overturning loads by a single

pile and a pile group 182.7 (a) Foundation on soil deposit underlain by sloping base layer; potential for

differential excitation of the two sides(b) Pile-group foundation in soft soil underlain by a sloping base; records at points I,

2,3,4 and 5 reveal the strong influence of base slope ("2-D" effect)(c) Calculated relative displacements induced on a pile embedded in a soil deposit

underlain by steep rock 202.8 (a) Large lateral displacement of Struve Slough Bridge piles; (b) resulting damage to

pier/ deck connection 222.9 Cross-section through 7th Street Terminal at Port of Oakland 232.10 Damage to tops of batter piles beneath Seventh Street Terminal, Port of Oakland......... 232.11 Slope failure along Union Pacific railroad right-of-way in 1949 Olympia,

Washington earthquake 262.12 Failure of steep coastal bluff in Daly City, California in Loma Prieta earthquake.......... 262.13 Failure of quay walls due to liquefaction in Niigata, Japan, and (b) of retaining wall

due to inertial forces in 1960 Chile earthquake............................................................. 273.1 Dynamic compaction at a site in Bangladesh using a 100 ton crane dropping a 16 ton

weight 30 m 323.2 Vibrocompaction equipment and process 333.3 Stone columns constructed by vibroreplacement 343.4 Schematic of compaction grouting.......................................................................................... 353.5 Caisson quay wall supported on a gravel pad densified by vibroflotation...................... 373.6 Surface drainage of a slope by a diversion ditch and interceptor drain............................ 413.7 Drainage blanket used for stabilizing shallow foundations............................................... 413.8 Subhorizontal and vertical drains used to lower ground water in natural slopes.......... 423.9 Common methods for draining retaining wall backfills..................................................... 423.10 Cross-section of gravel drains applied to a common reinforced concrete utility duct

in a loose sand in Japan 443.11 Drainage of an excavation by multi-stage well-points........................................................ 443.12 Collection of seepage in a slope by open ditches and sumps 453.13 A typical well-instrumented vertical drain installation for a highway embankment.... 453.14 Example of countermeasures used to mitigate the liquefaction hazard inHichiro, Japan 473.15 Types of Grouting...................................................................................................................... 503.16 Jet grouting (a) columns and (b) panels 513.17 Schematic of the deep soil mixing rig 513.18 The Swedish lime column method: (a) schematic of completed column and auger

system; (b) close up of mixing tool; (c) installation machine 533.19 Schematic of soil nailing for (a) excavations and (b) natural slopes.................................. 573.20 Component parts and key dimensions of reinforced earth wall........................................ 583.21 Possible reinforced. walls and slopes using geosynthetic reinforcement.......................... 593.22 Schematic cross section illustrating the use of root pile for stabilization of a slope 603.23 Stone columns and new piling installed to increase the seismic resistance of a

wharf structure................................................................................................................... 68

National Science Foundation Preceding page blank vii

SummaryOn August 19-21, 1991 a workshop

sponsored by the National Science Foundationon Soil Improvement and FoundationRemediation with Emphasis on SeismicHazards was held at the University ofWashington in Seattle. The objective of theworkshop was to provide a forum for theexchange of information and experienceamong experts with a wide variety of viewpoints and perspectives on soil improvementand foundation remediation as well asgeotechnical earthquake engineering. Invitedparticipants included consulting geotechnicaland structural engineers, specialty contractors,engineers representing all levels of government, and academic researchers.

The speCific goals of the workshopwere (1) to summarize the current state ofknowledge concerning soil improvement andits applicability to foundation remediation forvarious geotechnical hazards, especially thosewhich are earthquake-induced; (2) to identifycurrent research needs in these areas; and (3)to recommend future directions for researchon soil and foundation remediation. Thisreport is a written record of the workshop deliberations and its attempt to meet these goals.

The report begins with an introduction providing some background and organizational details of the workshop. Chapter 2 isan overview of the seismic hazards ofliquefaction, ground shaking, and foundation,slope and retaining structure failures, alongwith a brief discussioI\ of soil improvementand foundation remediation techniques formitigation of these hazards.

National Science Foundation

Chapter 3 describes all the commonsoil improvement and foundation remediationtechniques in terms of their current practiceand illustrated by case histories, historical use,observed effectiveness, and current levels ofconfidence in their results. Included are (1)densification techniques (dynamic compaction, vibro compaction and vibro replacement, compaction grouting, blasting, andcompaction piles); (2) drainage techniques(interception, pore pressure control, dewatering, and acceleration of consolidation); (3)physical and chemical modification techniques(grouting, soil mixing); (4) inclusion techniques (soil nailing, metallic and geosyntheticreinforcement, piles, and stone columns); and(5) structural foundation remediationtechniques.

Verification of the effectiveness of soilimprovement and. foundation remediationtechniques is addressed in Chapter 4.Included is a discussion of geophysical testingtechniques which hold considerable promisein this area of verification. Research needs aredescribed and prioritized in Chapter 5.Chapter 6 gives some suggestions as to howfuture research in soil improvement andfoundation remediation might be directed andimproved.

Probably the most important productof the workshop is a prioritized list of specificresearch needs for densification, drainage,physical and chemical modification, inclusions, and foundation remediation techniques.This list was carefully developed by an inter-

1

disciplinary group of experts with a broadrange of experience and perspective.

There are tremendous benefits toincreasing industry-university cooperativeresearch on soil improvement and foundationremediation techniques, and such cooperationshould be strongly encouraged. Thedeveloping National Test Sites program for

Geotechnical Experimentation appears to provide an excellent framework for sharingresearch costs and results in this area.

The results of future research on soilimprovement and foundation remediationtechniques will lead to their more widespreadacceptance in practice, and to their more reliable and economical use. §

2 Soil Improvement and Foundation Remediation

Acknowledgments

This workshop was made possible bygrant BCS-9107767 from the U.S. NationalScience Foundation, Siting and GeotechnicalSystems/Earthquake Hazard MitigationProgram, Dr. Clifford J. Astill, Director, andthe Geomechanical, Geotechnical, and GeoEnvironmental Systems Program, Dr. MehmetT. Tumay, Director. This support is gratefullyacknowledged. We especially appreciate theencouragement and ideas provided by Drs.Astill and Tumay, both while we were planning the workshop and at the workshop itself.

The Organizing Committee consistedof Dean G. W. Clough (Virginia PolytechnicInstitute), Mr. Edward D. Graf (GroutingConsultant), Dr. Richard H. Ledbetter (USAEWaterways Experiment Station), Mr. Joseph P.Welsh (Hayward Baker Co.), and ourselves.This committee selected the workshop participants and assisted in the initial planning. Inaddition, Ledbetter and Welsh did doubleduty as Discussion Group Leaders, for whichwe are grateful. We also thank the otherDiscussion Group Leaders for not only summarizing all the participants' reports, but alsofor ably leading their respective discussiongroups.

A number of UW staff and graduatestudents assisted with various aspects of theworkshop. Ms. Ellen Barker of Engineering


Continuing Education and Dr. Ron Bucknamof the Department of Civil Engineering tookcare of many of the local arrangements. Oursecretaries, Ms. Carole McCutcheon and Ms.Gretchen Carlson provided their usual competent help before, during, and after the workshop. Ms. Carlson also prepared the text ofthe final report. Our graduate students, AmyBeitel, Mathew Craig, Sujan Punyamurthula,Bonnie Savage, Nadarajah Sivaneswaran,Kandiah Sribalaskandarajah, Wen-Sen Tsai,and Jerry Wu served as discussion groupreporters, secretaries, copiers and runners.Ms. Mary Marrah and Mr. Ron Porter of theWashington State Transportation Center tookcare of the final graphics, cover design, andprinting of the report.

Mr. Juan Baez, Dr. Rudy Bonaparte,Prof. Roy Borden, Mr. Rich Faris, Dr. GusFranklin, Prof. George Gazetas, Mr. Ed Graf,Dr. Richard Ledbetter, and Prof. Geoff Martinprovided helpful comments on a draft of thereport. Profs. Dick Woods and Jean Benoitkindly wrote sections of Chapters 4 and 6.

We thank all these persons for theircontributions to this report and to the successof the workshop.

Steven L. KramerRobert D. Holtz

§

3

1. Introduction1.1 General

Over the past 25 years, the field of soilimprovement has developed tremendously.This development has resulted from animproved understanding of geotechnical hazards and the factors that control them, theeconomic benefit of the development of siteswith marginal to poor soil conditions, andfrom the use of innovative construction techniques developed primarily by specialty contractors. Most of the recent developmentshave been applied to foundation soils and sitesprior to new construction, although a few traditional procedures (e.g., grouting) have along history as a post-construction site andfoundation remediation technique. Whileremediation of existing structures, particularlywith respect to seismic hazards, has receivedconsiderable attention in recent years, foundation remediation has been comparativelyneglected. It is possible that some of therecently developed soil improvement techniques might, with appropriate research anddevelopment, also be suitable for remediationof existing foundations. With this in mind, thesummer of 1991 appeared to be an opportunetime for a careful examination and evaluationof soil improvement and foundation remediation techniques to set the stage for efficientand productive future research in the area.

On August 19-21, 1991 a workshop onSoil Improvement and FoundationRemediation with Emphasis on SeismicHazards was held on the University ofWashington campus in Seattle, Washington.


The workshop was sponsored by the NationalScience Foundation with support from boththe Siting and Geotechnical Systems program,Dr. Clifford J. Astill, Director, and theGeomechanics, Geotechnical and Geo-environmental program, Dr. Mehmet T. Tumay,Director. The workshop was attended by 35engineers from the United States, Canada, andJapan. A roster of workshop participants canbe found in Appendix A.

1.2 Background

Geotechnical hazards represent continuing threats to the performance of manytypes of structures and facilities. Certainhazards, such as expansive soils, act slowlyand insidiously without recognition by thegeneral public or policymakers, but causetremendous localized economic losses yearafter year. Other hazards, such asearthquakes, act very quickly and dramaticallycausing highly publicized economic loss andloss of life or injury. Advances in thecharacterization of such hazards have allowedthem to be identified and to be generally dealtwith in a rational manner. In many instances,logistical or economic factors may require thatthe soil or foundation conditions at the site beimproved to ensure satisfactory foundationperformance.

Certain sites possess geographiclocation or other characteristics thatsignificantly increase their economic value,even though subsurface conditions may be

5

Preceding page blank

such that their proposed development isdifficult and ,costly. The natun . thegeotechnical hazards is often rela thetype of the proposed developn Forexample, port facilities are often suPi~ onloose, saturated, cohesionless alluvial andfluvial soil deposits that may be subject toliquefaction. In such cases, soil improvement,foundation remediation or both are requiredto ensure satisfactory performance. Whetherwe are considering residential or industrialbuilding sites or dam sites, the natural processof development tends to utilize the most easilydeveloped sites first Thus, as the 21st centuryapproaches, geotechnical engineers are moreand more frequently being challenged todevelop poor and marginal sites. Since thistrend can only be expected to increase, theneed for improved methods of soil andfoundation improvement is urgent. This needis especially urgent for existing structuresfounded on poor soils in hazardous locations.As knowledge of geotechnical and structuralearthquake engineering has increased inrecent years, so have the standards ofperformance. Adding to this are the increasedexpectations of the general public to be riskfree.

In contrast with many other aspects ofgeotechnical engineering, soil improvementtechniques have developed largely due to theinitiative and imagination of contractors. Arelatively small number of specialty contractors have promoted many of the techniquesnow commonly used for soil and siteimprovement. While these techniques haveproven to be very effective, most have beendeveloped by empirical trial and error testing,and the mechanisms by which they work arenot clearly understood by all practicing engineers. Thus their selection, design and utilization may not be the most efficient or economical for the soils or structure under consideration. Research in this area, especially appliedto remediation of existing structure foundationis clearly required.

No National Science Foundationworkshops had previously been held in theUnited States or Canada on the specific topic

" of soil improvement and foundation

remediation. Workshops on related topicshave been held on siting and geotechnicalsystems (lllinois Institute of Technology, 1986),darn failures (Purdue University, 1988),national geotechnical test sites (University ofNew Hampshire, 1989), and sitecharacterization (University of California atDavis, 1990). Related conferences include theSymposium on Soil Improvement - A TenYear Update held at the ASCE Convention inAtlantic City in 1987, several on geosynthetics,the ASCE Foundations Congress atNorthwestern University in 1989, and theASCE specialty conference on Grouting, SoilImprovement, and Geosynthetics in NewOrleans in 1992.

1.3 Objectives

The scope of the Workshop on SoilImprovement and Foundation Remediationwith Emphasis on Seismic Hazards was quitebroad. Because experience with non-seismichazards and their related mitigation techniques is so extensive, the treatment of seismic hazards and the foundation soil remediation techniques available to mitigate them wasemphasized. Issues were addressed from thestandpoint of the hazards themselves, available mitigation techniques of all types, thehazards for which they are applicable, and thelikely feasible mitigation measures.

The objective of the workshop was toprovide a forum for the exchange ofknowledge and experience among expertswith a wide variety of viewpoints andperspectives on soil improvement andfoundation remediation as well asgeotechnical earthquake engineering. Thespecific goals of the workshop were (1) tosummarize the current state of knowledgeconcerning soil improvement and its applicability to foundation remediation for variousgeotechnical hazards, especially those whichare earthquake-induced, (2) to identify andevaluate current research needs and opportunities in these areas, and (3) to recommendfuture directions for research on soil andfoundation remediation.


1.4 Workshop Organization

Because of the interdisciplinary natureof the topic, the workshop participantsinvitation list was designed to provide a broadrange of experience and perspective.Invitations were divided nearly evenly amongacademic researchers, government agencyeng.ineers, contractors, and consultingengtneers.

Because of the breadth of the topics tobe considered and the diversity ofbackgrounds and interests of the invitedparticipants, the workshop was organized in away to encourage productive interaction andactive participation among all attendees.

Discussion groups were formed toaddress both hazards (static and seismic) andsoil improvement and foundation remediationtechniques. Discussion leaders, each of whomwas a recognized authority in the topic of thatgroup, were assigned to each discussiongroup. Each participant sat on one hazardsdiscussion group and one techniques discus~

sion group. The participants were shiftedaround in the hazards and techniques discus~

sion groups, i.e., members of a particular haz~ard discussion group were distributed amongall of the techniques discussion groups and

vice versa. This format allowed eachparticipant to interact with as many otherparticipants as possible.

The makeup of the various discussiongroups was as shown below.

Prior to the workshop, eachparticipant was asked to prepare a brief (2-3page double spaced) report on the applicationof a particular class of techniques to aparticular class of hazards. These reports wereto discuss the current state of knowledgeregarding the applicability of the technique tothe hazard (including both advantages andlimitations), and methods for verification ofimprovement or remediation effectiveness.The reports were also to list, where possible,examples (referenced case histories) of goodand/or poor performance of the technique tothe hazard, and to discuss research needs andopportunities. These short reports wereassembled and summarized by the discussiongroup leaders with the summaries andindividual reports made available to allparticipants for review at the time ofworkshop registration. They provided a goodstarting point for discussions in the discussiongroups and for the written reports producedby each discussion group.

Hazards Liquefaction Ground Shaking Foundation Failure SlopelRet. StructuresDiscussion Martin" Ledbetter" Woods" Bonaparte"Groups Baez Abghari Darragh Collin

Denby Anderson Faught FarisFinn Koga Gazetas ForrestKilian Lum Ho HoltzYang Taylor NgStevens Mayne Wightman

Franklin.. Discussion Grafgroup leader Manning

Techniques Densification Drainage PhysiChem Mod. Inclusions Foundation Rem.Discussion Welsh" Holtz" Borden" Finn" Franklin"Groups Mayne Baez Abghari Collin Anderson

Stevens Bonaparte Faris Denby DarraghWightman Ho Faught Forrest Kilian

Koga Yang Gazetas NgBenoit Graf Lum

.. Discussion Woods Manninggroup leader Taylor

National Science Foundation 7

Workshop Schedule

DateMon., August 19

Tues., August 20

Wed., August 21

Time7:308:008:209:009:309:4510:0012:001:001:302:453:008:009:4510:0012:001:002:453:008:009:4510:0011:50

- 8:00- 8:20- 9:00- 9:30- 9:45- 10:00- 12:00- 1:00- 1:30- 2:45- 3:00- 5:00- 9:45- 10:00- 12:00- 1:00- 2:45- 3:00- 5:00- 9:45- 10:00- 11:50- 12:00

ActivityRegistrationWelcoming remarks and workshop introductionOverview of seismic hazards - Prof. S. KramerOverview of dynamic foundation response - Prof. G. GazetasNational geotechnical test sites program - Prof. J. BenoitBreakHazards discussion groups meetingsLunchHazards discussion groups meetingsPlenary session - hazards discussion groups reportsBreakTechniques discussion groups meetingsTechniques discussion groups meetingsBreakPlenary session - techniques discussion groups reportsLunchHazards discussion groups written report preparationBreakTechniques discussion groups written report preparationTechniques discussion groups written report preparationBreakPlenary session - general discussionQosing remarks and adjournment

The workshop schedule was brokeninto a series of sessions that took place over a2.5 day period. The workshop schedule wasas shown above.

The purpose of the hazardsdiscussions was to set the stage for productivediscussions of soil improvement andfoundation remediation techniques.Consequently, more workshop time wasdevoted to the techniques discussions than tothe hazards discussions.

1.5 Organization of Report

This report summarizes the currentstate of practice in soil improvement andfoundation remediation for seismic hazards,and presents the discussions and conclusionsof the various discussion groups. It reliesheavily on written reports prepared dUringthe workshop by each of the discussiongroups, but is not simply a compilation ofthose reports. Draft copies were reviewed and

edited by the members of the orgamzmgcommittee and each of the discussion groupleaders prior to publication. Some sections ofthe report were also reviewed by variousmembers of the discussion groups.

Chapter 2 presents an overview ofseismic hazards and their causes, anddiscusses the requirements of soilimprovement and foundation remediationtechniques for mitigation of these hazards.Chapter 3 describes current soil improvementand foundation remediation techniques interms of their historical use and effectiveness,and the existing levels of confidence in theirresults. The issue of verification of theeffectiveness of soil improvement andfoundation remediation techniques isaddressed in Chapter 4. Research needs aredescribed and prioritized in Chapter 5, andfuture directions for research in soil improvement and foundation remediation are discussed in Chapter 6. A summary and theconclusions of the workshop are presented inChapter7.§


2. Overview of Seismic Hazards2.1 Introduction 2.2 Ground Shaking

A great number of geotechnical hazards have been associated with seismic activity. These hazards have historically resultedin considerable geotechnical damage, but havealso strongly influenced other types of damagesuch as collapse of buildings, bridges, anddams, destruction of lifelines, and initiation offires. These hazards may be broadly dividedinto those primarily associated with groundshaking and those associated with soil deformation. However, for purposes of workshoporganization, hazards were considered to bethose associated with liquefaction, groundshaking, foundation failure, and slope andretaining structure failure.

In order for soil improvement or foundation remediation techniques to mitigate thesehazards, their causes must first be understood.Some hazards may be attributed to a singlecause, others may have several potentialcauses. In some cases, the causes of the hazardare relatively straightforward, in others thecause may be unclear. Further research intothe causes of various geotechnical seismichazards is in progress and the results of thatresearch will undoubtedly influence the practice of soil improvement and foundationremediation. This chapter presents anoverview of seismic hazards with examples oftheir effects, and discusses their causes as currently understood by the geotechnical engineering profession.


Ground shaking hazards may bedescribed as those that lead to the excessiveresponse of soils and structures to earthquakemotions. Ground shaking effects are implicitin all geotechnical seismic hazards since it isthe shaking that leads to the development ofthe hazards. Ground shaking characteristicscan be broadly divided into three categoriesground motion amplitude, frequency content,and duration of strong ground shaking - andthe related hazards can be interpreted in termsof these categories.

1. Excessive ground motion amplitude

Amplitude can be expressed in termsof acceleration, velocity, or displacement.Large ground motion amplitudes can lead todamage to various types of structures locatedon or within a soil deposit. Large forces maybe induced in relatively stiff (low naturalperiod) structures by high acceleration amplitudes, while damage to more flexible (highernatural period) structures may be less influenced by acceleration amplitude than byvelodty or even displacement amplitude.

While ground motions are typicallydescribed at a single point, they actually varyboth vertically and horizontally. Structures oflong horizontal extent, such as bridges ordams, or structures that extend over considerable ranges of depth, such as shafts or piles,may be subjected to differential groundmotions. The differential motions, whethercaused by travelling wave effects or material

9

and geometric heterogeneity, can induceresponse that is different from that predictedwith the common assumption of coherentground motion.

Ground shaking can also lead to permanent deformations of slopes, retainingstructures, foundations, etc.; however, thesehazards will be discussed in other sections ofthis report

2. Unfavorable frequency content of .theground motion

The dynamic response of structures,whether building- or bridge-type structures orgeotechnical structures such as slopes, retaining walls and dams, is influenced by the frequency content of the imposed loading. Thegeometry and material properties of a soildeposit combine to form a "filter" to incomingseismic waves that can allow some frequenciesto be amplified while others are attenuated.The process is often complicated, but theeffects of soil conditions on the frequency content of ground motion are now fairly wellestablished.

3. Excessive duration of strong groundmotion

The duration of strong ground shaking is most strongly influenced by earthquakemagnitude, however, it has also been shownto be influenced by local soil conditions(Chang and Krinitszky, 1977). Certain types ofstructures, and certain types of soil deposits,accumulate damage with increasing numberof cycles or stress reversals. In such cases, theduration of strong ground motion can becomevery important.

2.2.1 Examples

The influence of local soil conditionson levels of ground shaking has been illustrated on many occasions. The report of the1906 San Francisco earthquake (Wood, 1908)stated that the observed level of damage wasprimarily associated with the underlying geologic conditions. In the 1957 San Francisco(Idriss and Seed, 1968), illustrated inFigure 2.1, and 1967 Caracas earthquakes

(Seed and Alonso, 1974), strong motioninstruments recorded significant differences inground motion amplitude and frequency content for sites on rock and/or shallow, stiff soildeposits than for sites on thick deposits of softsoil. In 1985, a large earthquake off the Pacificcoast of Mexico induced modest accelerationsin hard soils in and below Mexico City, some350 km away. In the parts of Mexico Citywhere the hard soil was overlain by 35 to 40 mof soft clay, however, ground surface accelerations were nearly five times higher, andstructures with natural periods equal to thepredominant period of the soft clay (and withtypical damping characteristics) were subjected to horizontal accelerations approaching1 g, as shown in Figure 2.2. In the 1989 LornaPrieta earthquake in the San Francisco Bayarea, ground motions at soft soil sites, whereconsiderable damage to bridges and viaductswas observed, were significantly greater thanthose recorded at rock and stiff soil sites(Figure 2.3).

The term "soil amplification" has beencoined to describe the filtering that seismicwaves undergo as they pass through the soil.On the other hand, soil filtering might alsodepress those harmonic components of theincident seismic wave whose frequencies substantially exceed the natural frequencies of thesoil deposit. De-amplification of shaking isthus also possible. Documented case historiesof de-amplification of seismic motion bysoft/loose soil layers have been presented bySeed and Idriss (1970) and Gazetas, et al.(1990).

2.2.2 Causes

Excessive levels of ground shaking arecaused by the response of geologic materialsof various geometries and material propertiesto excitation from an earthquake source. Theenergy released by the earthquake is largelymanifested as seismic waves that propagateaway from the source. These waves travelthrough various geological materials producing ground motion that varies, due to geologicstructure and radiation and material damping,with distance from the source. The resultingexcitation at the base of a soil deposit is afunction of, among other things, source


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mechanism, source geometry and orientation,earthquake magnitude, and travel pathcharacteristics.

When these seismic waves reach thesoil/bedrock interface near a site of interest,they impose a form of loading on the soildeposit at the site. The response of the soildeposit is influenced by its geometry and bythe material properties of the soil. The material properties that control the dynamicresponse of the soil are density, stiffness, anddamping characteristics. Of these, theresponse is far more sensitive to stiffness anddamping than to density. Contrasts betweenproperties at various layer boundaries alsoinfluence the nature of shaking at the groundsurface.

2.2.3 Mitigation

Modification of soil properties, particularly of the soils nearest the ground surface,will change the ground motion characteristicsduring an earthquake. It is important to note,however, that the effects of soil modificationon ground shaking hazards are not like theeffects on the other hazards described in thisreport. When soil modification techniques areused for mitigation of the other hazards, e.g.,liquefaction, foundation failure, slope andretaining structure failure, etc., they areintended to increase the soil resistance to thehazards. They tend to move the "state" of thesoil from a condition that may be close to failure to a new state further from failure.Ground shaking hazards, on the other hand,do not generally involve failure. Rather thanmoving the state of the soil farther from orcloser to a failure state, soil modificationresults in some alteration of the groundmotion characteristics. The effects of suchalteration may, depending on the characteristics of the site and any structures on it, notalways be beneficial. Thus, mitigation ofground shaking hazards represents a complexand difficult task.

2.2.4 Discussion

Ground shaking hazards affect structures (including earth structures) and facilitiesfounded on, within, and adjacent to the prob-


lem ground. Ground shaking, and consequently the resulting damage, is highlydependent on site geology, soil conditions,earthquake characteristics, and structuralresponse characteristics. All methods forevaluation of hazards and all designs for hazard mitigation are critically dependent on theanticipated level of ground shaking; yetground motions are probably the single largestunknown in earthquake analyses. Methodsfor characterizing ground motions withrespect to their potential for inducinggeotechnical hazards are urgently needed.

Additional research on dynamicproperties of soils, particularly for gravels,silts, and very soft organic soils, is alsoneeded. Advancements in methods of groundresponse analysis, including the properconsideration of topography and groundmotion coherence, will also provide significantbenefits in the identification and evaluation ofgeotechnical seismic hazards, and in thedesign of measures for mitigation of thosehazards.

2.3 Liquefaction

Uquefaction is a process by which theshearing resistance of a loose, saturated, cohesionless soil is reduced by the buildup ofexcess porewater pressure. Liquefaction canoccur under static loading conditions(Terzaghi, 1956) but is more commonly associated with earthquake activity (Seed and Idriss,1967; Seed, 1968; Ambraseys and Sarma, 1969;National Research Council, 1985). The hazards associated with liquefaction can begrouped into four main categories for level ornear level ground:

1. Surface manifestation effects

Subsurface liquefaction often manifests itself on the ground surface in the form ofsand boils, ground cracking, or ground lurching. Such effects can result in loss of bearingcapacity, structural damage due to differentialdisplacement, foundation uplift, and groundsettlement.

13

2. Post-liquefaction settlement

Even for cases where surface manifestation effects do not occur, post-earthquakedissipation of high excess pore pressures inthe subsurface soils may cause excessive settlement. Post-liquefaction settlement generally increases with decreasing soil density andincreasing thickness of liquefied zones.

3. Lateral Spreads

For gently sloping ground or whereliquefiable strata "daylight" in river banks, thelow residual undrained shear strength of aliquefiable shallow, superficial soil may resultin the accumulation of lateral ground displacements during earthquake shaking. Suchdisplacements will usually cease at the end ofthe earthquake, and are generally characterized by surface ground cracking and differential ground movement. Displacements mayrange from a few centimeters to severalmeters.

4. Flow Slides

Where residual undrained shearstrengths of liquefied soils are less than the

static shearing stresses required for equilibrium in sloping ground, very large lateraldeformations may occur. In some cases, dueto the effects of post-earthquake redistributionof excess pore pressure, such flow slides maybe triggered after the earthquake event.

2.3.1 Examples

The effects of liquefaction were firstbrought to the attention of most geotechnicalengineers in 1964 by the dramatic failures atNiigata, Japan (Japan National Committee onEarthquake Engineering, 1965; Yamada, 1966;Yokomura, 1966) and Anchorage, Alaska(Ross, et al., 1969; Seed and Wilson, 1967;Scott, 1973) during separate earthquakes. Inthe Niigata earthquake, widespread liquefaction occurred in low-lying areas of the citycausing widespread damage as illustrated bythe well-known photograph of Figure 2.4.Liquefaction caused bearing failure of foundations for buildings and bridges and failureof quay walls. Lateral spreading of liquefiedsoils damaged bridge approach embankments,buildings, buried structures and pipelines.The Good Friday earthquake in Alaska causedlocalized landslide damage in Anchorage and

Figure 2.4. Tilting of apartment building in Niigata, Japan (after Seed and Idriss, 1982)


a massive flow slide in the Turnagain Heightsarea of Anchorage shown in Figure 2.5.Additional liquefaction failures produced considerable damage to port and waterfront facilities of Seward and Valdez.

Liquefaction failures have beenobserved on many occasions and at manylocations since 1964, most recently in the SanFrancisco Bay area during the 1989 LomaPrieta earthquake (Housner, et a1., 1990).Widespread liquefaction in natural and manmade soil deposits was responsible for muchof the damage sustained in the earthquake.

2.3.2 Causes

The causes of liquefaction of saturatedcohesionless soils due to earthquake inducedcyclic loading are fairly well understood andhave been documented in a comprehensivemanner in a National Research Council (1985)report. Basically, liquefaction is caused by the

buildup of excess pore pressure associatedwith cyclic straining induced by earthquakeshaking. This buildup of pore pressure resultsfrom the tendency of liquefiable soils to densify, or contract, upon shearing. Evaluationof liquefaction hazards has historically developed along two lines - evaluation of thepotential for initiation of liquefaction andevaluation of the stability of liquefied soils.The former approach involves carefulconsideration of the process of pore pressuregeneration that occurs at relatively low strains,while the latter is concerned with the residual,or steady state, strength mobilized at largestrains. Currently, the factors influencing, andconsequently the procedures for evaluation of,the initiation of liquefaction are betterunderstood than those influencing the residualstrength of liquefied soil. Regardless, theapplication of soil improvement techniques toliquefiable soils will influence both thepotential for initiation of liquefaction and theresidual strength of the soil.

Figure 2.5. The Turnagain Heights landslide near Anchorage, Alaska (after Seed and Idriss,1982)


2.3.3 Mitigation

Methods for mitigation of hazardsassociated with the occurrence of liquefactioncan deal directly with the factors that causeliquefaction to be initiated, or they can addressthe potential effects of liquefaction. Most mitigation techniques, however, influence both.

Liquefaction is triggered by thebuildup of positive excess pore pressures;consequently, methods for prevention of itsoccurrence must reduce the tendency forbuildup of these excess pore pressures.Reduction of pore pressure generation can beaccomplished in a number of different ways,including:

1. Densification

Since the tendency for pore pressurebuildup is strongly related to the density ofthe soil, increasing its density will decrease thetendency for positive pore pressure buildupand consequently reduce liquefaction potential. Densification will also increase the residual strength of the soil, thereby reducing theeffects of any liquefaction that should occur.Densification has historically been the mostcommonly used technique for improvement ofliquefiable soils. However, its application tomitigation of liquefaction hazards in areas ofexisting structures and utilities is limited bythe tolerance to the settlements inevitablyassociated with densification.

2. Drainage

Since the process of liquefactioninvolves the buildup of excess pore pressures,mitigation of liquefaction hazards can also beaccomplished by the provision of drainagewithin the liquefiable soil. A drainage pathextending through a liquefiable soil layerallows water to flow from the layer whenexcess pore pressures are generated. Suchflow reduces the pore pressures in the liquefiable layer and increases the density of the soilin that layer both during and after the earthquake. Some amount of settlement, whichmay be intolerable in some instances, may beassociated with this drainage. Though somesoil improvement methods, e.g., stone

columns, introduce drainage paths into liquefiable soils, it is often the densification associated with their installation rather than theirdrainage capabilities that is relied upon formitigation. Alternatively, the generation ofexcess pore pressures can be greatly retardedby reducing the degree of saturation of thesoil.

3. Chemical Modification

The tendency for contraction of a loosesoil, and hence for pore pressure buildup,could also be reduced by increasing thestrength of particle-to-particle contacts.Strengthening these contacts by chemicalmeans will increase both the stiffness and thestrength of the soil, thus reducing the amplitude of cyclic strains and the contractive tendency of the soil.

Combinations of two or more of theabove methods, or perhaps the developmentof new and innovative methods, can also beused to mitigate liquefaction hazards. In somecases, it may be more economical to undertakemitigation measures that limit the damagingeffects of liquefaction. This is most readilyaccomplished by increasing the residualstrength that the soil would have if it were toliquefy, most commonly by densification. Theuse of stabilizing buttresses and berms orstructural inclusions, however, may also beused to prevent large deformations of liquefied soil.

2.3.4 Discussion

Mitigation of liquefaction hazards isoften expensive and evaluation of its neceSSityis critically dependent on understanding ofand reliable assessment of both the causes andthe potential effects of liquefaction. A greatdeal of research has addressed these latterissues, however, empirical methods based oncase histories (aided and retained by theoretical and experimental data) currently form thebackbone of design approaches for assessingboth the potential for liquefaction hazards andfor acceptance of design criteria for soilimprovement or foundation remediationtechniques.


In order to improve methods foridentification and evaluation of liquefactionhazards, additional research is needed.Seismically induced pore pressure generationin clean sand is reasonably well understood;our understanding of the pore pressureresponse of gravels and silty and clayey sandsis not as well established. Incorporation ofearthquake-induced pore pressures in stabilityanalyses is not done uniformly, nor does thereexist a consistent definition of a factor of safetyagainst liquefaction. In addition, due to complex geology and hydrogeology, the prediction of seismically-induced pore pressures innatural slopes is uncertain. Though lateralspreading of such slopes has caused considerable damage in past earthquakes, it has beenpoorly understood until relatively recently(Youd and Perkins, 1987; Dobry and Baziar,1992). ImprOVed methods for identification ofliquefiable soils, particularly when they occurin thin lenses or seams, are needed. Furtherdevelopment of in situ test methods is likely tocontribute to improved capabilities in thisarea. Advances in characterization of theresidual strength of liquefied soils are alsoneeded.

Additional research, in the form offield investigations and instrumentation ofliquefaction-susceptible sites and laboratoryinvestigations such as the NSF-supportedVELACS project, will allow improvedunderstanding of the liquefaction process.Such research is needed to improve theaccuracy of liquefaction evaluation methodsand the economy and reliability of liquefactionhazard mitigation techniques.

2.4 Foundation Failure

The performance of foundations during earthquakes is critical to the performanceof the structures they support. Foundationfailure hazards are most conveniently classified under the traditional foundation categories of deep foundations and shallow foundations. Deep foundations consist of relatively slender structural members that transferload from the base of a superstructure todeeper soils with greater supporting capacity,


or that develop supporting capacity alongtheir length. Shallow foundations derive support from the soil immediately below thestructure.

Satisfactory performance of deepfoundations dUring earthquakes requires thatthey not deflect excessively during the earthquake, and that they retain their supportingcapacity after the earthquake. They musttherefore, provide sufficient resistance to bothlateral and axial loads. Foundation failuresmay therefore be categorized as hazards associated with capacity, settlement, and dynamicresponse.

1. Capacity

Pile-supported structures usually havea high factor of safety against failure due todownward-acting vertical loads.Consequently, failure or excessive deformation of piles in compression is unusual.Research and experience have also indicatedthat pile buckling is virtually impossible, evenwhen the piles extend through very weakmaterials. Earthquake-induced groundmotions may also induce bending and torsional loading in individual piles, particularlynear the interface between very soft and verystiff soils. However, their effects in pilegroups are generally less significant than theeffects of lateral and axial loading since pilegroups are able to mobilize resistance to overturning moments through their axial capacities(Figure 2.6). When overturning moments onpile groups are large, some piles may berequired to resist tensile, or uplift, loads.Uplift loads are generally more critical thandownward-acting loads, both with respect tothe pullout capacity of the soil/pile systemand with respect to the structural capacity ofthe pile/pile cap system itself.

Since earthquakes primarily imparthorizontal accelerations, the lateral resistanceof soils surrounding deep foundations may befully mobilized, particularly in loose orcompressible zones near the ground surface.In loose sands below the groundwater table,partial or complete liquefaction may result in adramatic loss of lateral resistance. Movements

17

Figure 2.6. Mobilization of resistance to vertical, lateral, and overturning loadsby a single pile and a pile group (resistance to overturning of pilegroup induces vertical and uplift loads in exterior piles)

of liquefied soil, or of non-liquefied soil"floating" on liquefied soil, can impose largelateral loads on foundations. In soft clays,particularly those which are sensitive, seismicmotions may propagate as mud waves withconsiderable lateral flow and spreading, andthis may cause excessive horizontal movementat the pile heads. Overstressing may alsoresult from large bending moments induced inthe foundation elements.

Shallow foundations are typicallydesigned with large factors' of safety againstbearing capacity failure. Consequently, theyare usually able to resist transient increasedloads without distress, as long as thesupporting capacity of the soil is maintained.Seismically induced reduction of shallowfoundation bearing capacity can result frombuild-up of excess pore pressures in saturatedgranular soils and from a possible reduction inshear strength of cohesive soils (i.e., cyclicdegradation). On the other hand, the currentpractice in the construction of storage tanks isto design the foundation with a factor of safetyof about one, though subsequent water testing

of the tank may cause consolidation thatincreases the factor of safety against bearingfailure. This design practice, however, canleave storage tanks in a potentially vulnerablestate with respect to earthquake loading.

2. Settlement

Structures properly supported ondeep foundations are normally not prone tolarge settlements during or after an earthquake, but connected appurtenant facilitiesthat are not similarly supported may settlesignificantly. The resulting differential settlements may be damaging to the connectedfacilities. Those include lifeline items such asutilities (water, gas, sewer pipes, electricalcables, communications cables, etc.) and otherimportant facilities such as stairways, accessramps, loading docks, etc. For a new structure, provisions for differential settlement canbe readily incorporated in design; however,they may be difficult to accommodate economically in an existing structure.

Loose, dry sandy soils tend to contract, or densify, when subjected to ground


shaking that exceeds the threshold shearstrain. Saturated sands will also settle as aresult of earthquake ground shaking, whenexcess pore pressures dissipate. Settlementscan be very large if liquefaction occurs.Significant foundation settlements have alsoresulted from liquefaction-related lateralspreading of soils supporting shallow foundations. Thus, shallow foundations supportednear the ground surface may experience bothtotal and differential settlement due to seismicshaking. Even a few centimeters of settlementmay cause of distress in some deformationsensitive foundation systems.

Pile and sheet-pile driving has alsobeen known to cause densification of loosesandy layers (Lacy and Gould, 1985; Picornelland del Monte, 1985). Ground surface settlement associated with such densification mayhave detrimental effects on sensitive neighboring structures or facilities.

Foundation settlement problems mayalso result from combinations of foundationtypes. Installation of new foundations adjacent to old or deep foundations next to shallow foundations may be subject to significantdifferential settlement. Such problems arewell known for static conditions but may beexacerbated by the dynamic conditions associated with earthquake shaking.

With complete liquefaction of the surrounding foundation soil, buried facilitiessuch as tanks and pipelines may becomebuoyant and float. The resulting differentialmovements can cause significant damage toconnected appurtenant facilities. In extremecases, buried structures can float to the groundsurface (Seed and Idriss, 1967).

3. Dynamic Response

Foundations must often supportinstruments, equipment, or structures whichare particularly sensitive to vibration at specific frequencies. When failure is interpretedas non-conformity of actual performance tosome design criterion, excessive dynamicresponse of foundations may be considered asa potential mode of failure.


Batter, or inclined, piles are oftenincluded with vertical piles in a pile group toprovide increased resistance to lateral loads.The seismic design of batter piles requiresinteraction between structural and geotechnical engineers. Their design in areas of highseismicity requires careful consideration ofgeotechnical factors including:

• the nature of the ground motionswhich will introduce bending, axialand lateral loads in the piles.

• the influence of the connectionbetween the pile and the pile cap onthe deformation characteristics of thepile.

• the increased lateral stiffness of batterpiles that will stiffen the overall structure and probably induce higherseismic forces.

In most ground response and soilstructure interaction analyses, the cornmonpractice is to assume horizontal soil layers ofinfinite lateral extent. More often than not,however, the base layer is actually sloping asin the COrnmon case of bridge crossings of narrow rivers illustrated in Figure 2.7. The wavepropagation phenomena in such cases arecomplicated. Waves striking the foundationcome at various angles, directly or after multiple reflection, and include Rayleigh waves(Figure 2.7a). Even at sloping base angles of10-20 deg, the motion induced on two sides ofa foundation of large lateral dimensions willbe of different amplitude and clearly will beout ofphase. Differential foundation motionsthat cause unfavorable response of the structure-foundation system may result. Similarly,the kinematic deformations imposed on pilesmay be unfavorable. Figure 2.7(b) shows awell-instrumented case history involving abridge foundation in Japan, (Gazetas, et aI.,1992) underlain by a base slope of 15 deg,which clearly demonstrated this possibilitythrough records of motion. Figure 2.7(c)shows a comparison of analytically predictedhorizontal displacement profiles for one- andtwo-dimensional analyses near the edge of asoft soil deposit underlain by a sloping stifferlayer (Faccioli, 1991).

19

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20

Figure 2.7. (a) Foundation on soil deposit underlain by sloping base layer; potential fordifferential excitation of the two sides

(b) Pile-group foundation in soft soil underlain by a sloping base; records atpoints 1,2,3,4 and 5 reveal the strong influence of base slope ("2-D" effect)(from Gazetas, Fan, and Makris (1992) and Tazoh, et al. (1989»

(c) Calculated relative displacements induced on a pile embedded in a soildeposit underlain by steep rock (adapted from Faccioli, 1991)

Soil Improvement and Foundation Remediation

2.4.1 Examples

Most examples of classical bearingcapacity failures during earthquakes havebeen associated with liquefaction of near-surface soils. The overturned apartment buildings in Niigata, Japan, shown previously inFigure 2.4, provide the best known example ofthis type of failure.

Treasure Island was formed in SanFrancisco Bay during the 1930s by placementof up to 90 feet of hydraulic fill over soft claydeposits. Over the years, light to moderatelyheavy structures have been built on the islandusing a variety of foundations includingdriven piles, stiffened shallow footings on filldensified by vibroflotation, displacementpiles, or terraprobes, and conventional spreadfootings. There has been slow subsidence ofthe island due to consolidation of the underlying soft clay. During the 1989 Loma Prietaearthquake, sites where soil improvement hadbeen performed did not experience differentialsettlement, lateral spreading, or sand boils, allof which were widespread over the rest of theisland. Lateral spreading did occur adjacent tothe shoreline slope. The site of the Medical!Dental building, where vibroflotation hadbeen performed, settled uniformly about 25mm. The densified approach area near Pier 1exhibited no signs of ground deformationwhile sand boils, sink holes, and spreadingcracks developed in adjacent areas. Severalbuildings supported on conventional footingsin areas where ground improvement had notbeen performed experienced significant differential settlement and foundation cracking.Pile supported buildings performed well butnon-pile supported floors settled about 150mm. Damage to buried utili ties was extensive.

Failures of foundations due to lack oflateral or uplift resistance have also beenobserved. Sensitivity of the confining soil mayalso play a significant role in both lateraldeformations and tensile capacity of the pile.For example, during the 1985 Mexico Cityearthquake, pile foundations were subjected toapproximately 20 cycles of loading. Some of


the soils in the vicinity of the piles wereidentified as having a sensitivity of about 15.After 2 or 3 cycles of loading, the cohesion ofthe soil significantly decreased, reducing theskin friction. This reduction resulted in a lowpullout resistance. Resistance to lateraldeformation was also most likely reduced.

Large lateral displacement of pilessupporting the Struve Slough Bridges nearWatsonville, California in the 1989 LomaPrieta earthquake (Housner, et aI., 1990)caused excessive rotations at the bridgepier/ deck connection. Each bridge consistedof a flat slab spans with approximately 22 baysof 11 m length, each supported on fourRaymond step taper piles that were approximately 21 m long with 250 mm toes and 380mm heads. The piles extended through a 11 mthick peat deposit and into a silty clay whosedensity increased with depth. A 3.6 to 4.6 mlong, 380 mm diameter circular concretecolumn extended up to the bridge deck. Theextension and at least the top portion of theRaymond pile were reinforced with 4 to 6 No.6 bars. After the earthquake, approximately40% of the spans had separated from the pilesor had broken the piles and fallen into the peatunder the bridge (Figure 2.8).

Severe damage occurred at Berths 3558 Public Container Terminal in Oakland dueto the Loma Prieta Earthquake (Figure 2.9).Batter piles were badly damaged in the sectionbelow the deck in brittle shear partly as aresult of insufficient confinement reinforcement. Settlement and lateral spreading ofhydraulic placed fills behind a rock dike andloose saturated sands beneath the dike due toliquefaction contributed to the damage (Figure2.10).

An example of foundation failure dueto excessive settlement was the failure of prestressed tension piles under a drydock inGreece during a cluster of three earthquakeshocks producing accelerations of 0.15 - 0.20 geach. The densification of a thick sandy layerunder the drydock slab was one of the consequences of that failure (Tassios, 1987).

21

Figure 2.8. (a) Large la,teral displacement of Struve Slough Bridge piles; (b) resultingdamage to pier/deck connection (after Seed, et al., 1989)

2.4.2 Causes

There are a number of causes of foundation failure hazards. Some are related to thesoil surrounding the foundation, some to thefoundation, and some to the interactionbetween the soil and foundation.

Foundation failure due to lack of supporting capacity is typically a soil strengthproblem. A lack of sufficient strength to resistdynamic loading may be inherent in the soil,though the customary use of large factors ofsafety in foundation design suggests that thismechanism is not likely to be common.Reduction of the available strength, however,due to excess pore pressure generation orcyclic degradation effects, can produce failuresof both deep and shallow foundations.Structural failure of one or more elements of afoundation system may also be caused by

defects, splices, or joints, or may result frominadequate consideration of the compatibilityof different foundation elements in design.The latter can be illustrated. by the historicallypoor performance of foundations using batterpiles, which introduce stiffness into the foundation system that must be accounted for inthe structural design of the pile cap.

Foundation failure due to excessiveearthquake-induced settlement is caused bythe contractive nature of loose, cohesionlesssoil. When such soils are dry, seismic compaction-induced. settlements can occur veryquickly, predominantly during the period ofstrong shaking. When they are saturated., positive excess pore pressures are induced bycyclic straining. Even if these pore pressuresdo not reach levels sufficient for the initiationof liquefaction, their subsequent dissipationleads to densification and settlement.


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Figure 2.9. Cross-section through 7th Street Terminal at Port of Oakland (after Seed,et al., 1989)

Figure 2.10. Damage to tops of batter piles beneath Seventh Street Terminal, Portof Oakland (after Seed, et al., 1989)


Unsatisfactory foundation performance can result in the transfer of excessivelevels of shaking to structures or equipmentsupported on the foundations. The unsatisfactory performance, which may be characterizedby excessive ground motion amplitudes or frequency contents that produce resonanceeffects, is related to the characteristics of boththe soil (stiffness, damping characteristics) andthe foundation (stiffness, mass, depth), and theloading (ground motion amplitude, frequencycontent, and duration).

2.4.3 Mitigation

Mechanisms for mitigation of potential foundation failure hazards depend on thevarious anticipated modes of failure.Reduction of the potential for bearing failuresrequires strengthening of the soil and/orreduction of pore pressure generation potential. Both can be accomplished by densification or grouting. Drainage techniques can beused for reduction of pore pressures duringand after earthquake shaking, but provide noincrease in the inherent strength of the soil.

Excessive settlement of foundationsdue to earthquake shaking can be mitigated bymethods that improve the ability of the soil toresist densification due to cyclic shearing.Such methods include densification prior toconstruction, physical or chemicalmodification, or the use of stiff inclusions thattransfer at least some of the support of thestructure to soil at greater depths.

Hazards associated with excessivedy' ;~ic response of foundations can bemitL;",ted by improving the soil in the vicinityof the foundation or by modifying the foundation itself. The stiffness of the soil can beincreased by a variety of standard soilimprovement techniques. Techniques thatincrease soil stiffness generally decrease hysteretic damping; however, hysteretic dampingis usually much smaller than radiation damping for most foundation systems. The naturalfrequency of a footing can be increased byincreasing its mass or decreasing its dimensions. Decreasing the footing mass or increasing its dimensions will have the oppOSiteeffect. Increasing the depth of embedment of

a footing will increase both its natural frequency (except for the horizontal translationmode of vibration) and damping ratio, provided that good contact between the sides ofthe foundation and soil is maintained.

2.4.4 Discussion

Foundations have, in the absence ofliquefaction, generally performed well duringearthquakes. Perhaps the most critical problems with respect to mitigation of foundationfailure hazards are those associated with existing structures. Evaluation of the conditionand physical characteristics of older foundations that have been in service for extendedperiods of time remains a difficult task.Remediation of such foundations, particularlyin congested urban areas where sensitivestructures or buried utilities are common, isalso difficult.

2.5 Slopes and RetainingStructures

A great deal of the geotechnical damage observed during and after earthquakeshas been associated with slopes and retainingstructures. Slopes may be divided into threecategories: (i) natural slopes; (ii) excavatedslopes; and (iii) compacted earth and rockfillembankments. Any of these types of slopesand retaining structures may fail by grossinstability or by excessive cumulative deformation, the latter of which can lead to eitherunsafe conditions or loss of serviceability.

1. Slopes

Natural slopes consist of geologicallydeposited soil or rock strata that can fail as aresult of inadequate static or seismic stability.The periodic instability of natural slopes is anatural process of landscape evolution, andfailures of natural slopes occur frequentlyeven in the absence of earthquakes. At anygiven time, unstable or marginally stableslopes are common in many areas. Whenthese slopes are subjected to strong earthquake shaking, slope failures may occur.


Excavated slopes consist of geologically-deposited soil or rock strata throughwhich an excavation has been made. As withnatural slopes, excavated slopes may undergogross instability or excessive cumulativedeformation resulting from inadequate staticor seismic stability.

The primary hazards associated withcompacted earth and rockfill embankmentstructures are failures of the foundations onwhich these structures rest. Failure may resultfrom liquefaction or pore pressure buildup insaturated sandy or silty foundation soils orinadequate undrained strength of clayeyfoundation soils. There should be little hazardassociated with slope failure of modernembankment fills and dams due to the strictengineering controls that go into their designand construction. Older embankment structures, particularly those constructed by thehydraulic fill technique, however, may be susceptible to seismically-induced failure.

2. Retaining Structures

Earth retaining structures retain eithergeologically-deposited materials or fill soil orrock. These structures may also undergo failure as a result of inadequate static or seismicstability. Mechanisms of failure include sliding or overturning as a result of the build-upof excessive thrust from the retained soil as aresult of ground motion induced inertialforces, loss of shear strength in the retainedsoil due to pore pressure buildup, or loss ofshearing resistance in the foundation soil as aresult of pore pressure buildup or cyclicdegradation. Furthermore, hydrodynamicforces may contribute to failure of waterfrontstructures.

Experience shows that most earthretaining structures that have been designedfor static loading with conventional factors ofsafety perform satisfactorily in an earthquake.Most retaining structure failures that haveoccurred have been the result of liquefactionof the retained or foundation soil resulting forexample in excessive lateral thrust or loss ofresistance of anchors or deadmen. Numerousexamples of this kind of failure are available in


Japanese Port and Harbor Authority publications.

An important category of retainingstructure is reinforced backfill structures, i.e.,structures constructed with steel strips andwire mesh, geogrids, geotextiles, etc.Adequate performance of these structuresrequires proper design and performance of thestructural reinforcing elements as well as thesoil in which they are embedded. Failure ofthese elements may lead to gross instability orexcessive deformation of the face of the retaining structure. Similar to the structural elements used in reinforced soil structures,tiebacks and soil nails used to support excavated slopes may also fail due to excessiveshear or tension, although examination of anumber of tieback walls in various stages ofconstruction during the 1987 Whittier earthquake revealed no instances of failure or significant distress (Ho, et aI., 1990). Failure ofstructural reinforcing elements may lead togross instability or excessive deformation ofthe face of the excavation.

2.5.1 Examples

The 1964 Turnagain Heights slide inAlaska, shown in Figure 2.5, is a well knownexample of an earthquake-induced landslide.The Turnagain Heights slide involved a lengthof about 2.6 km of coastline and extendedinland an average distance of approximately275 m, encompassing an area of more than4000 m2. Some residential structures in theslide zone moved laterally as much as 150 to180 m. Earthquake-induced failures of naturalslopes are shown in Figures 2.11 and 2.12.Failures of excavated slopes were observed inthe banks of the All American Canal and theSolfatara Canal in California during the 1940EI Centro earthquake. Liquefaction ofhydraulic fill forming the upstream slope ofthe Upper San Fernando dam dUring the 1971San Fernando Valley earthquake illustrates thepotential for earthquake-induced failure ofsuch embankments.

Most failures of retaining structuresduring earthquakes have been associated withliquefaction of backfill and/or foundationsoils. Many failures of quay walls in the

25

26

Figure 2.11. Slope failure along Union Pacific railroad right-of-way in 1949 Olympia,Washington earthquake (photo by G.W. TIlOrsen)

Figure 2.12. Failure of steep coastal bluff in Daly City, California in Lorna Prietaearthquake (after Seed et aI., 1989)


harbor area of Niigata were observed in 1964(Figure 2.13). Failures of bridge abutmentwalls have also been observed as a result ofearthquake shaking.

2.5.2 Causes

Slope and retaining structure hazardsmay be broadly divided into two categories failures due to earthquake-induced inertialstresses that temporarily exceed the availablestrength of the soil and failures due to earthquake-induced weakening of the soil.

Strong ground shaking can inducelarge dynamic stresses irt slopes and retainingstructures. When these dynamic stressesexceed the strength of the soil, deformationswill develop. The sliding block analog is useful for illustration of the resulting accumulation of permanent deformations and hasformed the basis for methods of analysis ofpermanent deformations of slopes <Newmark,

1965) and retaining structures (Richards andElms, 1979). Such analyses clearly indicate thedependence of permanent deformations onshear strength and identify insufficientshearstrength as the principal cause of thosedeformations.

Seismically-induced shear strengthdegradation is another factor that may contribute to a slope or retaining structure hazard.Excess pore pressures in a slope or in thebackfill or foundation of a retaining structurecan be generated by earthquake shaking.These pore pressures can have three adverseaffects: (0 increased driving forces in theslope or retaining structure; (ii) decreased soilstrength; and (iii) decreased soil stiffness.Strength degradation may result from the porepressure buildup associated with an earthquake as well as from changes in soil structuredue to prior shearing. The technical literaturecontains information on residual strength ofsands and clays.

Figure 2.13. Failure of (a) quay wall due to liquefaction in Niigata, Japan, and (b) retainingwall due to inertial forces in 1960 Chile earthquake (after Seed, 1970)


Changes in slope geometry can alsocause a hazard. Changes in geometry mayresult from human activity, as in the case ofconstruction excavations near the base of aslope, or natural actions in the form of thefamiliar process of erosion.

2.5.3 Mitigation

For a soil improvement technique tobe effective for mitigation of slope and retaining structure hazards, it must influence thecauses of the hazards by either: (i) alterationof the forces affecting equilibrium (i.e., reducing the stress level acting on soil elements);(ii) alteration of soil or rock material behaviorand properties; or, (iii) control of the geometryof the slope or retaining structure so thatactions by man or nature do not adverselyimpact stability.

With respect to the forces affectingequilibrium, requirements for hazardmitigation may include decreasing thedynamic forces acting on the slope or retainingstructure through control of geometry andmaterial properties. Mechanisms formitigation of slope and retaining structurehazards also depend on the nature of theanticipated mode of failure. Mitigation offailure due to exceedance of soil strength bydynamic stresses requires improvement of thestrength of the soil. Failure due to generationof excessive pore pressure must be mitigatedby measures that either reduce the tendencyfor pore pressure generation or that allowdissipation of any generated pore pressuresduring and after earthquake shaking.Hydrodynamic forces on waterfront structurescan be reduced through the use of energydissipation methods.

With respect to geometry, therequirements for hazard mitigation mustresult in control of the factors affecting geometry. For example, stream erosion of naturalslopes should be prevented and slope cutsshould not occur without proper considerationof stability and erosion protection.

2.5.4 Discussion

Despite advances in analytical methods and material characterization, the stabilityof slopes and retaining structures is mostcommonly evaluated using pseudostaticmethods, which represent the complex effectsof earthquake shaking by a single seismic coefficient. Even with these simplified methods,uncertainty remains in selection of the appropriate strengths, particularly for sands, silts,sensitive clays, and for natural slopes withfault gouge and joint infilling. The development and accumulation of permanent deformation during periods of strength exceedancemay lead to unsatisfactory performance, andcases of excessive slope, retaining structure,and foundation deformations are common inthe literature. Methods for estimation of permanent deformations are available and areused in practice, at times for conditions otherthan those for which they were originallydeveloped. Additional research on the application of such methods to slopes other thanembankment dams would be beneficial.Development of methods for prediction ofpermanent deformations in reinforced earthand soil nailed walls and embankments isneeded. The stability of slopes in lined wasteimpoundments is a growing concern in modem geotechnical engineering practice anddeserves increased attention from researchers.

2.6 Summary andConclusions

Earthquake damage can be caused bya variety of geotechnical hazards. Broad division of these hazards into the categories of liquefaction, ground shaking, foundation failureand slope and retaining structure failure provides a framework for consideration of theirbasic causes. Consideration of these causes,however, indicates several recurring themesthat are common to many of the hazards.


Many geotechnical hazards are causedby the temporary exceedance of the availablestrength of the soil by earthquake-inducedshear stresses. It is necessary to understandthe magnitude and distribution of thesestresses if seismic stability and seismicallyinduced deformations are to be evaluated.Excessive shear stresses can be caused byunfavorable combinations of soil propertiesand geometry, and input motion amplitudeand frequency content. Presently, however,significant uncertainty exists with respect tothe estimation of these dynamic stresses, indicating that further research in ground motioncharacterization, ground response and soilstructure interaction analysis is needed.

Many geotechnical hazards are causedby an earthquake-induced reduction of shearing resistance, as in the case of liquefaction.Failures of this type are caused by excessivepore pressure buildup during and after earthquake shaking, and can have catastrophicconsequences. Mitigation of liquefaction


hazards is known to require reduction of porepressure generation or improvement of theresidual strength of the soil; however, anumber of issues related to evaluation ofliquefaction hazards require additionalresearch.

The requirements for mitigation ofgeotechnical hazards depend on many factorsincluding the nature of the hazards, thegeotechnical conditions, the cost of mitigation,and the nature of any nearby development.Because soil improvement and foundationremediation may be required in close proximity to sensitive structures and facilities, it isnecessary to ensure that the mitigation processdoes not cause any damage to such facilities.Methods for mitigation of seismic hazards bysoil improvement or foundation remediationtechniques must also take into account thespecific causes and mechanisms associatedwith the various hazards. The methods mustdirectly influence those causes and mechanisms and, in order to be used with confidence, their effects must be verifiable.§

29

3. Soil Improvement and FoundationRemediation Techniques

After each technique is brieflydescribed, its historical use, a few pertinentcase histories, and a discussion of the observedeffectiveness of each is given. Finally, currentissues in the application of each technique toseismic hazards and foundation remediationare mentioned.

As the number of soil improvementand foundation remediation techniques andtheir applications is quite broad, we havedivided them into (1) densification, (2)drainage, (3) physical and chemical modifica,.tion, (4) inclusions, and (5) foundation reme-

.diation techniques.

3.1 Introduction 3.2.1 Description of DensificationTechniques

Dynamic Compaction

Dynamic compaction is a simple andeconomical means of densifying loose granular deposits. Typically, steel or concreteweights of 5 to 20 tons are repeatedly droppedon a. site using the single cable of a standardcrawler crane from heights of between 10 to 30m. In one special U.S. case, weights of 35 tonswere used with two lines on two separatedrums with synchronized clutches and brakes.In another unusual case, a special crane wasable to lift and drop 35 tons on a single line.Figure 3.1 shows dynamic compaction inprogress.

3.2 Densification Techniques

Since most desirable soil propertiesimprove with increasing soil density, densification is one of the most commonly used techniques for soil improvement. A number ofdifferent densification methods have beendeveloped and used successfully, and theiruse and effectiveness are discussed in this section. The densification methods consideredare dynamic compaction, vibro compactionand vibro replacement, compaction grouting,and other techniques such as compaction byexplosives and compaction piles.

Dynamic compaction has essentiallyevolved on a trial-and-error basis as a methodfor solving site development problems. Sinceno special equipment is required, the methodcan be quite economical, especially for treatment depths over 3 m and for areas over 2»00m2. It has been estimated that about 2,000sites worldwide have been treated by dynamiccompaction. In a majority of these projects,the process was used for the improvement ofloose natural sands, granular fills, cohesivefills, rubble, and miscellaneous materials for avariety of civil engineering projects, generallyin non-seismic areas. A number of cases arereported in the literature where dynamiccompaction was used to reduce liquefactionpotential. In these cases, repeated drops were

National Science Foundation Preceding page blank 31

Figure 3.1. Dynamic compaction at a site inBangladesh using a 100 ton cranedropping a 16 ton weight 30 m.(Holtz and Kov.acs, 1981).

used to densify loose sands to a sufficientdegree that it appeared that liquefactionwould no longer be a concern. Dynamic compaction has also been successfully utilized toimprove collapsible soils in Utah, Montana,Arizona, Bulgaria and the USSR.

Since the weight is dropped onto theground surface, the method is limited in itsability to treat soils at great depth becausestress wave amplitudes attenuate with depthdue to radiation and material damping.According to Welsh, et al. (987), the maximum practical treatment depth is about 12 mdepending upon soil conditions and the size ofthe project.

Vibro Techniques

Loose deposits of clean granular materials may also be densified by inserting sometype of vibrating tube, probe, or blade at different locations and depths in the deposit.

Sometimes water jets and/or air pressure areused to aid in insertion, and usually the probeis repeatedly inserted and withdrawn at thesame location. In vibroflotation and sandcompaction pile systems, water and additionalclean sand are added during withdrawal ofthe probe so that a column of denser materialremains after the process is completed. Figure3.2 shows a schematic of the vibro compactionequipment and process. In addition to thevibroflotation and sand compaction pile systems, other vibro compaction systems includethe Foster Terra-Probe, Vibro Rod, VibroWing, and the FrankiY-Probe. Welsh, et al.(1987) and Massarsch (1991) prOVide goodsummaries of vibro densification techniques.

Densification by vibro techniques andits limitations are fairly well understood.Important parameters include soil type andgradation, distance from vibrating source,vibration levels and whether the soil liquefiesduring the installation procedure. Two mechanisms have been recognized as responsiblefor densification in granular soils: (1) porepressure generation leading to controlled liquefaction, and (2) displacement of soil due tointrusion of gravel or sand.

For cohesive soils, vibration alone willnot improve the soil, so some other materialsuch as sand (sand compaction piles), sandand gravel (sand-gravel piles), or gravel alone(stone columns) is introduced into the soil anddensified, usually by displacing gravel or sandaround the probe. Sand and stone columnsare made in a manner very similar to the vibrocompaction methods described above (Figure3.3). Depending on the spacing, usuallybetween 15 and 35% of the volume of the softsoil is replaced by stone. Columns areinstalled in a triangular or rectangular gridpattern at typical center-to-eenter spacings of1.5 - 3.5 m. Vibro replacement methods areappropriate 'for a rather wide range of soilsincluding silty sands to clays. Stratified sandsor sands with some clay that cannot be effectively treated by vibro compaction methodsare good candidates for stone columns. Theyare not recommended for highly sensitiveclays nor for peats or other organic materials.Vibro concrete columns can be used in thesesituations.


It \ Water supply

Figure 3.2. Vibrocompaction equipment and process (Welsh, et a1., 1987)

DiMaggio (1978) and Barksdale andBachus (1983) provide good descriptions ofthe design and construction of stone columns.

Compaction Grouting

Compaction grouting involves theinjection of low slump mortar grout underrelatively high pressure to displace and compact in-situ soils. Silty sand is used with orwithout Portland cement and additives suchas fly ash, fluidizers, and accelerators. Onlyenough water for a 0 - 25 mm slump is used.Figure 3.4 illustrates how compaction groutingdensifies loose soils by displacement.

Compaction grouting has been usedmost often to arrest or eliminate foundationsettlements, to lift and level slabs, and forfoundation slabs on grade. It has also beenused very successfully to control surface settlements occurring during soft ground tunneling, to rectify sinkhole problems, and to densHy liquefiable soils under both new and existing structures.


Other Densification Techniques

Other densification teclmiques such ascompaction by blasting and by driving displacement piles have been occasionally andsuccessfully used to densify loose deposits.Although quite respectable relative densitiescan be economically achieved by blasting, theresults may be somewhat erratic in depositswhich are not very homogeneous. Denselayers and lenses are likely to be loosenedsomewhat, but the overall improvement willprobably be satisfactory. An almost immediate surface settlement of between 2 to 10% ofthe layer thickness will occur after blasting.

Driving displacement piles into aloose deposit also densifies the soil by displacement and vibration. Sometimes suchpiles are called compaction piles. Low costtimber piles (e.g., telephone and power polerejects) are commonly used for this purpose.They are not necessarily attached to any cap orsuper structure but merely used for in situdensification. Most of the time the piles are

33

'- ~

)WATER JETS/",

STONEAI

r-... .. .~, \ ....

'b ..

~.

-:~:

.}~ COMPACTED.~. COLUMNt<:.,

Figure 3.3. Stone columns constructed by vibroreplacement (DiMaggio, 1978)

left in place, but if they are withdrawn, thenthe hole is backfilled during withdrawal of thepile. Backfilling automatically takes placewith the type of compaction piling describedby Solymar and Reed (1986). They used a 0.5m diameter closed-end pipe pile driven torefusal then filled with sand. Then the pilewas withdrawn about 2m so that the hingeddriving shoe opened and allowed sand to flowinto the hole. The pile was repeatedly drivenand withdrawn to density the sand.

3.2.2 The Practice of SoilImprovement by Densification

Densification techniques are amongthe most commonly used methods for soilimprovement. A great deal of practical experience has been obtained with many densification techniques; some newer methods are lesswell tested.


Figure 3.4. Schematic of compaction grouting(Welsh, et a1., 1987)

3.2.2.1 Historical Use of DensificationTechniques

Dynamic Compaction

Dynamic compaction is one of theoldest known densification techniques. Thereis good evidence that compaction of loosedeposits by repeatedly lifting and droppingstone weights took place in China around 1000A.D. In modem times, dynamic compactionwas used in the U.S.S.R. and Germany in the1920s and '30s, primarily to densify loosesands and loess deposits. In the late 1930s, theU.S. Army Corps of Engineers used dynamiccompaction to densify certain parts of FranklinFalls Dam, New Hampshire, which was constructed by hydraulic filling.

After those early uses, the techniquewas under-utilized until the early 1970s whenL. Menard in France and R.G. Lukas in theU.s.A. started using large weights droppedfrom crawler cranes to densify all types ofloose deposits and waste fills.


Vibro Techniques

Vibro compaction (vibroflotation)techniques are among the oldest densificationmethods. Vibroflotation was developed andpatented in Germany in the 1930s, and introduced into the U.S. in 1941. The VibroflotationCompany of Pittsburgh was one of the firstU.s. contractors Specializing in foundation soilimprovement techniques. Other vibro compaction techniques such as the Terra-Probeand Vibro Wing were developed much later,in the 19705. Mitchell (1970, 1981) describessome of the early developments in vibro compaction systems. Recent developments aredescribed by Welsh, et al. (1987).

Stone columns are somewhat older.For example, according to Handa (1984), handdug stone columns were used to support theTaj Mahal in India. Modem use of stonecolumns was a logical outgrowth of sandreplacement and densification methods(vibroflotation), and they have been used inincreasing numbers in the last 10 to 15 years,according to Munfakh, et a1. (1987) andBarksdale and Bachus (983).

Compaction Grouting

In contrast to many other soilimprovement techniques, compaction grouting is a uniquely American development.Warner (1982) provides an excellent historicaldescription of the development of compactiongrouting, while Graf (1992a) and Warner, eta1. (1992) provide overviews of currentpractice.

Other Densification Techniques

Blasting as a means of densifyingloose granular deposits is a rather old densification technique. According to Lyman (1942),blasting was used to densify Franklin FallsDam in New Hampshire in the late 1930s. Ithas also been used for many years in theSoviet Union to densify loess soils (Mitchell,1970).

Compaction piles are not well documented in literature. According to Mitchell(970), references about using piles to densify

35

granular soils only appeared in the 1950s,although the first edition (1948) of Terzaghiand Peck (1967) mentions the practice.Apparent]y, also, loess in the Soviet Unionwas compacted by compaction piles in the1930s and '40s.

3.2.2.2 Case Histories

Recently, Mitchell and Wentz (1991)evaluated the performance of improvedground during the 1989 Loma Prieta earthquake, where treatment methods such as vibroreplacement, vibro compaction, sand compaction piles, non-structural timber displacement piles, deep dynamic compaction, compaction grouting, and chemical permeationgrouting were used. Twe]ve sites from SanFrancisco Bay to Santa Cruz were studiedwhere maximum peak ground accelerationsranged from as low as O.11g at Marina Bay inRichmond to as high as 0.45g at two sites inSanta Cruz. Mitchell and Wentz reported thatwithout exception, there was no distress ordamage to the improved ground or the structures supported on this ground. In manycases, untreated adjacent ground settled,cracked or showed signs of liquefaction sandboils.

Dynamic Compaction

Dynamic compaction has seen considerab]e use for remediation of embankmentsand dams supported on potentially liquefiablefoundation soils. A good example is MormonIs]and Auxiliary Dam, a part of the FolsomDam and Reservoir Project, located nearFolsom, CA. Preliminary investigationsdownstream of the embankment indicatedthat the dredged alluvial foundation materialsconsisted primarily of gravels with cobblesand sand, and cobbles with gravel and sand.As a result, the Becker Penetration Test (BPT)method was selected by the Corps ofEngineers as the primary too] for evaluatingthe liquefaction potentia] of these materials. Itwas concluded that extensive liquefactioncould be expected and that catastrophic loss ofthe reservoir might result

Drought conditions and resulting lowreservoir levels in 1990 provided an opportunity to perform at least partial remedial treat-

36 5 0 i] Imp r 0 v e men t

ment along the upstream portion of theembankment. Liquefaction remediation wasrequired to depths as great as 18 m and densification by dynamic compaction was selectedfor remediation based on the amount of timeavailable for construction, technical feasibility,and overall cost. Dynamic compaction wasperformed on a grid pattern consisting of primary, secondary and tertiary impact points.Primary impact points were on 15 m centerswith 30 drops specified at each point. Thegrid was completed with 30 secondary and 15tertiary drops at intermediate drop points.

After treatment, the Becker data indicated that a high level of compaction effectiveness was achieved in the upper 9 to 12 mof the treated zone. Below 12 m, analysesindicated a marked reduction in improvement,although increases in baseline b]owcount va]ues were obtained. There was some questionas to whether the marginal increases observedat some locations and at greater depths wasthe effects of friction on the Becker hammercasing as it penetrated the highly treated anddense upper 9 m of the treated zone. Anincrease in the number of BPT(Nl)60 valuesless than the established criteria for a factor ofsafety against liquefaction (FSL) of 1.0 wasobserved in the lower 6 m of the treatmentzone.

Generally, the middle third of the 46m wide treatment zone was provided adequate treatment to preclude the occurrence ofliquefaction, under the assumption that friction has a minima] effect on the BPT (Nl)60values at depth. Areas in which the desiredresults (FSL=1.0) were not achieved werealong the undredged/dredged alluvial contactand at depth along the edges of the treatmentzone. Fie]d observations indicated that excessive pore pressures were developed withinmaterials along the undredged/dredged contact during later phases of the compactionprocess. These may have resulted from somecombination of confinement of water againstthe relatively impermeable undredged materia], lack of drainage features (i.e., perimetertrench, wick drains, etc.) to reduce pore pressure development and excessive energy inputper drop in this area.

and Foundation Remediation

Lukas (1986) presents three case histories where detailed research was done ondensification using dynamic compaction.Other case histories are reported by Leonards,et aI. (1980), Menard and Braise (1975), Welsh,et aI. (1987), Hussin and Ali (1987), and Keller,et aI. (1987). The latter two papers describedtests to improve the seismic stability of loosefoundation soils.

Vibro Methods

At the Fairview Terminal in PrinceRupert, B.C., in order to provide a quay wallin 13 to 21 m of tidal water, concrete box caissons were placed on a gravel fill pad about 7m thick on bedrock. The initiaI design calledfor the foundation pad to be rockfill compacted in place, in two lifts, using dynamiccompaction underwater. The final schemeadopted used a gravel fill that was compactedin situ in one lift using the vibroflotation technique. The design earthquake PGA was 0.14g, and PGV = 0.3 m/s. Figure 3.5 illustratesthe application.

Dobson (1987) describes two case histories in which vibrotechniques were used inan attempt to reduce the risk of liquefaction.In one case, foundations for a 40 m diameterLNG tank supported on timber piles werestrengthened. The foundation conditions wereloose sandy silts, silty sands and sands and thestabiliZing system was by a ring of stonecolumns surrounding the tank out to a distance of approximately 25 m. In the secondcase, stone columns were used quite successfully to stabilize the foundations for a dam inSouth Carolina.

Other case histories on vibrotechniques used to improve seismic resistancehave been reported by Solyrnar, et aI. (1984),Keller, et al. (1987), Hussin and Ali (1987),Hayden and Welch (991), Neely and Leroy(1991), Ergun (992), and Egan, et al. (1992).Mitchell and Wentz (1991) reported satisfactory performance at seven sites where vibrotechniques were used to mitigate the risk ofliquefaction. On the other hand, an unsuccessful attempt at using vibroflotation to densify

General fill

Y High tide

.............. Concrete caissonRock fill

y Low tide

Figure 3.5.

~ // Gravel padx. compacted in-situ~ ..~'"\..-~----

.......~' ~~-.:. - ~..:-o-._._~ ,-

Caisson quay wall supported on a gravel pad densified by vibroflotation(Mr. A. Wightman, Klohn Leonoff Ltd., personal communication, August 1991)


loose deposits of silty sands to increase theirseismic resistance has been reported byHarder, et aI. (1984).

Compaction Grouting

An excellent case history on the use ofcompaction grouting for tunnel constructionhas been reported by Baker, et aI. (1983) andsummarized by Welsh, et aI. (1987). Anothercase history, summarized by Welsh, et al.(1987), was the use of compaction grouting todensify the foundations for a power plant inJacksonville, Florida (Schmertmann, et al.,1986). Warner (1982) also describes two casehistories of compaction grouting.

The use of compaction groutingspecifically to mitigate the potential for earthquake damage is not common. A successfultesting program conducted to determine thefeasibility of compaction grouting a loose sandlayer beneath at Pinopolis West dam in SouthCarolina was described by Salley, et aI. (1987).It is not clear whether the technique was utilized' under the entire dam. Mitchell andWentz (1991) reported that no damage wasobserved after the Lorna Prieta earthquake atKaiser Hospital and surrounding paved areason loose sandy soils previously improved bycompaction grouting. Graf (1992b) describeda case history in which liquefiable sands underthe proposed addition at a California hospitalsite were improved by compaction grouting.The existing hospital building was pilesupported, and the need to keep the facilityopen during construction precluded the use ofdriven piles, vibrofiotation, and dynamiccompaction. Cost analyses showedcompaction grouting was significantly cheaperthan the other feasible alternates considered(excavation and replacement with adiaphragm wall and dewatering; chemicalgrouting; drilled shafts). After a successfultest program, the final grouting pattern washoles on 3 m centers up to 11 m deep.Intermediate holes at 1.5 m spacing weregrouted between 2 and 5 m deep. In all cases,grouting was staged at 0.9 m intervals.SPT(N) values increased from 10 to 20 beforegrouting to 30 to 35 afterwards. Interestingly,the structure experienced the 1989 LomaPrieta earthquake with no damage (Graf1992b).

Other Densification Methods

Solymar, et al. (1984) presented aninteresting case history of the use of blasting todensify a deep liquefiable deposit of sandunder a dam in Nigeria (see Sutton andMcAlexander, 1987, for a brief summary). LaFosse and von Rosenringe (1992) reported thesuccessful use of blasting to densify a potentially liquefiable loose sand layer 6 to 15 mdeep below a site for building foundations inMassachusetts. Mitchell (1970) presentedsummaries of five case histories on compaction piles which were documented in theliterature up until that time. Since then, theonly published case history of their use thatcould be located was reported by Egan, et al.(1992).

The Sealand pier in the Port ofTacoma, Washington, is supported on precastconcrete piling driven through loose tomedium dense silty sand and bearing in densesand at some depth. The upper 15 m of soilalong the pierhead line was particularly loose(SPT(N) values typically were less than 8)prior to construction. Using Seed's method ofanalysis, the liquefaction potential was foundto be high for even a small to moderate earthquake. Timber "pinch" piles were driven at 2m spacings along the pierhead line to densifythe soils in this area. After the piles weredriven to the required depth they were broken("pinched") off at the mudline to prevent interference with shipping. No postconstructionverification of improvement was conducted.

3.2.2.3 Observed Effectiveness ofDensification Techniques

The observed effectiveness of the various densification techniques described in thissection varies considerably. Solymar andReed (1986) made a detailed study of threeproject sites with underlying loose granularsoils, at which dynamic compaction, vibrocompaction, compaction (sand) piling, anddeep blasting were used. Their observationsand conclusions provide a good indication ofthe effectiveness of these densification techniques.

Dynamic compaction is suitable forsands, even if the deposit "contains layers with


some cohesive properties." (On the otherhand, Leonards, et aI., 1980, found that theenergy of impact appeared to be damped outby a thin clay layer.) The technique requirescommonly available equipment that is notdifficult to operate. Dynamic compaction wasthe most practical for depths between 15 and20 m. Dr values of 65 - 75% are attainable atdepths greater than 10 - 12 m and at shallowdepths, Dr can be up to 75 - 85%.

Vibro compaction appears to be bestsuited to loose, clean, saturated sands.Specialized equipment and expertise arerequired, particularly at greater depths. Drvalues of 70 - 80% are possible below 25 m,with even greater densities at shallowerdepths.

Compaction (sand) piling is suitablefor layered or stratified materials, throughsome expertise as well as a source of suitablesand backfill is required. Depths greater than20 m are possible with Dr values of 75-80%.

Deep blasting is appropriate for partlyor fully saturated clean loose sands and saturated silt deposits. Blasting expertise isrequired, as is suitable drilling equipmentcapable of casing holes to the desired depth.Maximum Dr values seem to be about 65-70%.

During its 50 year development, compaction grouting has been successfully used inthousands of projects. In addition, there is thesatisfactory performance of numerous compaction grouted structures in the 1989 LomaPrieta earthquake.

3.2.3 Current Issues in Applicationof Densification Techniques

Many densification techniques havenot been standardized or codified. Very littleresearch has been undertaken to establish thefundamental mechanisms by which theywork, or to evaluate the effects of various soil,material, and construction method parameterson their effectiveness. However, despite thefact that many of these parameters varywidely in practice, the methods appear towork quite well.


3.2.4 Summary

Because most soil properties improvewith increasing density, densification is one ofthe most commonly used soil improvementmethods. Densification techniques describedin this section include dynamic compaction,vibro compaction, vibro replacement, compaction grou ting, deep blasting, andcompaction piles. The suitability of each technique depends on soil and site conditions,economics, whether it is new construction orremedial work, etc. A very important parameter in determining effectiveness is the relativehomogeneity of the soils at the site. Althoughmost soil improvement projects using densification have been for static conditions, a fewcase histories were found in which the procedures were successfully used to mitigate potential seismic hazards.

3.3 Drainage Techniques

It is well known that in many situations lowering the groundwater table can veryeffectively reduce the potential for failure offoundations, slopes, and retaining structures.Several techniques have been developed tofacilitate the drainage of these features, andthe design and installation of most drainagetechniques is now fairly well established ingeotechnical practice. This section summarizesthe drainage techniques that may be appropriate for the mitigation of the hazards of liquefaction and ground shaking as well as failures of foundations, slopes, and retainingstructures when subject to earthquake forces.Although drainage design and construction isrelatively straightforward, a number of questions remain regarding the performance ofdrainage systems during seismic events andthe suitability of some drainage techniques forthe mitigation of seismically induced hazards.

3.3.1 Description of DrainageTechniques

A wide variety of techniques havebeen used for drainage of soil deposits, foundations, slopes and structures, and these arelisted in Table 3.1. It is convenient to considerthese techniques in terms of four functional

39

Table 3.1. Drainage Techniques Function

Drainage Technique Interception Pore Pressure Dewatering AccelerateControl Consolidation

Gravel Drains X X

Sand Drains X X

Prefabricated Geocomposite X X X("wick") DrainsWells X X

Surface Drains X

Toe Drains X

Horizontal Drains

- Borehole Drain X X

- Blanket Drain X X

Gallery Drain Systems X X

ElectrokineticlElectro-osmosis X X X

Vacuum Extraction X X

drainage categories indicated at the top of thetable: interception, pore pressure control,dewatering, and techniques used to accelerateconsolidation.

Interception

Drains which redirect surface flow oralter the seepage patterns in slopes, embankments, or behind retaining structures are verycommon and have been used for many years.Surface drains include diversion and interceptor ditches, and near surface interceptor drainsconstructed in shallow trenches. These latterdrains are sometimes called French drains.Because of its high stabilization efficiency inrelation to design and construction costs,drainage of surface and groundwater is themost widely used and generally the most successful method for stabilizing slopes. Figure3.6 illustrates these types of surface drains.

Subsurface drainage to lower thegroundwater table is a very commontreatment method for potentially unstable

slopes. Traditional procedures include a)drainage blankets and trenches, b) drainagewells, c) drainage galleries, adits or tunnels, d)subhorizontal drains which may be drilledeither from the slope surface or drainage wellsor galleries, and e) subvertical drains drilledupward from drainage galleries. Some ofthese systems are illustrated in Figures 3.7 and3.8. Most often these systems drain by meansof gravity flow; however, pumps areoccasionally used from low level collectiongalleries or wells. Descriptions of drainagesystems applied especially to landslides aregiven by Gedney and Weber (1978) and byHoltz and Schuster (1992).

Drainage of the backfill behind retaining structures is essential for virtually allwalls, and Figure 3.9 illustrates some of thesesystems. Geocomposite drains may be substituted for granular filters and drain rock.

Pore Pressure Control

Other drainage techniques (e.g.,gravel drains, blanket drains, etc. - Table 3.1)


I+O-..,.--'NTERCEPTOR ORA'N

Figure 3.6. Surface drainage of a slope by a diversion ditch and interceptor drain (Gedney andWeber, 1978)

Soft, saturated--...,..,.,~~~"...--:;r:)r;/~k~'~<'i1:7;;'T--/ /)),(\.O}

Q

PipeDrainage

.~blanket

Geotextile on top and bottom of. drainage blanket

Fill...~'"

!~N~.)//oJ!.///Seepage

Firm ground

Figure 3.7. Drainage blanket used for stabilizing shallow foundations (after Cedergren, 1989)


42

UPSLOPEDRAINAGE DITCHES~

Figure 3.8. Subhorizontal and vertical drains used to lower ground water in natural slopes(Gedney and Weber, 1978)

./

Figure 3.9. Common methods for draining retaining wall backfills


may also be used to reduce excess porepressure resulting from ground shaking insoils that are already saturated. Thesemethods are based on the concept that thepore pressures on one or more surfaces withinthe soil deposit are maintained at hydrostatic,so that any excess pore pressures generated byearthquake shaking will be rapidly dissipatedby flow toward the drainage surface. Thisreduces the pore pressure within the soildeposit, thereby maintaining the effectivestresses at higher values than they would bewithout drainage.

Gravel drains installed using an augercasing do not produce significant vibrationsand densification; consequently their performance is based on the capability of the gravelcolumn to control excess pore pressures, Le.,provide drainage. Simplified analyses ofgravel drain response to cyclic loading for design purposes are available (Seed and Booker,1976). Required parameters include those forthe earthquake, stress state, soil properties,and drainage and filtration parameters.Limitations of the analyses include the factthat they are one dimensional, and the assumptions of (1) a constant coefficient of volume compressibility, and (2) that the graveldrains have a permeability large enough toprevent pore pressure generation andimpedance to flow at the interface between thedrain and the surrounding soils.

Basements, tanks, utility corridors,and other similar structures founded near thesurface in potentially liquefiable deposits canbe effectively stabilized through encapsulationby drainage materials (Figure 3.10) and bydrainage blankets under the facility (Figure3.7). Drainage blankets are especially effectivefor facilities in which the width of the facilityis very large.

Pore pressure control can also beachieved in finer grain deposits by the use ofvertical sand drains and prefabricated geocomposite ("wick") drains although thesetechniques are more commonly used for accelerating the consolidation of soft cohesive soils.


Dewatering

Conventional dewatering techniqueslower the groundwater table or removegroundwater by active pumping by means ofwell-points (Figure 3.11), sump pumps (Figure3.12), or submersible pumps in deep wells.Other types of surface and shallow drainshave also been successfully used for this purpose. Although most dewatering systems aretemporary, passive dewatering systems aremore appropriate for permanent construction.

Details of dewatering for constructionare given by Mansur and Kaufman (1962) andCedergren (1989).

Lowering a high groundwater table ina potentially liquefiable deposit is a viable design alternative. Appropriate dewatering systems for this case include traditional wellpoints or sump pumps, french and/or trenchdrains, blanket drains, vertical geocomposite("wick") drains and deep wells.

Dewatering can very effectivelyimprove foundation soils by increasing theirshear strength and bearing capacity, reducingtheir potential for volume change, and increasing their resistance to seismic forces.Conventional drainage is also very importantfor retaining structures and abutments.

Among the dewatering techniquesapplicable to slopes are electrokinetic stabilization and electro-osmosis. Drainage ispromoted by these techniques, which increases the shearing strength and thus thestability of the slope. Design and operationalprocedures, particularly for electro.,.osmosis,are well established.

Acceleration of consolidation ofsoft soil

Vertical drainage is often utilized inconjunction with preloading or surcharge fillsto accelerate the consolidation of soft compressible cohesive soil layers in the foundation(Figure 3.13). Total settlements are reduced,soil shear strength and therefore foundationbearing capacity is increased, and resistance toseismically-induced deformations is therebyimproved. Although conventional sand drains

43

Common Utility Duct

fl /A..~

~ j .,.d'"

0OCI>OCOO

00

DDggg

- Crushed StoneM g

:; gg

'"8

0

0.8 6.0 ~I,g·~I'" ~I'"

..

Figure 3.10. Cross-section of gravel drains applied to a common reinforced concrete utility ductin a loose sand in Japan; units in meters (Dr. Y. Koga, Public Works ResearchInstitute, personal communication, August, 1991)

Impervious stratum

Figure 3.11. Drainage of an excavation by multi-stage well-points (Mansur and Kaufman, 1962)


Figure 3.12. Collection of seepage in a slope by open ditches and sumps (Mansur and Kaufman,1962)

INCLINOMETER~DRAINAGE BLANKET

BERM___..c:.._~

II

~SURCHARGE FILL

- -" GROUNDWATEROBSERVATION WELL

"/

/

DEEPSETTLEMENT

POINTS

SETTLEMENTPLATFORM

PERMANENTFILL

PIEZOMETERS NOT TO SCALE

Figure 3.13. A typical well-instrumented vertical drain installation for a highwayembankment (Rixner, et al., 1986)


have been used for nearly sixty years, duringthe last twenty years, they have essentiallybeen replaced by prefabricated verticalgeocomposite ("wick") drains, principallybecause they are much less expensive toinstall. Design principles and installationmethods for wick drains are quite wellestablished (Holtz, et aI., 1991).

3.3.2 The Practice of SoilImprovement by Drainage

3.3.2.1 Historical Use of DrainageTechniques

Interception

According to Cedergren (1989), theCalifornia Division of Highways used subhorizontal drains to stabilize highway slopes asearly as 1939. Other interception techniquessuch as surface drainage and interceptordrains are probably even older. Undergrounddrainage galleries have been used for generations to stabilize slopes, and well systems,particularly relief wells, have been used tostabilize levees and dams and other hydraulicstructures. Therefore, none of the interceptiontechniques currently in use are particularlynew.


Systems for pore pressure control aresomewhat more recent than those for interception. Sand drains were invented in the late1920s, while gravel drains (stone columns) aresomewhat newer. The use of gravel drains asa possible method of stabilizing a potentiallyliquefiable soil deposits was first suggested bySeed and Booker (1976; 1977). Prefabricatedgeocomposite drains were developed in the1970s (Holtz, et aI., 1991), and as far as isknown, they have not so far been used to reduce seismically generated pore pressures.

Dewatering

Mansur and Kaufman (1962) describethe early history of dewatering. The firstrecorded use of well points and large deepwells to lower the water table was in connection with the construction of the Berlin sub-

ways around the tum of the century. Theyalso describe other early dewatering examplesfrom Egypt, Europe, and the U.S.A.Dewatering has mostly been applied to stabilize temporary slopes and excavations.

Acceleration of the Consolidation of SoftSoils

Sand drains were invented by D.].Moran, who received a U.S. patent on the concept in 1926. The first practical installationswere in California a few years later (Porter,1936). In Sweden, Walter Kjellman began experiments in the mid 1930s with woodenpipes; he was issued patents on the first prototype prefabricated drain made entirely ofcardboard in 1938. The first modem geocomposite prefabricated drain was developed byO. Wager of the Swedish GeotechnicalInstitute in the early 1970s. The first drainsused kraft paper filters; later models wereprovided with nonwoven geotextile filters.Competition has decreased the cost of geocomposite drains appreciably, and contractorshave been very innovative in developing installation procedures. Consequently, the installed cost of prefabricated geocompositedrains is now relatively low and sand drainsare obsolete (Holtz, et aI., 1991).


Interception

As mentioned above, interception as adewatering technique has primarily been utilized to improve the stability of slopes.Although most of the published case historiesabout the successful use of drainage for thispurpose did not involve seismic hazards, thereis no fundamental reason why such techniqueswould not also be suitable for the stabilizationof slopes under seismic conditions. A numberof case histories illustrating successful interception of groundwater in slopes are presented by Schuster (1992) and Holtz andSchuster (1992). Although not specificallydesigned to mitigate seismic problems, manyof these cases are from seismically active areas<e.g., California, Italy, New Zealand).

46 Soil Improvement and Founda3ion Remediation


Very few well documented case histories are available that describe the use ofdrainage techniques to mitigate liquefactionpotential of loose granular deposits. Graveldrains (stone columns) have been utilized at anumber of potentially liquefiable sites inJapan, but to date the system has not beentested, as no significant shaking has yetoccurred at these sites. Still, the technique isrelatively popular in Japan.

Dewatering

Dewatering techniques were used inthe restoration work of Hachiro-gata reclamation dykes that were heavily damaged by liquefaction of the foundation in the 1983Nihonkai Chubu Earthquake. About 20% ofthe total length of 51.4 km of the dykes settled1 m or more. Since the damage was assumedto have been caused by liquefaction, the liquefaction resistance of the foundation was investigated. The following methods were employed to mitigate the liquefaction hazard:1) counterweight fill was used to add abearing load to the front and the back of thedyke; 2) cutoff sheet piles were placed at thetoe of shore side slope and drains wereprovided in the dyke on land side for

dewatering, and 3) impermeable asphaltfacing was laid over the top and on the frontslope of the dykes to prevent seepage of lakewater and rain water through the dykesurface. Figure 3.14 illustrates thesecountermeasures.

Acceleration of the Consolidation of SoftSoils

The use of drainage techniques toaccelerate soft soil consolidation has becomecommonly accepted in geotechnical engineering practice. Case histories on this techniqueare given by Johnson (1970), Rixner, et al.(1986), Cedergren (1989), and Holtz, et al.(1991).

3.3.2.3 Observed Effectiveness of DrainageTechniques

Interception

As noted by Gedney and Weber(1978), Schuster (1992), and Holtz andSchuster (1992), interception of surface andgroundwater in unstable slopes is one of themost effective methods of increasing theirstability. Numerous case histories can befound in the literature describing the successful performance of interception drainagetechniques.

. Counterweight fill

Ri?rap7

Asphalt facing

Counterweight fill

Figure 3.14. Example of countermeasures used to mitigate the liquefaction hazard in Hichiro,Japan (Dr. Y. Koga, Public Works Research Institute, personal communication,August, 1991)



As noted above, no case histories areavailable to demonstrate the successful utilization in the field of vertical drainage to mitigateliquefaction potential in loose granulardeposits. In the laboratory, some investigators(e.g., Yoshimi, 1980; Sasaki and Tanaguchi,1982; Iai, 1988) have used large shaking tabletests to evaluate the performance of modelgravel columns. These studies indicated thatin loose (Dr = 33%) soils subjected to a peakacceleration of O.lg, the area of influence of thegravel drain was limited to one diameter fromits center. At greater accelerations, the drainswere not as effective, leading to concern aboutthe flow capacity of gravel drains.Theoretically, they should work by limitingthe development of pore pressure; however,the accuracy of commonly used pore pressuregeneration models (e.g., Seed and Booker,1976; 1977), has not been extensively verifiedby field performance.

Dewatering

As noted in the case history describedabove, dewatering has been apparently successful in at least one site in Japan. As withthe use with gravel drains to control porepressures, dewatering as a means for mitigating seismic hazards is theoretically viable.

Acceleration ofConsolidation ofSoft Soils

If properly designed and installed,vertical drainage of all types can be successfully utilized to accelerate the consolidation ofsoft cohesive soil deposits (Holtz, et al., 1991).One of the more spectacular failures illustrating improper drain selection was reported byHannon and Walsh (1982) and Hoover (1987).These papers emphasize the need for propertesting, specification, and selection of the filterfor the drains.

quake shaking conditions. Because of thissome investigators (e.g., Yoshimi andTokimatsu, 1991) have suggested that excesspore pressure ratios be limited to 0.4 when designing gravel drains for mitigation of liquefaction hazards. In some cases, this willrequire spacings so small as to render the technique uneconomical.

Another issue is the use of drainagetechniques for rehabilitation, under existingstructures because their installation would bedifficult. For example, the installation of stonecolumns directly under existing structureswould be impossible, although they have beeninstalled as close as 3m from new and existingstructures by Japanese and U.s. contractors.Their installation in city streets would bealmost impossible due to buried utilities.However, drainage could be potentially provided by prefabricated geocomposite drainswhich have much smaller profiles and can beinstalled in much more restrictive conditionsand at almost any angle. Although less permeable than gravel drains, prefabricated geocomposite drains may still be viable as adrainage technique because they can beinstalled economically at closer spacings.

3.3.4 Summary

As noted in this section, drainage isone of the oldest and surest means for mitigating potential instability in foundations, slopes,and behind retaining structures. Drainagemethods which are suitable for static stabilityconditions seem to also be entirely appropriatefor many of the same situations when they aresubjected to seismic shaking. Although theoretical analyses indicate that drainage techniques should work, well documented casehistories of their successful utilization underseismic conditions are not available.

3.3.3 Current Issues in Applicationof Drainage Techniques

3.4 Physical and ChemicalModification Techniques

Although a number of drainage methods have been widely employed to mitigateliquefaction potential, their effectiveness hasnot been demonstrated under strong earth-

48

Physical and chemical modification(PCM) techniques play a significant roleamong available methods for improving looseand soft soils, foundation remediation, andmitigation of potential damage due to seismic


events. PCM methods alter the engineeringcharacteristics of the soil and thereby cause itto be inherently more stable. Primary physicalmodifications include densification and thatcomponent of improvement that results frominjecting or mixing another material in withsoil. For example, the injection of a grout intothe pores of a soil may decrease its liquefaction potential by virtue of decreasing itspotential for volumetric contraction, inaddition to any hardening or chemicalbonding that may occur.

Chemical modification includesstrengthening of soft clay by mixing in placewith cementitious materials, and the creationof high strength grouted soil masses, columns,walls or grids using permeation or jet groutingtechniques. Because the distinction is somewhat arbitrary and there is significant overlapin the definitions of the techniques, compaction grouting is considered in Section 3.2,Densification Techniques.

All of the peM techniques have hadextensive use in practice, and the advantagesand limitations for each technique are relatively well established. In most cases, thestrength improvement of the treated soil canbe predicted with some accuracy. However,their use in some seismic hazard applicationsis not yet very common.

3.4.1 Description of Physical andChemical ModificationTechniques

The physical and chemical modification techniques discussed in this report will,for descriptive purposes, be divided intogrouting techniques and soil mixing techniques.

Grouting

Numerous methods and materials canbe used for grouting. In any discussion ofgrouting it is important to distinguish betweendisplacement or densification grouting, suchas compaction grouting (described in Section3.2.2), and non-densification grouting. Theprocesses by which non-densification grouting


is conducted are intrusion grouting, permeation grouting, and jet grouting (Figure 3.15).

1. Intrusion grouting is a technique inwhich fluid grout is injected underpressure with the intention of introdUcing controlled fracturing of theground. Sometimes this is calledfracture grouting. Theoretically, thefirst fractures are perpendicular to theminor principal stress direction, butobservations show that they usuallyfollow the weaker bedding planes.Repeated injections will tend to fracture the densified ground along different planes. The permeability of thesoil decreases, and some densificationprobably occurs, although it is secondary to the strength increase resulting from hardened grout lenses.

2. Permeation grouting is a technique bywhich the voids of a soil are filled,with little change in the physicalstructure of the soil. Materialsinjected are (1) particulate grouts,including Portland and micro-finecement, fly ash, clay, and mixtures ofthese components, and (2) chem1calgrouts, such as lime, sodium silicates,acrylates, etc., in solution. Chemicalgrouts can permeate smaller poresizes, as they usually are liquids witha low viscosity and no solid particlesto clog the pores. Along with cementgrouts, they offer the added benefit ofcementing particles together.However, they are more costly thanparticulate grouts. Although manyformulations of chemical grout areavailable, sodium silicate based groutsare the most widely used.

3. Jet grouting (Figure 3.16) cuts the soilwith high pressure, high velocity airor water jets, and mixes it with an injected grout material such as Portlandcement. Single or multiple jets arerotated around a central drill stem tocut the soil and mix it with the grout.Jet grouting is able to construct relatively uniform columns of improvedsoil in a wide variety of soil conditions. Continuous walls or panels of

49

SLURRY GROUT

(Intrusion)

CHEMICAL GROUT

(Permeation)

COMPACTION GROUT

(Displacement)

JET GROUT

(Replacement)

50

Figure 3.15. TypeS of grouting (courtesy of Hayward Baker Co.)


(a) Column

t

Grout

(b) Panel

Figure 3.16. Jet grouting (a) columns and (b) panels (after Munfakh, et aI., 1987)

/ ~onstruction,/ rig

:::::=~t==

i'i=;:;:::::>-.-_lfif·--==---- Stiffener

~:-;"';'''''''4:,..J-Triple axishollow stemauger

Figure 3.17. Schematic of the deep soil mixing rig (O'Rourke and Jones, 1990)


solidified soil can be connected toform cells. Jet grouting is applicableto sand, silt and clay deposits.

A good general description of cementand chemical grouting can be found inChapter 14 of Hausmann (1990). Cementgrouting for dam foundations is discussed indetail by Houlsby (1982) and Weaver (1991).Karol (1982) gives a complete description ofchemical grouts, while Baker (1982) presentsdetails for carrying out structural chemicalgrouting.

Detailed information on jet grouting isgiven by Gallavresi (1992) and Kauschinger, etal. (1992).

Soil Mixing

Soil mixing is a technique wherebysoil is physically mixed with cementitious materials using a hollow stem auger and paddlearrangement. Available configurationsinclude single shaft augers (0.5 to 4 m indiameter) or in gangs (2 to 5 shafts) of augersabout 1 m in diameter. Figure 3.17 is aschematic .of t.he Japanese triple axis augersystem while FIgure 3.18 shows the equipmentused to make the Swedish lime columns. Thesingle-row multiple shaft auger is generallyused for in situ soil mixed walls while doublerow multiple shaft augers (8 shafts) are usedfor areal treatment of soft or contaminatedground. As the mixing augers are advancedinto the soil, grout is pumped through thehollow stem of the auger shaft and injectedinto the soil at the tip. The auger flights andmixing paddles blend the soil with grout inpugmill fashion. When the design depth isreached, the augers are withdrawn and themixing process is repeated on the way up tothe surface. Left behind are stabilized limesoil-eement, or soil-eement-bentonite colu~or panels. Depths of soil improvement arelimited only by the available eqUipment. TheSwedish lime columns are lo-15m deep, whiledepths greater than 30 m have been achievedin the United States and more than 60 m inJapan with soil-eement mixing.

Soil mixing produces an improvedvolume of soil that is very well defined. Themechanical mixing ensures constant element

size with depth and enables columns, walls,and cells to be constructed in virtually alltypes of soils. The strength of the soil cementis dependent on the type of soil treated, mixdesign of the grout, and degree of mixing.Broms and Boman (1979) and Broms (1985)provide a summary of the lime columnmethod (see also Holtz, 1989). Taki and Yang(1991) present an overview of the deep soilmixing technique, while Broomhead andJasperse (1992) describe shallow mixing.

3.4.2 The Practice of SoilImprovement by Physical andChemical Modification

3.4.2.1 Historical Use of Physical andChemical Modification Techniques

According to Hausmann (1990),grouting with day, lime, and cement has beenaround a few hundred years. It was primarilyused to repair masonry walls, fill cracks inload-bearing structures, and stop unwantedseepage in rock fissures. Chemical groutingstarted in the mid 1920s with silicate basedgrouts. Acrylamide-based grouts (example:AM-9) were first developed in 1951(Karol,1982). They were almost an ideal grout interms of their penetration, viscosity, gel time,and strength. However, they are toxic, andthus are no longer manufactured. Since then,most of the chemical grouts utilized aresodium silicate based.

According to Weaver (1991), grouting~or seepage cutoff in dam foundations beganIn the U.S. before 1890, and became quitecommon and highly developed by the 1930s.Applications to tunnels and foundations developed concurrently.

Jet grouting, on the other had, is relatively new; it started in Japan about1965 (Kauschinger, et aL, 1992).Developments occurred rapidly and the technol0?y quickly spread to Italy, Germany, and~razil. Although jet grouting was introducedIn the U.s. as early as 1979, it has had a ratherslow development, with relatively few projectscO,mpleted SO far. ,It does appear to be gainingWIder acceptance In recent years.


"--'A- Ume andI - - ~ compressed air

~I II I

b)

Soft clay

Mixing tooi

IcL Unslaked lime

.Kelly

-t-Completed limecolumn

Maximum10 to 15m

a)

c) Mast

Rotarytable

T Kelly

Up to 15m Mixing tool

l O.5m

Figure 3.18. The Swedish lime column method: (a) schematic of completed columnand auger system; (b) close up of mixing tool; (c) installation machine(after Broms and Boman, 1978; Broms,1985)



Although deep soil mixing began inthe U.S. in the 1950s (Ryan and Jasperse, 1989),it was really developed in Sweden and Japanin the 1970s and '80s. The Swedish lime column system (Figure 3.18) was initially used toreduce the settlements of shallow foundations,especially for highway embankments andsmall structures, on soft clays. Later, applications to improve the stability of slopes andexcavation have proven to be feasible (Bromsand Boman, 1979; Broms, 1985). The multipleshaft auger systems for deep soil mixing ofsoil-eement developed by Japanese contractorshas been a significant advancement. Theserigs greatly increased both the productivityand maximum depth of mixing. So far, mostdeep soil mixing projects have involved seepage control under dams and levees, containment of hazardous waste sites, and large areastabilization of contaminated sites.

Shallow soil mixing is a very recentdevelopment (Broomhead and Jasperse, 1992).


Very few case histories could be foundin which grouting was utilized to increase theseismic resistance of a structure or foundation.Among theses are the case of Roosevelt HighSchool, built in San Francisco in the 1920s using spread footings founded on clean sands.California regulations required all schoolbuildings to be analyzed for earthquake resistance and, among other problems, it was discovered that the shallow footings were bearingon loose sands above the water table that weresubject to earthquake "shock" densificationand excessive settlements. The loose sands below the footings were stabilized with chemicalgrout (zacher and Graf, 1979). No settlementsof this building were observed following the1989 Lorna Prieta earthquake. Graf (1992b)summarizes three additional cases from theSan Francisco area in which chemical groutingwas successfully utilized to stabilize foundations against potential seismic damage. Allthree projects were completed prior to the1989 Lorna Prieta earthquake and all structures successfully withstood at least that test.Bruce (1992a), in a comprehensive paper aboutgrouting for the rehabilitation of dams, mentions using grouting to improve seismic stability.

54

None of the six papers with case histories on jet grouting presented at the ASCESpecialty Conference on Grouting, SoilImprovement, and Geosynthetics (Borden,Holtz, and Juran, 1992) mentions seismic hazards as a reason for using jet grouting.

According to Mitchell and Wentz(1991), chemical grouting at the RiversideAvenue Bridge in Santa Cruz, California,which was subjected to a peak ground acceleration of 0.45g in the 1989 Lorna Prieta earthquake, was very effective. No settlement ordamage to the bridge was reported.

In Kawasaki, Japan, a combination ofslurry walls and deep soil-mixed walls wereconstructed to create a containment withinwhich permanent drawdown of groundwaterwas performed. Liquefaction potential wasreduced by causing a portion of the liquefiablesoils to be above the groundwater table and byincreasing the effective stress in the soilswhich remained below the groundwater table.

At another project in Vancouver,British Columbia, a large tank was founded ona ring of deep soil-mixed columns placed tangent to one another to create a containmentbarrier similar to that in Kawasaki. In this instance, however, the containment was designed to hold in soil that would liquefy during an earthquake. The strength of the improved soil and the volume of treatment weredesigned to resist the shear stress inducedduring and after ground shaking, assumingthe zone of concern to be either completely orpartially liquefied.

In none of the case histories on the useof lime columns described by Broms andBoman (1979) and in the references in Holtz(1989) did seismic hazards appear to be a consideration. That is not the case, however, formultiple auger deep soil mixing with soil cement. The Jackson Lake Dam modificationproject described by Ryan and Jasperse (1989)and Taki and Yang (1991) was undertakenspecifically to reduce the potential for liquefaction in the granular alluvial soils in thefoundation. This was also the objective of theproject reported by Babasaki, et al. (1991) inwhich large cells of deep soil mixing wereconstructed under a building founded on a

deposit of very loose sand in Kagoshirna City,Japan. Taki and Yang (1989) report on twosuccessful uses of deep soil mixing for supportand to control ground water in tunnel construction. In the shallow soil mixing casereported by Broomhead and Jasperse (1992),potential seismic loading was a designconsideration.

3.4.2.3 Observed Effectiveness of Physicaland Chemical ModificationTechniques

The effectiveness of grouting and soilmixing technologies has been confirmed bysuccessful field performance and by excavation of stabilized soils in conjunction with construction.

Permeation and fracture groutinghave been used for more than 70 years forfoundation soil improvement and remediation, and the characteristics and limitations ofmaterials and methods of application are quitewell understood. Permeation grouting is aproven method of strengthening soils andcutting off groundwater movement.Application of this process is limited to granular materials and/or fractured rock. Strengthand stiffness of the grouted soil can be predicted in advance. Permeation grouting hasbeen used to mitigate soil liquefaction, and toprevent failures of foundations, slopes and retaining walls. It has also been used to modifythe response of soils to vibratory loads. Inparticular chemical grouts have been usedsuccessfully to stabilize loose sands subject toshock densification. Individual columns ofgrouted soil or continuous walls or panels canbe connected to form cells. Intrusion or fracture grouting has been shown to be effective incontrol of seepage and, to a lesser extent, surface settlements. Success to date is largelybased on empirical observations. Groutingcan be used to achieve design objectives with ahigh degree of confidence, provided adequategeotechnical investigations are done inadvance.

Although, as noted above, there are nocase histories illustrating the effectiveness ofjet grouting to mitigate seismic hazards, itsefficacy as a foundation soil improvementtechnique is well documented in the literature.


Judging from the properties and performanceof jet grouted foundations and cutoff walls, itshould be a viable technique in seismicallyactive regions.

The proceedings of the ASCESpecialty Conference on Grouting, SoilImprovement and Geosynthetics (Borden;Holtz and Juran, 1992) presents a number ofrecent case histories on grouting, but exceptfor those reported by Graf (1992b), nonespecifically concerned seismic hazardmitigation.

The effectiveness of soil mixing hasalso been fairly well documented in theliterature, with applications to resist seismicloading conditions described by Taki andYang (1989; 1991) and Broomhead andJasperse (1992). These projects were designedto minimize pore pressure generation andcontain liquefied deposits in order tominimize surface displacements. However,although the design concept suggests thatthese methods should be effective, none of theinstallations is known to have experienced anactual seismic event.

3.4.2.4 Potential Applications to SeismicHazards

Liquefaction of low density saturatedcohesionless soils can be mitigated by increasing their shear strength through cementation(permeation grouting, jet grouting or soil mixing) or through the addition of cementedlenses in conjunction with increased density(intrusion or fracture grouting). Recent applications of soil mixing walls suggest similarapplications are possible for jet grouting andpossibly permeation grouting. As at theJackson Lake Dam in Wyoming, the improvedsoil can be installed in a grid or cellular pattern to isolate the liquefiable soils into individual zones, such that the buildup of porewater pressure during and after seismicground shaking is reduced to a level so thatliquefaction does not occur. Conceptually, jetgrouting and perhaps permeation groutingcould be used to create the same improved soilzones, depending on soil profile characteristics.

55

In addition to liquefaction, earthquakemotions can result in densification of soils anddamaging surface settlements and displacements. All of the PCM techniques have thepotential to increase soil stiffness which mayreduce the amplification of ground motions.Of course, permeation grouting will not be effective in cohesive soil profiles, and if continuous "shear walls" are required, either jet grouting or soil mixing will be required.

PCM techniques are widely used forboth enhancing the stability of existing foundations and for the repair or renovation ofdamaged foundations. Most particularlygrouting tools and techniques are welladapted to the low headroom and close quarters common to foundation work on existingstructures. The use of permeation or fracturegrouting to improve strength or to transferloads to more competent layers at depth iscommon. These techniques can also be usedto lift settled foundations.

PCM techniques are also applicable toimproving seismic performance of slopes andretaining structures by reducing the potentialfor liquefaction of granular soils as well as byimproving strength characteristics of bothgranular and fine-grained materials. Due tocost, PCM is generally considered only forlimited areas or to construct stabilized zonesor cells within a larger soil mass. PCM techniques have been considered for the seismicstabilization of earth embankments which aresupported by relatively soft fine-grained soilswhen the associated improvement of theseunderlying layers is desired.

3.4.3 Current Issues in Applicationof Physical and ChemicalModification Techniques

Grouting Techniques

The verification of the effectiveness ofgrouting appears to be one of the most important issues. Appropriate techniques for evaluating the characteristics of grouted soil are alsoneeded. The effectiveness of grouting toreduce pore pressure generation duringseismic events is not well established, nor isthe performance of stabilized zones and

protecting structures from detrimentalmovements during shaking.

Soil Mixing Techniques

Although deep soil mixing appears tobea viable technique for groundwater cutoffand support of temporary excavations, thebehavior and performance of deep soil mixedwalls under seismic conditions is not well understood. Because soil-cement mixtures appear to be rather brittle, the movements generated during a seismic event may inducecracking and fracture in the structures, whichof course would compromise their integrityand effectiveness.

3.4.4 Summary

As noted in this section, physical andchemical modification techniques are potentially very effective means for improving looseand soft soils for mitigating the damage tofoundations and slopes due to earthquakes.Both conventional grouting techniques(permeation, intrusion, and jet) and both deepand shallow soil mixing appear to be viabletechniques which should be considered in anyseismically sensitive region or area.

3.5 Inclusions Techniques

The concept that in some situationssoils require reinforcing is not particularlynew. Tree roots, for example, can be quiteeffective as slope reinforcement. Other examples include adobe bricks, brushwood andbamboo facines, corduroy roads, and reliefpiles used to stabilize embankments on softfoundations. These examples indicate that thestrength and stiffness of a soil mass in a slopeor foundation can be increased by the inclusion of some other material with a muchhigher tensile strength. Modem examplesinclude soil anchors and nails, reinforced soilretaining structures, and stone columns. Allthese techniques are called inclusions. Forpurposes of this report, four types of inclusions are considered: (1) soil nailing, (2) metaland geosynthetic reinforcement, (3) concreteand steel piles, and (4) stone columns.


3.5.1 Description of InclusionsTechniques

Soil Nails

Soil nails are steel bars or cables whichare either driven or grouted into a drill hole ina vertical or sloping soil face (Figure 3.19). Inthe case of excavationS, soil nailing is a "topdown" construction technique in which rowsof nails are sequentially installed as the excavation proceeds downward. The face of thewall is then shotcreted. The nails are not pretensioned, but rather gradually take up load asthe excavation deepens and the soils behindthe face deform. Significant depths of exposedsoil face must not be left without promptnailing, and the soil must have sufficientstrength to stand unsupported for severalhours. When installed in existing slopes(Figure 3.19b), the nails will gradually pick uptension as soil deformation occurs. Soilnailing has been used in temporaryexcavations to depths up to 23 m in U. S. A.and up to 27 m in France. Permanentinstallations so far are in the 10 to 15 m range.

a)

EXCAVATION BY STEPS

~~?~~"':1---L$J++~8rtf±\:.. -- rm.."'<1/

- - - - /11\.'<:VTT

J:.:.;~~~+~~::t - - -'m::..W/

---7~WI

..~~~~~

'., ."

b)

The design of soil-nailed structures isusually based on limiting equilibrium orkinematic methods which incorporate tensionelements to simulate the nails. The nail itselfmust have sufficient cross-section to avoidfailure in tension and must also resist pullout.In typical installations, the length of the nail isbetween 0:7 to 1.0 times the height of slope ordepth of excavation. The spacing of the nailsis usually about 1.5 or 1.8 m center to centerboth horizontally and vertically.

In practice, seismic design involveschecking the adequacy of the static design under seismic conditions by incorporating anadditional horizontal static force given by theMononobe-Okabe procedure. Static factors ofsafety are typically about 1.5 and seismicsafety factors are in the range of 1.1 to 1.2.Therefore, in most cases the excess capacityunder static conditions can absorb the additional seismic force with a safety factor greaterthan 1.2. However, seismic design may control under very strong shaking, especially forhigh or deep structures.

Figure 3.19. Schematic of soil nailing for (a) excavations and (b) natural slopes (Munfakh, etaI., 1987)


References on soil nailing includeMunfakh, et aI, (1987), Mitchell and Villet(1987), Christopher, et al. (1990), Elias andJuran (1991), Schlosser, et al. (1992), and Byrne(1992).

Backfill Soil Reinforcement

Soil reinforcement by metal (Figure3.20) or geosynthetic (Figure 3.21) inclusions isnow a mature technology with a generalconsensus on design procedures andconstruction methods. There is also animpressive record of satisfactory performancein a wide variety of applications over longperiods of time, including seismic conditions.For details the reader is referred to, forexample, Ingold (982), Jones (985),Bonaparte, et al. (1985), Mitchell and Villet(1987), Christopher, et al. (1990), Koerner(1990), Allen and Holtz (1991), andChristopher and Leshchinsky (1991).

Reinforcement Piles

Concrete and steel piles have occasionally been used to limit the movements ofslopes. Included are large diameter heavily reinforced drilled shafts installed to form tangent pile walls at the toe of an unstable slope,secant pile walls (often with tie back anchors),and more widely spaced driven piles anddrilled shafts. See Holtz and Schuster (992)for a description of these systems for landslidestabilization together with some case historiesof their use.

Another approach to in situ reinforcement is the use of micropiles, pin piles, or"root piles." These pile systems form a monolithic block of reinforced soil that extends below the critical failure surface (Figure 3.22).The principal differences between root pilesand soil nailing are that root piles are usuallylonger, of larger diameter, and have a morethree-dimensional geometric arrangement.

~ •.l

Figure 3.20. Component parts and key dimensions of reinforced earth wall (after Lee, Adams,and Vagneron, 1973)


/.:.~=~.<#·.;':T··· _

a) Vertical geotextile facing

~.. ~::::.~.; '.~:':...::~.:.:.. :-0:.:'.,· .0.::

o - 0 ••••• - •• :.: ••••.": ;.:.. ~ .~.:·.· ..·....o:~

.~~,t:t?~\:;;~ .

b) Vertical precast concreteelement facing

c) Vertical cast in-placeconcrete/masonry facing

. -.. "

411,,~,,

, ....1fI!, ...~

II

".-'----"e) Sloping geotextile facing

.... I,"j

,,.,111-'~,

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/~-)

f) Sloping gunite or structuralfacing

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,AI

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g) Sloping soil and vegetationfacing

.. -; - .

d) Vertical masonry facing h) Geotextile gabion

Figure 3.21. Possible reinforced walls and slopes using geosynthetic reinforcement (afterMitchell and Villet, 1987)


•

....

. ... ..o

•

~ .

•

:~-: ... , .... ~ .. ~..." . , ....

'.

Figure 3.22. Schematic cross section illustrating the use of root pile for stabilization of a slope(after Lizzi, 1977)

Design of these systems requires anunderstanding of the composite pile-soil interaction, particularly the mechanism of loadtransfer from soil to piles as deformation takesplace.

Stone Columns

Stone columns have three functionsthat enhance the seismic performance of afoundation during an earthquake: densification of the surrounding soil, drainage, and itsown action as a stiffer component of the foundation soil system. Installation of stone

columns and the functions of densification anddrainage were discussed earlier in Sections 3.2and 3.3. Stone columns also improve theability of the soil to carry added structuralloading with little reduced settlements.Because the primary benefit of stone columnsin loose sands results from densification, theability of the soil to carry added loading israrely evaluated and considered. However,theory indicates that a reduction factor shouldbe applied to cyclic stress ratios induced in thesoil (Priebe, 1990). Because of this, theliquefaction resistance of soil-stone columnsystems should be enhanced.


3.5.2 The Practice of SoilImprovement Using Inclusions

3.5.2.1 Historical Use of Inclusions

Soil Nailing

According to Munfakh, et al. (1987),soil nailing was first developed in France andGermany in the early '70s. The method is anextension of the New Austrian TunnellingMethod, which utilizes a combination of rockbolts and lightly reinforced shotcrete as a support system for underground excavations.Application to the stabilization of excavatedslopes is termed soil nailing. A number ofwell instrumented experimental full scalestructures were constructed primarily inFrance and design rules established. In thiscountry, Shen, et al. (1978) described a temporary support system for excavations which isthe precursor of today's soil nailing systems.They also developed calculation procedureswhich have been widely used in the U.S.

Metal and Geosynthetic Reinforcement

Although very early examples of similar soil reinforcement systems can be found(Jones, 1985), the first modem developer andpromoter of soil reinforced retaining systemswas H. Vidal, who developed his system terrearmee ("reinforced earth") in the mid 1960s(Vidal, 1969; Schlosser and Vidal, 1969; andLee, et al., 1973). Since then, more than 16,000reinforced soil structures have been successfully built throughout the world. The use ofgeosynthetics to reinforce the backfill behindretaining walls was developed independentlyin the U.S.A. by the U.S. Forest Service (Belland Steward, 1977) and in Sweden (Holtz andBroms,1977). Since then, geosynthetics havebeen used quite successfully to stabilize natural slopes (Margason and Bonaparte, 1985;Bonaparte et al., 1989), fill slopes, and for theconstruction of embankments on very soft andunstable foundations (Holtz, 1989; 1990).

Reinforcement Piles

The first use of piles as a soilimprovement technique is rather obscure.Timber relief piles have been commonly used


for at least 50 years and perhaps longer to stabilize embankments on soft foundations inScandinavia. The outer rows of piles aredriven battered, typically 1H:4V to IH:I0V,while the inner rows of piles are driven vertically (see Bjerrum, 1972). (The historicaldevelopment of this stabilization technique isalso discussed by Holtz, 1989). Under whatconditions concrete and steel piles became aviable alternative to timber piles is not known;however, it must be a matter of economics andload carrying capacity. Long timber piles donot have very much lateral load capability, sosteel piles and properly spliced precast concrete piles or drilled shafts would be moresuitable in those situations.

"Reticulated Root Piles" were originally developed in the 1950s by F. Lizzi (1977)and were patented by the Italian constructionfirm Fondedile of Maples, which has installedthe system all over the world, mainly forunderpinning. It has only been in the past 20years that root piles have been used for slopestabilization, and most root-pile slope-stabilization works have been constructed withinthe past 10 years.

According to Bruce (1992a), during thepast 20 years, U.S. practice has developed using micropiles to a state quite different thanthe original European developments. He provides details from some 20 projects which illustrate recent developments of pinpile technology.

Stone Columns

Stone columns followed from the development of vibroflotation and vibroreplacement, in which instead of sand beingadded to the cavity, course crushed stone orgravel was utilized. According to DiMaggio(1978) this development occurred in Germanyin the early 1950s.


Soil Nailing

According to Felio, et al. (1990), anumber of soil-nailed structures were shakenduring the Loma Prieta earthquake atacceleration levels around O.lg and they

61

performed very well. The structures weredesigned for an equivalent seismic loadcorresponding to horizontal accelerationcoefficient Kh = 0.159 which was estimated as1/3 of the expected peak acceleration of 0.45g.Similar satisfactory performance of tiebackwalls during the 1987 Whittier earthquake wasobserved by Ho, et al. (1990).

Reinforcement

Five steep slopes and walls reinforcedwith geogrids were all located within 100 kmof the epicenter of the 1989 Lorna Prieta earthquake. The slopes ranged in height from 3 mto 24.5 m; the 3 m high slope was within 11 kmof the epicenter. Peak ground accelerationsranged up to 0.4g. According to Collin, et al.(1992), the performance of all five structureswas satisfactory.

Reinforcement Piles

A number of case histories of the useof piles to stabilize landslides are given inHoltz and Schuster (1992), but in no case werepiles specifically installed to enhance theseismic resistance of the slope.

However, piles are proposed as one ofthe remediation measures for Sardis Dam inMississippi to control potential deformationsinduced by seismic liquefaction. During thedesign earthquake the saturated portion of thecore and a part of the upstream slope arelikely to liquefy. These, however, do not havea significant impact on stability. The primefactor resulting in excessive deformations is alayer in the foundation which could lose alarge percentage of its strength during shaking. It is proposed to prevent sliding on thislayer by driving 0.6 m square concrete pilesthrough the layer to provide adequate restraint in the alluvial sands below. There aretwo major technical questions posed by the solution:

• how will the composite section behave?

• how should the piles be designed toresist both hard driving, cyclic load-

ing, and the large static horizontalforces that develop after liquefaction?

A proposed replacement dock at theGeorgia-Pacific facility in Bellingham,Washington, will be supported on piles driventhrough loose to medium dense sand andbearing in bedrock at depth. The upper 6 mbehind the bulkhead line is loose, hydraulically-placed sand. Offshore soils along theslope are generally medium dense. The concern at the site is that lateral movement of liquefied onshore soils will cause excessive horizontal movement of the new dock. Usingmethods presented by Byrne (1991), 0.6 to 0.9m of lateral soil movement was estimated. Toreduce liquefaction potential and its effects onthe new dock design, driving two rows of 15m-long timber piles spaced at 0.9 m on-centeralong the bulkhead line was recommended.Lateral soil movement after the improvementwas estimated to be about 30 cm during amoderate-sized earthquake. The timber pileswill also be used to support the dead weightof the lightly-loaded bulkhead wall.

For case histories of pin piles for slopestabilization, see Bruce (1992b). An interestingcase history on the use of pin piles to controlslope movements was described by Pearlman,et al. (1992).

Stone Columns

Case histories on the use of stonecolumns to mitigate liquefaction potentialhave been reported by Dobson (1987), Hussinand Ali (1987) and Hayden and Welch (1991).Additional case histories on stone columns arereported by Watts and Charles (1991) andMitchell and Wentz (1991); see also Section3.2.2.2.

An interesting case of a failure of a stonecolumn-supported structure was reported byMathis and Munson (1987). A reinforced earthbridge abutment constructed on a stone column foundation failed during construction.An investigation revealed the causes of failureincluded: (a) too rapid construction, (b) contamination of the columns, which lengthenedthe consolidation time, (c) weakening of the


foundation soils by vibration and water jetting, and (d) an overestimation of the safetyfactor for shorHerrn stability. A concentrationfactor (Barksdale and Bachus, 1983) of twowas assumed in design; back analysis after thefailure indicated this factor was only aboutone. Thus the load of the reinforced earth wallwas not successfully transferred to the stonecolumns from the low-strength clays and siltsin the foundation.

Egan, et al. (1992) report on the use ofstone columns as a part of the seismic retrofitof a wharf at the Port of Oakland after the 1989Lorna Prieta earthquake (see also Section3.6.2.2).

3.5.2.3 Observed Effectiveness

Soil Nails

From the reports by FeUo, et al. (1990)and Ho, et al. (1990), nailed soil walls shouldperform relatively well during earthquakesbecause of their flexibility and ductility.

Metal and Geosynthetic Reinfarcement

Performance of reinforced soil structures in seismic events has been very good.This was most recently demonstrated. in the1989 Lorna Prieta earthquake. Several reinforced. slopes and walls were subjected. to thisearthquake and performed. very well (Collin,et al., 1992). One example is a 15 m high 1 to 1slope, reinforced. with HOPE geogrid, located.on Highway 9 in San Lorenzo, about 26 kmfrom the epicenter. Although the horizontalacceleration at the site was estimated to beabout 0.4g, the slope showed. no signs of distress after the event.

There is not much other direct evidence available as to the seismic stability ofmetal and geosynthetic reinforced. slopes andwalls. The rigid precast concrete facings ofconventional reinforced. earth and similar systems (e.g., VSL walls, Georgia stabilized. earthwalls, etc.), although they are relatively flexible, suggest that they may have some difficulty during a seismic event. The weak link inthe system is the attachment of the reinforcement to the facing. Wrapped. face geosyntheticreinforced structures, on the other hand,


should not encounter this particular difficulty,although if their facings are protected byshotcrete or precast concrete panels, then similar difficulties may also arise with these typesof walls.

Concrete and Steel Piles

Because no case histories of the performance of pile systems used for soil improvement and subsequently subjected toseismic events could be identified, there is nodirect evidence available on their effectiveness. This is also true for root piles, althoughthere are reports of several successful installations in seismically active areas of Italy.

Stone Columns

Comments on the effectiveness ofstone columns during seismic shaking weregiven previously in Sections 3.2.2.3 and 3.3.2.3.

3.5.3 Issues in Use of InclusionsTechniques

Soil Nailing

There is currently no consensus as tohow soil-nailed structures should be designedfor both static and seismic conditions.Performance of facings during seismic events(in permanent installations) is not well understood. For permanent installations, the longterm durability of the nails is subject to question, although experience with permanenttieback installations should provide considerable confidence in this respect.

Metal and Geosynthetic Reinforcement

Metallic reinforcement systems arequite well established in engineering practice,and design techniques for both static andseismic conditions appear to be well accepted..Questions concerning long-term durabilityunder adverse geochemical conditions stillremain, although the research by Elias (1990)and Allen (1991) has shed considerable lighton this subject. Connections between the reinforcement strips, bars, etc. and precast concrete facings are the weak link in reinforcingsystems subject to seismic shaking.

63

Design procedures for geosynthetic reinforcement are less well established, althoughconsiderable progress has been made (Allenand Holtz, 1991; Christopher and Leshchinsky,1991). Limited work has been done on seismicdesign of slopes and embankments withgeosynthetics (Bonaparte, et aI., 1986;Yamanouchi, et aI., 1986). Long-term durability is a serious impediment to the future use ofgeosynthetics reinforcement in permanentconstruction (Allen, 1991), and the FHWAnow has a major study underway on this subject.

Reinforcement Piles

The lateral stability of these foundation elements during seismic loading, andtheir connections, either pile splices or connections to footings and pilecaps, are the most serious questions remaining.

Stone Columns

The primary issue remaining in theuse of stone columns as foundation reinforcement is their actual performance under seismicshaking. Aside from the Loma Prieta casehistories presented by Mitchell and Wentz(1991), no direct evidence of stone column performance during earthquakes is available.

3.5.4 Summary

In this section, the use of inclusions(soil nails, metal and geosynthetic reinforcement, piles, and stone columns) to reinforceand strengthen foundations and slopes againstseismic loading has been discussed. Metallicreinforcement and piling systems appear to bethe best understood, especially with regard fotheir performance during earthquakes. OnUleother hand, we have so little experience withsoil nailed walls, geosynthetic reinforcedwalls, and stone columns during earthquakesthat their performance during seismic loadingremains difficult to predict.

3.6 Foundation RemediationTechniques

This category includes those remediation techniques that are not directed towardmodification of the properties of the foundation soils and thus are not classed as inclusions. Consequently, except for the case of removal and replacement of unsuitable soils,which needs little attention here, these techniques are implemented by means of structural elements - piles, walls, footings, etc. that form the interfaces between structuresand foundation soils (or rocks).

While issues of characterization andverification are spread across the whole matrixof hazards and remediation, the use of structural elements has an inherent advantage inthat the engineer generally has a high degreeof confidence in what actually has been put in .place in the ground. On the other hand, surveying damage to structural elements due toearthquakes or environmental deteriorationcan be very difficult or expensive, becausethese elements are normally not visible or easilyaccessible.

Remediation may consist either of thetreatment of unsuitable soil or site conditionsduring the original construction, or theretrofitting of existing structures, either to correct later-discovered deficiencies or to repairdamage. The applicability of various remediation techniques will depend on which of thetwo situations we are dealing with. For example, removal and replacement of unsuitablesoils may in some cases be the method ofchoice in original construction, but would nolonger be an available option once the structure is in place.

3.6.1 Description of FoundationRemediation Techniques

From a structural point of view, theaction of structural elements in connection~ith foundation remediation include providIng lateral support and stiffness, increasing theov.eral~ stiffness of the foundation, transferringseIsmIC loads to stronger bearing layers,


changing the resonant frequency or increasingthe damping of the foundation, etc.

When considering the use of foundation remediation techniques to control ormodify seismic response of a structure, the response characteristics of primary interest arethe acceleration, velocity, and displacement ofthe structural system. Foundation remediation systems include various types of piles,sheet piles, and related structural members.Although base isolation is not addressed inthis report, it offers significant benefits in controlling motions in a structure. Research in thearea of base isolation, however, is considered astructural design issue.

Structural elements and systems forfoundation remediation are usually selected toimprove static performance, rather than toimprove seismic response. The unsatisfactorystatic performance most often involves prediction of excessive settlement or inadequatebearing or lateral capacity. As design proceeds, the foundation system will often bemodified to satisfy seismic loading requirements. However, as long as the foundation isable to meet shear and bending moment requirements from the structural loading, littleeffort is normally made to modify the foundation characteristics (stiffness, damping, mass)to control accelerations, velocities, or displacements in the structure. Rather the structure is designed to handle those motions.

On the other hand, it has been suggested that modification of the structural characteristics of the foundation offers potentialbenefits for "tuning" the soil-structure systemto avoid excessive ground motion amplification. The amplification potentially resultsfrom coincidence of the natural frequencies ofthe soil-structure system and the seismic motion. The structural modification could include changes in the mass, stiffness, damping,and effective depth of the foundation. Becausesuch changes are conventionally used to improve the performance of machine foundations, in principle there is no reason why similar methods could not be applied to seismicloading.

Strengthening of earthquake deficientfoundations involves either additions or re-


placements. In all cases, the design shouldconsider the effect of the new structural elements on remaining foundations and changesin the overall structural response.

Appropriate foundation remediationtechniques may involve deeper foundations tobypass a troublesome soil layer that can not bemore economically replaced or improved.Various types of piles, drilled piers, and underpinning walls are examples. These additions must be designed considering both inertial forces and ground motions.

For shallow foundations, if the deficiency is related to inadequate bearing capacity, increasing the foundation area could solvethe problem provided load eccentricity andstiffness factors are properly considered.

3.6.2 The Practice of FoundationRemediation

3.6.2.1 Historical Use of FoundationRemediation Techniques

Foundation remediation systems suchas piles offer a means of improving seismic response, particularly in the case of bridges.andbuildings that must be upgraded to meethigher seismic load requirements. The controlling need for these upgrades will likely bea reduction in the potential for yield of the soilunder higher structural loading. On the otherhand, economic studies may indicate thatmodification of mass, stiffness, damping, oreffective depth of the foundation system maybe less expensive than designing individualcomponents of the structure to accommodatehigher accelerations, velocities, or displacements. Fundamental to effective remediationis the elimination of all hazards such as a largedisplacement landslide or a foundation bearing capacity failure that would cause collapseof the structure or otherwise endanger publicsafety.

Foundation remediation techniquesapplied to liquefaction susceptible sites typically consist of structural systems designed toresist lateral and vertical loads occurring during the earthquake event. The system mustmitigate loss of bearing capacity and the

65

,

effects of ground settlement, and it must beadequately designed for compressive anduplift, downdrag, and lateral loads. Lateralloads from lateral spreading of liquefied soilmay exceed the capacity of deep foundations.Stiffened mats or other heavily reinforcedshallow systems may be required in suchcases.

Systems that can be used at liquefaction sites include bored and driven piling,sheet piles, diaphragm walls (e.g., slurry andcylinder pile walls), encapsulation cells, andstructural elements to increase foundationstiffness. Micro piles are not considered appropriate for this hazard due to the relativelysmall structural section compared to the applied loads causing bending.

Common practice in the design ofbored and driven piling, diaphragm walls,and sheet piles is to ignore any lateral or vertical capacity of the liquefied soil mass. Thusthe purpose of the remediation technique is totransfer load to underlying stable soi1. Thetechniques used for determining lateral loadcapacity of pile systems are similar to the p-yanalyses used for static load cases. Thesemethods have not been developed nor fullyverified for the dynamic case. Downdragloads are often ignored in the liquefied zone incurrent designs, and uplift capacity is accounted for only below the liquefied zone.

Stiffness of pile groups in rotation andfor axial loads are often accounted for by vertical and horizontal springs and by neglectingthe liquefied soil mass. For bridges and marinestructures, batter piles are often designed toresist lateral loads due to earthquakes. Theiranalysis neglects the effects of the vertical soilcolumn above the piling and the effects ofearthquake ground motions causing bending.No suitable means is available to estimatethose loads for partial or full liquefaction, andthe piling should be designed for bending.

In many cases settlement is accountedfor indirectly by consideration of downdragloads on the structural system. However, inthe case of bridge abutments, area settlementmay cause settlement of approach fills resulting in extreme vertical and lateral loads on

piles. These forces need to be included in thestability analysis of the foundation system.

The techniques described above aregenerally applicable to liquefaction sites onlyif they are new sites. Retrofitting of existingstructures by these methods is very difficult,for example, due to the typical size of pilingand the location of diaphragm walls.Additionally, connecting the new system toexisting pile caps or footings can be difficultand expensive. The question is how to make apositive connection that remains flexible andductile.

It is important to recognize that mitigation of the effects of low probability events,such as earthquakes, does not require reduction of all associated potential damage.Accordingly, overdesign of foundation remediation measures should be avoided sincesuch action could cause other adverse effects- e.g., stiffening the foundation could causean increase in the inertial forces in thestructure and its foundation.


Encapsulation cells have normally notbeen designed to mitigate liquefaction.However, a building with a sheetpile "skirt"wall survived the Niigata earthquake in Japanwhile adjacent buildings failed. This has led tothe concept of using encapsulation cells to reduce the liquefaction potential of sites as wellas to improve foundation stiffness. No methods are currently available to design this typeof system or to estimate its effect on liquefaction potential of the confined soil.

The normally difficult problem of resisting strong ground motions for wharves canbe greatly exacerbated by ground failure beneath the wharf. Some of the damage in the1989 Loma Prieta earthquake at the Port ofOakland 7th Street Terminal was due to liquefaction failure of hydraulic fills at the wharfsite (Egan, et a1., 1992). The appropriate repairprogram for this wharf, designed by Ben C.Gerwick, Inc. and Geomatrix Consultants, involved use of vibro replacement stonecolumns to correct the liquefaction problemand ductile prestressed concrete piles to


replace the damaged batter piles. Figure 3.23shows the reconstructed wharf.

World War II era wharves at theOakland Army Base were also damaged bythe Loma Prieta earthquake. No soil failureswere observed or suspected. Based on theresults of non-linear soil structure interactionanalyses for code level earthquakes, Earl andWright designed. a retrofit scheme for theseseismically deficient wharves using large diameter steel pipe piles. Time histories ofground motion at variou~ levels below the soilsurface were required for these analyses.

3.6.2.3 Observed Effectiveness ofFoundation Remediation Techniques

Because we have so little direct experience with the effectiveness of foundationremediation techniques, no definitive conclusions are possible at present with regard totheir effectiveness.

3.6.4 Issues in the Use ofFoundation RemediationTechniques

Very few retrofitted foundations havebeen tested by major earthquakes.Nevertheless, engineers can learn importantlessons from case histories of successful andunsuccessful earthquake performance offoundation systems similar to the remediationscheme being considered. Examples rangefrom the successful behavior of non-brittle pilefoundations in major earthquakes such as SanFrancisco 1906 and Lorna Prieta 1989 to thefailure of some pile foundations due to upliftloads in Mexico Oty in 1985.


Analytical procedures for evaluatingthe effects of a foundation remediation systemon seismic response of a structure seem to bewell established. These include a variety ofsimple to highly complex numerical modellingmethods. What seems to be uncertain is theaccuracy to which' these procedures can beused to estimate soil-structure performance.'Any uncertainty in the method of analysis results directly in uncertainty in the benefits ofthe remediation method for modifying response of the structure to ground shaking.Limitations in the existing state of knowledgein the area of analytical procedures are the result of uncertainties in material characterization, as well as the analytical modellingmethod. Although various efforts are underway within the profession to improve confidence in these analytical methods, this area isstill a serious research need.

A related area of earthquake researchinvolves verification and improvement of siteresponse models for seismic analyses ofstructures on sloping ground.

3.6.5 Summary

This section has described foundationremediation techniques and issues that areimplemented primarily by means of structuralelements, including various types of piles,walls, footings, and related elements or systems. Even though their cost is often higherthan other soil improvement techniques, certainty of execution is often an overriding consideration in their selection. Research issuesare primarily related to retrofitted foundations, as very few have been tested by earthquake shaking. Analysis and design procedures are well established.§

67

a) SectionI 12m Ii (Total width of stabilized zone) II 4.1 m 1 m1 m 5.9 m I Existing wharfI I I I I--structure to remain

----..,'......v~'ew_'----~..,,__---------I

Sand fiJI

Sand Dike RII

Native Soils \ INew stonecolumns

.::: . .

i /.:.. New·:

" ;' stone ." i columns ~

:.: Existing pile '.:

'- New piles ,/

'Ex' . /rl, Istlng~

piles

Not to scale

b) Plan12m

0

New610mmoctagonalpiles

0

0

0

o 0

o

o

Bay ..

68

Figure 3.23. Stone columns and new piling installed to increase the seismic resistance of awharf structure (Egan, et al., 1992)


4. Verification of Soil Improvement andFoundation Remediation

4.1 Introduction

All attempts at soil improvement andfoundation remediation must be verified bysome laboratory or field testing technique. Inthe absence of such verification, owners andengineers may not have confidence that anyimprovement has in fact occurred.

As discussed in previous chapters, themost common goal of soil improvement forfoundation remediation is to increase theshearing resistance, or shear strength, of. thesoil. Therefore, verification of the effectIveness of measures intended to increase thestrength of the soil would ideally measure thestrength of the improved soil directly.However, increased soil strength is often accompanied by changes in other characteristicsof the soil such as increased stiffness or increased density. It then becomes possible toinfer improvement of soil strength by measurement of changes in one or more of theseother soil properties.

Verification may be accomplished bylaboratory or field testing techniques. Whilelaboratory testing has historically been themost common means for verification of soilimprovement, recent advances in field testingtechniques have provided additional meansfor verification. Field testing techniques maybe divided into in situ testing techniques andgeophysical testing techniques. L~~be~er

(1985) summarizes the common venflcationtechniques.


4.2 Laboratory TestingTechniques

Verification of the effectiveness of soilimprovement has commonly been perfonn~d

using various laboratory tests. Laboratorytesting techniques for verification purposeshave a number of advantages over othermethods of verification, but they also sufferfrom drawbacks that may, particularly for certain types of soil improvement, significantlylimit their usefulness. Verification of foundation remediation involves evaluation of the response of the entire soil/foundation system,and is consequently not amenable to conventional, small-scale laboratory testing. Perhapsthe most significant feature of laboratory testing techniques is the requirement of obtaininga sample of the improved soil. This requirement leads directly to many of the advantagesof using laboratory testing techniques and alsoto many of the disadvantages.

4.2.1 Advantages

Obtaining a sample of improved soilallows visual inspection of the effects of improvement. For many soil improvementtechniques, e.g., grouting, soil mixing, etc., theability to inspect the treated soil provides direct and invaluable evidence of the pervasiveness and effectiveness of the treatment.Samples may be inspected in the field at thetime of sampling or in the laboratory eitherbefore or after the performance of specificlaboratory tests.

69

Testing of some soil properties, e.g.,density, penneability, compressibility, etc., iseasier to perform in the laboratory than in thefield. In cases where these properties are important, considerable cost savings may be associated with verification using laboratorytesting techniques.

Laboratory tests provide great flexibility in testing conditions. For example, samplesof improved soil may be tested under stressconditions other than those that currently existin situ. If the foundation soils beneath aplanned embankment are improved by one ormore techniques prior to construction of theembankment, the performance of theimproved soil may be tested in the laboratoryunder stress conditions corresponding to endof-eonstruction conditions.

The stress conditions acting on a laboratory test specimen are generally much betterknown than those that exist in the field; inmany tests the boundary stress conditions aredirectly controlled. Accurate knowledge ofstress conditions is necessary for reliable interpretation of basic soil properties from testresults.

The strain conditions in a laboratorytest specimen are also better known and canbe controlled with much more flexibility thanthose in field tests. As previously mentioned,most geotechnical hazards are mitigated byimproving the strength of the soil. Since thestrength is mobilized at relatively large strains,its measurement requires testing methods thatcan induce large strains. This can often be accomplished much more easily in the laboratory than in field tests.

4.2.2 Disadvantages

There are also a number of disadvantages associated with the use of laboratorytesting methods for verification of soil improvement effectiveness. The disadvantagesare generally associated with the necessity ofsampling the soil to provide laboratory testspecimens.

One of the most important considerations is evaluation of the influence of sampledisturbance on test results. It is well knownthat some degree of sample disturbance is inevitable and that the results of many types oflaboratory tests are sensitive to the effects ofsample disturbance. The process of soilsampling, even with thin-walled, piston-typesamplers, results in straining and remolding ofthe perimeter portion of the sample. Thisstraining and remolding can change thestructure and/or the density of the soil, and itcan also obscure the influence of variousphysical and chemical processes that contribute to the in situ behavior of the soil. Sincesoil structure and density strongly influenceproperties such as strength, compressibility,stiffness, and permeability, sample disturbance can provide a misleading view of theeffectiveness of soil improvement.

Many soil improvement projects involve loose, cohesionless soil deposits. Thedensity changes associated with sampling ofcohesionless soils treated by various densification techniques are particularly troublesome.Research (Marcuson, et aI. 1977; Seko andTobe, 1977; Singh, et aI., 1979) has indicatedthat even thin-walled samplers can introducesignificant volume changes in clean sands.Loose sands are densified and dense sands areloosened by the sampling process. Inaccurateevaluation of in situ density, either before orafter soil improvement, can lead to considerable uncertainty in the evaluation of the effectiveness of the improvement, particularlywhen the effectiveness is interpreted usingcertain methods of liquefaction hazard evaluation (e.g., Kramer, 1989; Finn, 1991).

The process of drilling and samplingitself can be time-consuming and costly.Drilling rates can be quite slow in comparisonwith the rates of penetration that can beachieved with some in situ tests. Informationon the effectiveness of soil improvement is notavailable until the laboratory tests have beencompleted. When faced with the need forfield decisions on whether sufficient improvement has been obtained at a particulartime in a soil improvement project, laboratorytesting methods of verification are usually oflimited use.


Another disadvantage of verificationof soil improvement effectiveness using labo~

ratory tests is the lack of geometric continuityof the information produced by a typicallabo~

ratory testing program. In a typical drillingand sampling operation, soil samples are ob~

tained at discrete locations; continuous sam~

pIing is rarely used. As a result, measurements of soil parameters such as stiffness,strength, permeability, density, etc. are obtained at a discrete number of points and thevariation of the parameters between thosepoints must be estimated. For soil conditionsin which properties can vary significantly overrelatively small distances, the inferred properties may not reliably represent the propertiesof the entire soil mass. When soil improvement techniques are used to improve or eliminate localized zones or seams of weakness,verification by methods that require discretesampling may be ineffective or inappropriate.

4.3 In Situ Testing Techniques

As suggested above, the disadvantages of conventional sampling and laboratorytesting, especially in loose granular soils andother potentially unstable deposits, may beovercome by the use of in situ testing techniques. For this purpose, as well as in conventional geotechnical practice, the use of in situtests has increased dramatically in the last 15to 20 years. The traditional standard penetration test (SPT), Dutch cone penetration test(CPT), and plate load test (PLT) have beenused extensively to investigate treated ground.In recent years, the pressuremeter test (PMT),piezocone penetrometer, screw plate compressometer, and the dilatometer have come intotheir own as in situ tests for ground improvement evaluation. Ledbetter (1985), Mitchell(1986b), and Welsh (1986) discuss in situ testing as applied specifically to foundationtreatment methods, and they also present several case histories on the use of in situ tests toevaluate treated ground.

The CPT and especially the piezoconeare particularly useful because they provide acontinuous record with depth. Furthermore,these tests are rapid and cost effective, especially when used in sand and silt deposits.The results of penetration tests, however, must


be interpreted carefully, since time-dependentincreases in strength, stiffness, and penetrationresistance have been observed after densification (Mitchell and Solymar, 1984; Mitchell,1986a). Piezocone results are also especiallyuseful at sites in which macrofabric anddrainage boundaries are important, for example, when vertical geocomposite drains areused to accelerate the consolidation of softsoils.

The pressuremeter has been used extensively to determine the degree of improvement after dynamic compaction (Lukas,1986). The PMT provides estimates of the lateral stress, modulus, and shear strength, but itis performed in pre-bored holes (unless theself-boring model is used) and it is relativelyexpensive, especially if a large number of separate tests are required.

In recent years, the dilatometer testhas seen increasing use for determining the insitu stress and compressibility characteristicsof loose sands and moderately soft clays. It isnot very reliable for very soft clays and cannotbe used if the soil contains gravel. In this case,relative density and improvement in soilproperties after treatment must be derivedfrom empirical correlations (Mitchell, 1986b).

Welsh (1986) notes that in situ testingtechniques for establishing the degree of improvement of most grouting methods are stillvery primitive. Although conventional in situtesting can be used to evaluate some measureof the degree of improvement afforded bycompaction grouting, care must be exercisedin the interpretation of data. As described byJamiolkowski and Pasqualini (1992), thepenetration resistance of granular soils is not onlyinfluenced by relative density and depth(vertical effective stress) but very significantlyby lateral stress. To the extent that improvement methods increase lateral stress, they willproduce unconservative errors in the prediction of relative density based on commonlyused methods, unless correction for the newstress state is considered. As for slurry grouting, fracture grouting and chemical grouting,in situ tests do not appear to hold as muchpromise as do geophysical methods.

71

4.4 Geophysical TestingTechniques

Several geophysical methods availabletoday provide verification of the effectivenessof many ground improvement techniques.Other methods are being developed and testedfor use in the near future.

4.4.1 Seismic Techniques

Most soil improvement techniquescause changes in subsurface conditions whichare manifest in an increase in stiffness.Stiffness is directly related to soil modulus andmodulus can be measured with seismic techniques. Furthermore, for soils treated bygrouting, etc., there are empirical relationshipsbetween modulus and strength of the groutedsoil.

Soil improvement techniques whichresult in increased modulus include compaction grouting, chemical grouting, jet grouting, soil mixing, dynamic compaction, vibrocompaction, blasting, drainage and dewatering, and stone columns. The effectiveness ofall of these techniques can be verified withseismic techniques.

In most applications, it is best to perform seismic tests before and after soil treatment to measure improvement. In some instances, an increase in wave velocity is sufficient to prove improvement without absolutevalues of velocity. In other cases, it is sufficient to measure velocity after improvementbecause relationships between modulus (orwave velocity) and strength are available. Theshear wave velocity (or shear modulus) isusually measured because it is not influencedby water in the soil; however where water isnot present or not a factor, compression wavevelocity (Young's modulus) may also be useful.

The most common seismic techniquesfor proof testing soil improvement are crosshole and down-hole tests. Cross-hole testingmay be superior because it integrates velocityalong a wave path and represents a largervolume treated. However, a disadvantage isthat the seismic wave tends to travel through

the stiffest material and consequently, may nottravel on a straight path from hole to hole.Byle, et al. (1991) describe the use of geophysical methods to evaluate a site treated by compaction grouting.

Seismic tests are nondestructive, butthe cross-hole and down-hole test requireboreholes. The recently developed spectralanalysis-of-surface-waves (SASW) method canbe performed from any free surface (groundsurface or inside of a tunnel wall for example),thus eliminating the need for boreholes. Thismakes the SASW method both nondestructiveand nonintrusive (Gucunski and Woods,1991).

SASW can be used to test any materialfrom weak soil to rock and concrete. The procedure is the same; only the sensors, sourcesand spacing are different. For hard, strongmaterials, the spacing of sensors may be only20 to 30 mm, while for weak soils, the spacingmay be up to 30 m. This provides depths ofpenetration from a few millimeters to nearly5Om.

Geotomography approaches are attractive to explore the improvement of soil indetail, but are very expensive at this time.

4.4.2 Ground Probing Radar

Ground probing radar (GPR) canidentify the depth to strata of strong contrast.Ground improvement which causes sufficientchange in properties should represent a reflector to electromagnetic waves and show up inGPR. This method, however, detects onlyboundaries and does not provide informationon mechanical properties. It should probablybe used with discretion and only in very specific cases for verification of soil improvement.

4.4.3 Resistivity/Conductivity

Some ground improvement techniques may produce a change in the electricalproperties of the soil, and thereby causechanges in resistivity or conductivity. Thesemethods need to be explored in more detail todetermine their applicabiIity.§


5. Research Needs5.1 Introduction are described, and where possible, specific and

detailed requirements are given.

The field of soil improvement has developed. tremendously in the past two decadesprimarily because of three factors; (1) im~

proved. understanding of geotechnical hazardsand the factors that control them; (2) the economic benefit of the development of sites withmarginal to poor soil conditions; and (3) theuse of innovative construction techniques developed primarily by specialty contractors. Incontrast with many other aspects of geotechni~cal engineering, advances in soil improvementhave occurred largely due to the initiative andimagination of contractors. A relatively smallnumber of specialty contractors have developed many of the techniques now commonlyused for soil and site improvement and, whilemany of these techniques have proven to bevery effective, most have been developed. bytrial and error. Often, the mechanisms bywhich they work are not clearly understoodby all practicing engineers, and thus their se~

lection, design and utilization may not be themost efficient or economical for the soils orstructure under consideration. Additional research is clearly required.

As one of the overall objectives of theworkshop was to identify and evaluate currentresearch needs in soil improvement and foundation remediation, the workshop participantsdevoted considerable effort to what is currently not known about the various techniques, especially when seismic hazards areconsidered. In this section, research needs asdeveloped by the several techniques groups


5.2 Research Needs byTechnique Group

In this chapter, the needs for researchin soil improvement and foundation remediation are identified. These research needs weredeveloped during meetings of the workshopdiscussion groups and in plenary session discussions. In keeping with the previous structure of the report and the workshop itself, theyare presented in terms of the various classes oftechniques. The research needs are then prioritized in a manner that reflects the generalconsensus of the workshop participants.

The workshop identified three majorareas of research needs which are uniformlyapplicable across the entire spectrum of soilimprovement and foundation remediationproblems. These are:

1. A need for a well-documented database of quantitative information fromcase histories of both failures and successes. This information is needed forvalidation of analytical methods, validation of field performance of remedial techniques, and verification of ourconceptual understanding of the phenomena involved. Incorporation ofJapanese and other foreign experiencein the data base would be very useful;

73

Dynamic Compaction

5.2.1 Densification Techniques

3. A need for improved methods of verification of the effectiveness of the various soil improvement and foundationremediation techniques.

The densification techniques identified and discussed during the workshop havebeen developed primarily on a site-by-site basis by specialty contractors. As a result, existing design criteria lack the research backingrequired to fully understand the various densification processes.

Although the dynamic compactiontechnique has been successfully used on bothdry sites and sites with a high groundwatertable, the mechanisms of densification must bedifferent. At saturated sites several passes areused, causing local liquefaction around theimpact point and subsequent reconsolidation.Although the method has been used mainlyfor sandy soils, it appears that it may be alsoapplicable to silty and clayey soils, providedthat effective drainage measures provide fordissipation of excess pore water pressures.

Large repeated high-energy impactsgenerate ground vibrations, often with lowfrequencies of between 5 to 25 Hz. Currentthreshold particle velocities and structuraldamage criterion are based on U.S. Bureau ofMines studies of blast vibrations with frequencies in the 10 to 60 Hz range. Some workis needed to better understand low-frequencyvibrations and their attenuation with distance.

The measurement of deceleration ofthe weight upon impact could be used as ameans of quality control and verification ofimprovement. This idea is analogous to theuse of pile driver analyzers and high-strainmeasurements for evaluating pile capacities.

Vibro Techniques

Dynamic compaction requires in situtesting before, during, and after improvementin order to verify the depth and extent of improvement at a particular site. While this hasbeen previously accomplished using a varietyof in situ devices (e.g., SPT, CPT, PMT, CPTU,OMT, NOE, seismic, and Becker probes), timeeffects now recognized in clean sands(Mitchell, 1986a; Mesri, et al., 1988;Schmertmann, 1989) have clouded the truedegree of improvement. In situ tools whichcan effectively characterize unusual materials,such as landfills and waste materials, are alsoneeded.

Uncertainties in the application of vibro techniques still remain and should be thesubject of further research. Items for whichadditional knowledge is needed include anevaluation of the increase in lateral stressesdue to densification by horizontal expansion,

and Foundation RemediationSoil Improvement74

Currently, no well-developed theoretical model exists for quantifying the dynamiccompaction process. The basic mechanismscontrolling the effectiveness of the method arenot understood and, consequently, the methodhas probably not been optimized or "finetuned" to a variety of site improvement situations.

2. A need for better methods of characterizing and describing the conditionof soils and foundations in situ. Thisincludes methods of nondestructive insitu testing of structural materials,soils, and rock, and methods of analysis which can account for complexitythat is beyond our current ability todescribe completely; and

Current practice for estimating the effective depth of improvement is based on asimple empirical relationship that includes theenergy per drop of the weight. Not only isthis expression dimensionally incorrect, it failsto account for important factors such as thedimensions of the weight, the number ofdrops, level of groundwater, soil type, etc.Generally, weights used are often circular oroctagonal with flat bottoms. No research hasbeen conducted to determine the most efficientweight geometry to optimize of thetransmitting stress waves into the underlyingground.

evaluation of aging effects on probe penetration resistance, influence of soils that are believed to have potential for liquefaction, yet donot densify ("liquefy") by vibration, influenceof the fines content, effect of vibrator frequency, comparison of wet vs. dry construction methods, quantification of drainage effects, and non-destructive and in situ evaluation of treated ground.

Compaction Grouting

Like many other soil improvementtechniques, this technique was developed byspecialty contractors without the necessary basic research. Specific research needs for compaction grouting include (1) improved understanding of the general mechanics of injectinga low slump, sanded grout into a soil and howthe expanding element causes densification ofsoils, (2) development of a user's manual describing the theory and practice of compactiongrouting for densification purposes, with ananalytical procedure, typical design examples,and an updated bibliography, (3) determination of optimum spacing, pressure and mixdesign, and extension of the compactiongrouting technique to the densification ofmore cohesive soils, in which a non-eementitious material is injected under continuingsteady pressure.

5.2.2 Drainage Techniques

Although the design and constructionaspects of most drainage techniques are quitewell established, there still are several itemsthat require improvement and thus can beconsidered to be research needs.

Interception

Research needs in this area includeimproved installation techniques, especially incontrolling the alignment of subhorizontal andinclined drains. The durability and long termperformance of interception drains is uncertain and requires attention.


Pare Pressure Control

A number of aspects of site characterization and soil properties, as well as post installation properties need further study.

In terms of liquefaction and settlementproblems, the influence of grain size distribution and other properties such as permeabilityand compressibility, especially under seismicshaking, is not well understood. Conventional analyses of pore pressure controlproblems are at present unable to rationallyconsider site variability, nonlinearity of thecoefficient volume compreSSibility mv, itsdependence on stress level and number of cycles of shaking, macro fabric of the deposit,three-dimensional and real earthquake timeeffects, and coupling of pore pressure and deformation.

Heretofore, only stone columns andcoarse granular drainage blankets have beenproposed to mitigate liquefaction effects inloose sand deposits through enhanceddrainage. Research is needed on the rate ofpore water pressure dissipation and rate ofdrainage required to prevent liquefaction.Investigation of the use of geocompositedrains for this purpose is an important research need, especially because of their advantages for rehabilitation applications in urbanareas.

For evaluation and verification of porepressure control techniques, research isneeded on the durability and long term performance of these drainage systems and onevaluation of pore pressure dissipation, especially post liquefaction.

Dewatering

The primary research need in this areais in the development of improved hydrogeologic investigation models and analyses forsite characterization and design properties.

75

As with other drainage techniques,durability and long term performance of de,.watering systems, especially dUring and afterearthquakes, needs research.

Accelerate Consolidation

Probably the most important consideration in site characterization and propertiesis a good understanding of especially themacrofabric of the deposit. Improved techniques and procedures for rapidly and effectively mapping the subsurface macrofabric areneeded. Proper consideration of macrofabric,3-D and real earthquake time effects in designanalyses are also required.

Construction problems in this area arelimited to appropriate determination of theamount and effect of smear during wick draininstallation.

Wick drains are also subject to crimping and bending due to large settlements, andresearch is needed on how to properly consider these effects in evaluation of the effectiveness of the treatment technique.

Finally, research is needed in the longterm performance and durability of the drains.

5.2.3 Physical and ChemicalModification Techniques

Grouting and Soil Mixing

Verification of the effectiveness ofboth physical and chemical soil improvementtechniques is very important. The susceptibility of cohesionless and lightly cemented soilsto disturbance during sampling makes laboratory testing less reliable. The relative weakness and brittleness of most chemicallygrouted sands results in samples that appearto be uncemented; hand-earved block samplesare required to avoid the destructive affects ofsample disturbance.

In situ testing for verification can beappropriate provided comparable preconstruction testing has been done. However,relatively minor variations in soil propertiescan complicate interpretation and diminish

confidence in the results. Establishing appropriate techniques for evaluating the characteristics of improved soils is an important research need. Specifically, the following shouldbe done: (1) guidelines for the potentialstrength improvement of fracture groutedsoils as a function of soil characteristics, groutmaterials and application techniques shouldbe developed, (2) the environmental impact ofgrouts and solidifying admixtures used in injection/mixing techniques should be evaluated, and (3) the effectiveness of deep soil mixing for improving the seismic performance ofwalls and foundations in various soil typesshould be determined.

Impraued-Soil Cells or Grids

With regard to the use of improvedsoil cells or grids, a better definition of the following are important research needs: (1) investigation of the effectiveness of improvedsoil grids in redUcing pore pressure generationin saturated granular soils during seismicevents, (2) development of quantitative relationships between the thickness of liquefiablelayers and the allowable spacing between improved-soil grid walls, (3) evaluation of theperformance of stabilized zones in protectingstructures from floating, sinking, lateral sliding or lateral spreading, and (4) evaluation ofthe loading induced on soil-improved gridwalls constructed on slopes due to seismicdegradation of retained soils (Le., evaluationof residual strength) and potential flow of soilsoutside the cells.

Primary methods of study might include: (1) numerical modeling; (2) small-scalemodel tests using a dynamic centrifuge; and(3) larger scale model tests using a shakingtable.

Impraued Documentation

Improved documentation is requiredof projects where physical and chemical modification has been used to increase the seismicresistance of soil deposits, foundations, slopesor retaining structures.


5.2.4 Inclusions Techniques

Soil Nailing

No data is available on the performance of soil-nailed structures during shakingwith peak accelerations greater than aboutO.lg. Therefore, there is no check on the adequacy of current design practice under verystrong shaking. The moderate to high shakingof the 1989 Lorna Prieta earthquake providedno data to confirm the basic assumptionsmade in the seismic design of soil-nailed structures because no nails were instrumented andno walls suffered any apparent damage.

Designers are uncertain about the distribution of shear stresses and tension alongthe nail. The distribution of pressures thatshould be associated with the seismic force resulting from a Mononobe-Okabe analysis havenot been established for these structures. Thecurrent practice is to use distributions determined from shaking table tests on rigid retaining walls. Selection of design horizontal andvertical pseudo-static earth pressure coefficients can significantly influence the cost of asoil nailed structure. These coefficients arecurrently selected in a rather arbitrary mannerwhich, because of the implications for bothsafety and economy, should be verified byfurther research.

There are three procedures for developing suitable values of design horizontal andvertical pseudo-static earth pressure coefficients. First and foremost would be throughthe use of field data from instrumented walls.The infrequency of strong shaking and thedifficulty in maintaining instrumentation overlong time periods make it necessary to pursueother methods for clarifying seismic action ofsoil-nailed structures. Centrifuge tests offerthe most effective procedure for determiningseismic performance experimentally becauseeffective stress levels in the field can beachieved in the centrifuge model. This is especially important because the response of thesoil-structure and the action of the nails arecontrolled largely by the effective stressregime of the soil. Potentially useful for giving detailed insight into the behavior of soilnailed structures is the application of finite


element analyses. They do not appear to havebeen used very much so far to explore how thenails function or how the composite structuremight behave under seismic loading conditions. If successful, this approach would bethe most cost effective. The centrifuge testscould be used to verify the capability and results of finite element analyses.

Soil Reinforcement

Uncertainties in the values of dynamicearth pressure coefficients for design in a specific seismic environment have the same impact here as in soil-nailed structures. The basic design will be controlled by the earth pressure coefficients and research on their appropriate values is a high priority. Research onthe general validity of the Mononobe-Okabeapproach when applied to flexible reinforcedsoil structures is also needed.

Pull-out capacity is a key variablecontrolling the interaction between the soiland the reinforcement. The capacity underseismic loading requires verification. Thestrength and ductility of geosynthetics underseismic loading long after installation in thefield requires verification.

Long term durability of geosyntheticreinforcement is an important research need,especially in terms of any synergistic effects ofcreep and subsequent seismic shaking.

Reinforcement Piles

The Sardis Dam problem (Section3.5.2.2) has shown the need for research intothe response of pile reinforced structures. Formany similar applications, piles are a very attractive solution because of the ease of installation and the reliability of the properties ofthe inclusion. The latter is a major consideration in any situation involving significantissues of public safety.

Stone Columns

The feasibility of using stone columnsto provide improved shearing resistance, stiffness, and lateral capacity requires attention.

77

5.2.5 Foundation RemediationTechniques

Foundations

The primary research needs for foundation remediation systems relates to the verification of analytical predictive methods.These verification efforts should involve: (1)review of foundation performance followingseismic events, (2) instrumentation and monitoring of building and bridge foundations toobtain performance data, and (3) back analyses of performance information to calibrateanalytical methods.

The potential economics and practicality of "tuning" foundation systems to reduceseismic ground motions and provide betterseismic response should be investigated.There is a need for systems which can be easily added to existing foundations to changethe mass, stiffness, or damping. This investigation must consider the potential variation ininput motion characteristics (frequency andmotion amplitude) as well as source motiondirectionality.

There is also a need for research onprocedures which can be used to determine orconfirm the dynamic characteristics (stiffnessand damping) of existing foundations in orderthat retrofitting studies can be carried out withconfidence.

Improved methods need to be developed to analyze lateral load capacity of pilingunder seismic conditions.

Better methods are needed to determine the true residual strength of liquefiedsoils.

Known techniques currently considered for static stability in the remediation ofslope and retaining structures, (examples: reinforced concrete retaining walls, slurry walls,bulkheads, tieback walls, rockbolting systems,buttress walls, and piling) may also be appliedto seismic and dynamic design considerations.

There is a need for a better definitionof seismic design loads on retaining walls.Analytical techniques should be improved tofurther understand and define the ground motion field around retaining walls. This wouldapply to reinforced concrete retaining walls,bulkheads, buttresses, and slurry walls.Design guidelines, equations, tables and chartswould be helpful to practitioners.

Systematic procedures to determinethe seismic design forces and to evaluate theperformance of tieback anchors and rockbolting systems during earthquakes need to be established. Again, design guidelines, equations, tables and charts would be helpful topractitioners.

Effect of Code ChIlnges

There is a need for techniques toretrofit existing pile foundations that are undersized.

Determination of the effectiveness ofencapsulation systems to isolate and limit liquefaction, and to improve foundation stiffnessneeds to be made.

To better understand the behavior andfurther develop the current techniques for dynamic design applications, research is requiredto improve our understanding of the behaviorof soil and pile group interaction in a dynamicloading situation.

78 Soil Improvement

It is likely that forthcoming changes inbuilding code seismic design requirementsfrom working stress levels to limit state designalong with the associated changes in structuralductility factors will prompt a reevaluation ofexisting foundations and result in increasedfoundation vertical and lateral forces for newstructures. Research on the effects of thesecode changes on foundation design is timely.

Soil-Structure Interaction

Research is required on soil-structureinteraction effects such as non-rigid foundation response. This work should establish therange of reduction in force levels and the associated displacements, and establish simpli-

and Foundation Remediation

fied analytical methods for soil-structure interaction.

Nondestructive Tests

Densification Techniques

• Development of theoretical models forunderstanding the mechanics of densification

Improvement in the methods used toevaluate the appropriate soil-foundation parameters for earthquake design is needed.Geophysical techniques that would be applicable to various types of piling warrant furtherexperimentation. Related analytical studiesare required, preferably based on results ofwell-documented case histories. Some highstrain level and destructive tests should beperfonned to establish the limits of extrapolation of low strain test results to design earthquake levels.

5.3 Prioritization of ResearchNeeds

• Investigation of time-dependentstrength gain in densified ground

• Development of NDE methods forverifying densification effectiveness

• Further development. of testing toevaluate the liquefaction potential ofcoarse-grained soils

• Investigation of the role of vibratorfrequency and amplitude in the densification process

Drainage Techniques

The research needs identified and discussed in the previous section are listed intenns of the priority perceived by the workshop participants. On the basis of workshopdiscussions, the research needs were compared and divided into three categories: highpriority, medium priority, and low priority.The order in which they are listed within thesethree categories is arbitrary.

5.3.1 High Priority Research Needs

General

• Development of well-documenteddata bases of successful and unsuccessful field perfonnance

• Development of improved methods ofdetermining in situ soil characteristics

• Development of improved methodsfor verification of effectiveness of soilimprovement and foundation remediation techniques

• Enhancement of technology transferand infonnation exchange betweenUSA and other countries, particularlyJapan


• Improvement in the determination ofsoil properties required for drainagesystem design (horizontal and verticalpermeability, compressibility, etc.),both before and after drain installationand before, during, and after a seismicevent. Appropriate considerationshould be given to nonlinearity andspatial variability of theseproperties

• Investigation of the suitability ofgravel drains and prefabricated geocomposite drains that are installedwithout vibration to mitigate liquefaction potential in vibration sensitiveenvironments .

• Investigation of properties and performance of drains after large seismicevents

• Development of methods for quantification of drainage effectiveness ofstone columns

Physical and Chemical Modification

• Evaluation of long-tenn durability ofgrouts and cementing materials

• Investigation of the effectiveness ofcells or grids of improved soil

79

• Investigation of the environmental eff~ofmff~ent~sof~ouG

• Identification and characterization oflayered or stratified soils and evaluation of their eff~ on ~outability

Inclusions

• Evaluation of appropriate dynamicearth pressure coefficienG for nailedand reinforced structures

• Investigation of mechanics of reinforcement pile-soil systems

• Investigation of failure mechanismsfor soil-nailed and reinforced soilstructures

• Investigation of reinforcing effectiveness of stone columns

• Further development of physicalmodelling (shaking table and centrifuge) of nailed and reinforced structures

• Improvement of field instrumentationfor nailed and reinforced structures

• Development of simplified numericalmodels for analysis of nailed and reinforced structures, and for piles andmicro-piles

Foundation Remediation

• Improvement of instrumentation andmonitoring of building and bridgefoundations

• Calibration of analytical models byback-analyses of actual field performance

• Investigation of dynamic response ofpile~oups

• Investigation of methods forretrofitting undersized pile foundations

• Investigation of the effectiveness ofencapsulation systems

• Evaluation of effecG of building codechanges on foundation design

• Development of simplified methods ofanalysis for soil-structure interaction

5.3.2 Medium Priority ResearchNeeds

Densification

• Investigation of the role of residuallateral stress on the results of in situtests

• Identification of soil types that can effectively be densified by explosives

• Investigation of the effects of shear,compression and Rayleigh waves onstructural response to vibrations generated from dynamic compaction

• Evaluation of dynamic compactionefficiency at the surface by'means ofdeceleration measurements

• Development of a manual describingthe theory and practice of compaction~outing for densification purposes,with an analytical procedure, typicalexamples, and an updated bibliography

• Further development of the compaction grouting technique in which anon-cementitious element is injectedunder continuous steady pressure toimprove cohesive soils

Drainage

• Rapid in situ determination of soilproperties required for drain design

• Separation and quantification of thebeneficial effecG of densification anddrainage with vibroreplacement andvibrocompaction techniques


• Investigation of long-term performance and durability of all types ofdrainage systems, including materialdurability and potential for physical,chemical, or biological clogging

• Development of means for simpleverification of drainage system performance capability (i.e., that the drainis still functioning) many years afterinstallation

Physical and Chemical Modification

• Verification of effectiveness of physical and chemical modification at themicro-level

• Investigation of the seismic performance of slurry walls and cutoff walls

• Development of grouting criteria how much is enough?

Inclusions

• Investigation of mechanical reinforcement effects of stone columns

• Development of improved field instrumentation of piles, micro-piles andstone columns

• Development of simplified numericalmodels for stone columns

Foundation Remediation

• Investigation of the potential for "tuning" foundation systems to reduce dynamic response

• Development of methods for evaluation of dynamic characteristics of existing foundations

• Development of improved methodsfor evaluation of residual strength ofliquefied soils

• Development of improved definitionof seismic design loads on retainingwalls


• Development of improved proceduresfor seismic design of tieback androckbolting systems

• Development of improved methodsfor seismic analysis of retaining walls

5.3.3 Low Priority Research Needs

Den.sification

• Development of improved verificationby geophysical methods

• Optimization of dimensions andshape of the DC tamper weight

• Investigation of liquefiable soils notliquefying by vibration teclmiques

• Investigation of wet versus dry installation of stone columns

• Investigation of the effectiveness ofexplosives

• Investigation of the range of materialtypes for which vibrorod compactionis most suitable

• Investigation of the role of gas generation by explosives on laboratory andin situ test results

Drainage

• Development of biodegradable slurries for drain installation in loose deposits

• Investigation of bent, crimped andsmeared prefabricated vertical drains

• Improvement of hydrogeological investigations, models, and analysis fordewatering systems

Inclusions

• Improvement of physical models forpiles, micropiles and stone columns

§

81

6. Future Directions for Research on SoilImprovement and FoundationRemediation

6.1 Introduction 6.2 Small-scale Testing

When considering future directionsfor research on soil improvement and foundation remediation, it is important to keep itshistory in mind. Soil improvement and foundation remediation techniques have developed largely through the initiative and imagination of speciality contractors. While manyof these techniques have proven to be quite effective, their use and applicability is mostlygoverned by empirical, heuristic rules andprocedures developed through practical experience of the contractors. As a result, the actual mechanisms by which these techniqueswork are not always well understood, andthus their use may be more restricted thannecessary and not be as efficient or economicalas it could be. Both fundamental and appliedresearch is necessary to improve their efficiency and effectiveness; such research will result in significant progress toward attainingthe goals of the National Earthquake HazardMitigation Program.

The following sections discuss possible directions for future research on soil improvement and foundation remediation techniques.

Small-scale testing, including laboratory testing and physical model testing, hashistorically been used with great success insoil mechanics. Small-scale testing offers theopportunity to evaluate the behavior of geomaterials under carefully controlled environmental and stress conditions.

Small-scale testing has proven usefulin developing understanding of the physicalprocesses involved in, for example, densification of soils. During the past 25 years, the laboratory testing that has been directed towardinvestigation of liquefaction and the seismicresponse of granular soils has greatly increased our understanding of the mechanics ofdensification, including identification of manyof the factors that influence densification.Many of the important issues in densification,however, relate to the loading imposed on thesoil by various densification techniques, aswell as to the influence of boundary conditions. These factors are not easily modeled insmall-scale laboratory tests.

The effectiveness of drainage techniques are less easily evaluated in small-scaletests. The important influence of boundaryconditions, and the often dominant importance of material heterogeneity, render such


tests generally less useful for investigation ofthe effectiveness of drainage techniques thanmany other soil improvement and foundationremediation techniques.

In the area of grouting and admixturestabilization, significant work has been doneusing small- or bench-scale testing on laboratory prepared specimens of typical field materials, which has greatly increased our understanding of the mechanics of permeation andchemical grouting. In addition, this work has.permitted an evaluation of the likely improvement of waste materials and a determination of the influence of the In situ environment on stabilized materials and the concurrent influence of ~hese materials on the environment. This latter factor is being recognizedas of increasing importance. Issues relating tothe influence of material heterogeneity anddiscontinuities on physical and chemicalmodification techniques, however, are stillvery difficult to address in small-scale tests.

The use of inclusions and foundationremediation techniques involves interactionbetween structural elements and the soil inwhich they are in contact. Such interactionproblems are generally difficult to investigateby small-scale testing.

Small-scale testing does have an important place in soil improvement research. Inmany if not most cases, however, its applicability is limited and must be supplemented byother methods more capable of realisticallyrepresenting the environmental conditions,stress conditions, and boundary conditionsthat are present in the field. Centrifuge modeltesting can play an important role in such research by allowing many of these conditionsto be more accurately modeled in small-scaletests, and more tests can be conducted moreeconomically.

6.3 Large-scale Testing

Large-scale testing can include bothprototype-scale tests conducted in the laboratory as well as full-scale field tests. The formerare especially practical when symmetry requires only a portion of a field problem to

modeled. Although almost always less thanfully realistic, the large-scale laboratory testingaffords a control of variables not often possiblein the field. Full-scale field testing, though expensive and site specific, can produce extremely valuable information on the effectiveness of all soil improvement and foundationremediation techniques.

Research on soil improvement andfoundation remediation using large-scale testing can be divided into two main categories:research directed toward improved understanding the mechanics of the improvement orremediation technique, and research directedtoward a verification of the effectiveness of thetechnique. When the opportunity for largescale testing presents itself, the research planshould consider both of these categories.

Proper instrumentation of large-scalefield tests is particularly important as suchtests are frequently expensive and may be impossible to repeat. Instrumentation and fieldmeasurements must be extensive, reliable andproperly placed. In order to fully benefit fromthe large-scale tests, often extensive priorsmall-scale testing and analytical work may beuseful. Instrumentation of improved andunimproved sites and structures in seismicallyactive areas can be expected to yield veryuseful information on soil improvement andfoundation remediation techniques, and programs to properly document such sites arestrongly encouraged.

6.4 University-IndustryCooperation

The area of soil improvement andfoundation remediation is a particularly fertilerealm for interdisciplinary and cooperativeUniversity-Industry research programs. Thisis because so much of the development andpractical application of the common soil improvement and remediation techniques haspreceded the research necessary to explainhow or by what mechanism improvement isobtained. As is true of most geotechnical engineering activities, the process of constructionof the soil improvement technique significantly influences the characteristics of the end


product. Because the practitioners and contractors have the actual experience of successand failure in field applications, their participation in research programs in this area, fromplanning through to execution and interpretation, can be very beneficial to the overall success of the research.

The failure of the U.S. construction industry to invest in research and development,particularly in comparison with theirEuropean and Japanese counterparts, has longbeen lamented. While many soil improvementspecialty contractors, in spite of their relativelysmall size, are well above the national averagein investment in research and development,most technological advances in this area stillappear to be coming from overseas. One ofthe big reasons for this appears to be due to adifference in expenditures for research anddevelopment overseas in comparison with theU.S. Both government and the constructionindustry are equally responsible for the current situation.

For the. situation to improve, incentives must be provided to encourage U.S. contractors to actively participate in research anddevelopment of existing and new soil improvement techniques. The potential payoffsfrom this cooperative participation aretremendous. Construction in all aspects is avery large portion of the overall national GNP.The return to society of expenditures on construction is almost instantaneous. Therefore,increases in productivity and efficiency in anyaspect of construction has an almost immediate benefit to the local and national economy.Furthermore, there is a marvellous synergisticeffect when the construction industry anduniversity researchers work together. The research becomes more practical and immediately usable, the experience is good for studentengineers working on such projects, and contractors benefit from an immediate improvedunderstanding of the principles and processesinvolved and in improved credibility in latermarketing of soil improvement services.These benefits to specialty-contractors are realand thus are worth a significant financial andin-kind services contribution to the cost of theresearch project.


. We believe that a significant opportu-mty for the future development of soil improvement and foundation remediation techniques lies in the use of interdisciplinaryteams of university researchers and practitioners, particularly specialty contractors, to studyand solve the problems described in this report.

6.5 National Test SitesProgram

The development of a system of multiple-user test sites in the United States wouldbe of great benefit to geotechnical engineeringpractice. Access to well-eharacterized fieldsites and to a central data repository wouldhelp develop, evaluate, improve and betterunderstand laboratory and in situ tests, fieldinstrumentation, predictive capabilities, soilimprovement and foundation remediationtechniques, design and construction methods,earthquake response, and geoenvironmentalproblems. The use of such sites for researchwould also lead to more cost-effective utilization of available research funds and wouldpromote greater cooperation and exchange ofinformation between public agencies, universities, and the private sector. Such sites havebeen used to great advantage in a number ofother countries, resulting in Significant economic benefits to geotechnical constructionboth internally and in the international market.

In order to discuss the establishmentof a network of multiple-user geotechnical experimentation sites in the United States, theNational Science Foundation (NSF) sponsoreda workshop at the University of NewHampshire (Benoit and de Alba, 1988). About50 distinguished geotechnical engineers andprofessors from the U.S. and abroad were invited to examine this problem. The participants were presented with the results of a preworkshop survey of public agencies, universities and private firms in which the respondents had indicated the existence of 81 specificlocations for which some level of data existed,and which potentially could be developed intomultiple-user sites. One of the important conclusions of this meeting was that it was

85

imperative to continue the process initiated bythe workshop, and to establish a system ofmultiple-user sites, for which a central datarepository should be created.. This centralrepository would enable geotechnical researchers to select the most appropriate sitefor their needs. To oversee the establishmentand maintenance of the data repository, identify promising sites, and encourage their use, aSystem Management Board (SMB) was established that would include representatives ofdifferent elements of the geotechnical community. The 5MB would name a SystemManager, who in tum would be charged withmaintaining the central data base for the designated sites, promoting their use and helpingto develop funding to support the entire program.

To preserve the momentum of theworkshop and to put the process in motion,NSF provided the University of NewHampshire with funds to develop a summarycatalog describing those sites that have already been proposed to be distributed to public agencies, universities, and interested private concerns. Further support was obtainedfor this effort through a contract with theFederal Highway Administration (FHWA) aspart of the National Cooperative HighwayResearch Program, to establish a central datarepository for all available national geotechnical experimentation sites. The scope of thework consists of developing a user-friendlysystem shell, with on-line computer searchand data retrieval capabilities, to accommodate essential information about multiple-usertest sites (generalized soil conditions, list ofavailable test data, site logistics and limitations, published references, and any otherpertinent site information). An electronic bulletin board will be provided as part of the system to permit rapid exchange of informationabout ongOing projects and notify experimenters of the existence of limited-access"event" sites. The data base will be continuallyupdated as new information becomes available.

Another smaller workshop was convened in Orlando, Florida, in October, 1991, inorder to develop detailed guidelines for (1)selecting the first few primary NationalExperimentation ("Class A") Sites and (2) selection, organization, and operation of the5MB and System Manager. Discussions werealso started on the possible uses and localmanagement of the initial Class A sites.Finally, persons who might be willing to serveon the 5MB and as System Manager were proposed.

Since the Orlando meeting, the FHWAand the NSF have promised sufficient initialfunds to establish the 5MB, hire a SystemManager, and to establish a few (3 to 4) designated geotechnical experimentation sites. Itis envisioned that the sites will be characterized and the management system activateddUring the next two years. This initial activitywill establish a system management strategyto ensure (a) maximum and appropriate utilization of each site, (b) maximum exchange ofgeotechnical information about each siteamong users, and (c) minimum costs to users.

The National Test Sites program is potentially of great value to soil improvement research. To date, field studies have often suffered from a lack of geotechnical data relatedto geologic and subsurface conditions bothprior to and after improvement The NationalTest Sites should provide excellent subsurfacecharacterization in a variety of soil conditions.Furthermore, there is a reluctance to accept theresults of small scale or even moderate scalelaboratory testing on these techniques withoutsome measure of field or prototype scale verification. Thus, the concept of NationalGeotechnical Test sites provides a frameworkfor sharing of research costs and responsibilities as well as developing a cost effectivemechanism for comparing the performance ofvarious site investigation and verificationtechniques and soil improvement procedures.


6.6 Summary

In this chapter, the advantages anddisadvantages of both traditional small-scaleand large-scale or field testing used ingeotechnical engineering research are discussed. The importance of increasing industry-university cooperative research is described, particularly in terms of the benefits to


specialty contractors, university researchers,students, and even the local and nationaleconomies. The recent development of theNational Test Sites program for geotechnicalexperimentation provides an excellent framework for sharing research costs and results,and it seems especially well-suited for research on soil improvement and foundationremediation techniques.§

87

7. Summary and Conclusions

On August 19-21, 1991 a workshopsponsored by the National Science Foundationon Soil Improvement and FoundationRemediation with Emphasis on SeismicHazards was held at the University ofWashington in Seattle.

The specific goals of the workshopwere (1) to summarize the current state ofknowledge concerning soil improvement andits applicability to foundation remediation forvarious geotechnical hazards, especially thosewhich are earthquake-induced; (2) to identifyand evaluate current research needs and opportunities in these areas; and (3) to recommend future directions for research on soiland foundation remediation. This report is a

The objective of the workshop was toprovide a forum for the exchange of knowledge and experience among experts with awide variety of viewpoints and perspectiveson soil improvement and foundation remediation as well as geotechnical earthquake engineering. Invited participants included consulting geotechnical and structural engineers,specialty contractors, engineers representingall levels of government, and academic researchers. The workshop was one of the raretimes that such a diverse group met for indepth discussions. The result was a reallearning experience for the participants and agreatly increased appreciation for the problems and concerns each other faces in the solution of seismic hazard mitigation problems.

7.1 Summary written record of the workshop deliberationsand its attempt to meet these goals.

After a brief introduction providingsome background for and organizational details of the workshop, Chapter 2 presents anoverview of the seismic hazards of liquefaction, ground shaking, and foundation, slopeand retaining structure failures and theircauses. It then discusses the general requirements of soil improvement and foundationremediation techniques for mitigation of thesehazards.

Chapter 3 describes all the commonsoil improvement and foundation remediationtechniques in terms of their current practiceand illustrated by case histories, historical use,observed effectiveness, and current levels ofconfidence in their results. The chapter beginswith a discussion of densification (dynamiccompaction, vibro compaction and vibro replacement, compaction grouting, blasting, andcompaction piles). Next the common drainagetechniques (interception, pore pressure control, dewatering, and acceleration of consolidation) applicable to foundations, slopes andretaining structures are discussed. After physical and chemical modification techniquessuch as grouting and soil mixing, the chapterturns to inclusion techniques (soil nailing,metallic and geosynthetic reinforcement, piles,and stone columns) and ends with a discussion of structural foundation remediationtechniques.

The issue of verification of the effectiveness of soil improvement and foundation


remediation techniques is addressed inChapter 4. Included are the traditionalgeotechnical laboratory and in situ tests, aswell as a discussion of geophysical testingtechniques which hold considerable promisein this area of verification.

As one of the workshop goals was theidentification of research needs, these are described and prioritized in Chapter 5. Chapter6 provides some general suggestions as to howfuture research in soil improvement andfoundation remediation might be directed andimproved. Included in this chapter is a summary of the state of development of NationalTest Sites for Geotechnical Experimentation, aprogram which holds promise for, amongother things, industry-university cooperativeresearch in soil improvement and foundationremediation techniques.

7.2 Conclusions

Although considerable progress hasbeen made in the use of soil improvement andfoundation remediation techniques, their empirical application and use indicates that muchresearch is required for a fundamental understanding of their physical (and in some cases,chemical) mechanisms.

Probably the most important conclusion of the workshop is its development of aprioritized list of specific research needs thathas been carefully considered by an interdisciplinary group of experts with a broad rangeof experience and perspective. The list addresses research needs for densification,drainage, physical and chemical modification,inclusions, and foundation remediation techniques. These research needs are given inSection 5.3.

There are tremendous benefits to increasing industry-university cooperative research on soil improvement and foundationremediation techniques, and such cooperationshould be strongly encouraged. The developing National Test Sites program appears toprovide an excellent framework for sharingresearch costs and results in this area.

The results of future research on soilimprovement and foundation remediation .techniques will lead to their more widespreadacceptance, and to their more reliableeconomical use. As a result, research in thisarea will contribute to a realization of thegoals of the National Earthquake HazardsReduction Program.§


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§

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..

Appendix A

Workshop Roster

Dr. Abbas AbghariCalifornia Dept of TransportationOffice of Transportation Laboratory5900 Folsom Blvd.Sacramento, CA 95819T: 916-739-2485F: 916-454-6595

Dr. Donald G. AndersonCH2MHill777108th Avenue NEBellevue, WA 98004T: 206-453-5000F: 206-462-5957

Dr. Gifford J. AstillSiting and Geotechnical Systems ProgramNational Science Foundation1800 G Street NWWashington, DC 20550T: 202-357-9500F: 202-357-9803

Mr. Juan BaezHayward Baker, Inc.P. O. Box 7690Ventura, CA 93006T: 213-740-0605F: 213-744-1426


Prof. Jean BenoitDepartmentofCi~IEn@neering

Kingsbury HallUniversity of New HampshireDurham, NH 03824T: 603-862-1428F: 603-862-2364

Dr. Rudolph BonaparteGeoSyntec Consultants5950 Live Oak ParkwaySuite 330Norcross, GA 30093T: 404-448-5400F: 404-368-0447

Prof. Roy H. BordenCivil Engineering DepartmentBox 7908North Carolina State UniversityRaleigh, NC 27695-7908T: 919-737-7630F: 919-737-7908

Prof. G. Wayne Gough333 Norris HallCollege of EngineeringVir@nia Polytechnic Instituteand State University

Blacksburg, VA 24061-0217T: 703-231-6641F: 703-231-7248

Preceding page blank 103

Dr. James G. CollinTensar Corp.4631 River Bottom DriveNorcross, GA 30092T: 404-968-7600F: 404-968-5538

Mr. Robert D. DarraghDames & Moore221 Main St, Suite 600San Francisco, CA 94105T: 415-896-5858/415-243-3842F: 415-882-9261

Dr. Gordon DenbyGeoEngineers, Inc.8410 154th Avenue NERedmond, W A 98052T: 206-861-6000F: 206-861-6050

Mr. Kenneth L. FaughtSam Cal Corporation751 Laurel Street, No. 537San Carlos, CA 94070T: 415-594-9199F: 415-594-4972

Mr. James R. FarisNAVFAC, Western DivisionP. O. Box 727, Code 411San Bruno, CA 94066-0727T: 415-244-3451F: 415-244-3473

Prof. W. D. LiamFinnUniversity of British ColumbiaDeparbnent of Civil Engineering2324 Main MallVancouver, BC V6T 124CANADAT: 604-822-4938F: 604-822-6901

Mr. T. Wayne ForrestCELMK-ED-FA3515 1-20 Frontage RoadVicksburg, MS 3918~5191

T: 601-631-5633F: 601-631-5583

Dr. A. G. FranklinUSAEWESP.O. Box 631Vicksburg, MS 39180-0631T: 601-634-2658F: 601-634-3453

Prof. George GazetasDepartment of Civil Engineering212 KetterState University of New York at

BuffaloBuffalo, NY 14260T: 716-636-3634F: 716-636-3733

Mr. Edward D. Graf715 San Miguel LaneFoster City, CA 94404T: 415-573-0792F: 415-573-1270

Prof. Carlton L. HoDepartment of Civil and

Environmental EngineeingWashington State UniversityPullman, WA 99164-2910T: 509-335-3721F: 509-335-7632

Prof. Robert D. HoltzDepartment of Civil Engineering260 Wilcox Hall, FX-10University of WashingtonSeattle, WA 98195T: 206-543-7614F: 206-685-3836

Mr. Alan P. KilianWashington State Department

of TransportationMaterials Laboratory1655 S. Second, QM-21Tumwater, WA 98504T: 206-753-7111F: 206-586-4611


Dr. Yasuyuki KogaSoil Dynamics DivisionPublic Works Research Institute1, Asahi, Tsukuba-ShiIbaraki-Ken,305JAPANT: 0298-64-2211F: 0298-64-2840

Prof. Steven L. KramerDepartment of Civil Engineering265 Wilcox Hall, FX-10University of WashingtonSeattle, WA 98195T: 206-685-2642F: 206-685-3836

Dr. Richard H. LedbetterUSAEWESP. O. Box 631Vicksburg, MS 39180-0631T: 601-634-3380F: 601-634-3453

Mr. Walter B. Lum650 Ala Moana Blvd., No. 213Honolulu, HI 96813T: 808-521-2271F: 808-521-2271

Mr. Gerald ManningFoundation ConstructorsP.O. Box 97Oakley, CA 94561T: 415-754-6633F: 415-625-5783

Prof. Geoffrey R. MartinDepartment of Civil EngineeringUniversity ParkUniversity of Southern CaliforniaLos Angeles, CA 90089-2531T: 213-740-9124F: 213-744-1426

Prof. Paul W. MayneSchool of Civil EngineeringGeorgia Institute of TechnologyAtlanta, GA 30332T: 404-894-6226F: 404-894-2278


Mr. Charles NgDepartment of Public WorksBureau of EngineeringCity of San Francisco1680 Mission StreetSan Francisco, CA 94103T: 415-554-8350F: 415-554-8349

Mr. Mike StevensU. S. Bureau of ReclamationGeotechnical Engineering and

Embankment DamsBranch-D-3620P.O. Box 25007Denver, CO 80225T: 303-236-3886F: 303-236-6763

Mr. Henry T. TaylorHLA303 Second Street, Suite 630 N.San Francisco, CA 94105T: 415-543-8422F: 415-777-9706

Dr. Mehmet T. TumayGeomechanics. Geomechanical andGeo-Environmental Engineering ProgramNational Science Foundation1800 G Street NWWashington, DC 20550T: 202-357-9542F: 202-357-0167

Mr. Joseph P. WelshHayward Baker, Inc.1875 Mayfield RoadOdenton, MD 21113T: 301-551-8200F: 301-674-3063

Mr. A. WightmanKlohn Leonoff Ltd.10200 Shellbridge WayRichmond, Be V6X 2W7CANADAT: 604-273-0311F: 604-279-4300

105

Prof. R. D. WoodsDeparanent of Civil Engineering2360 G. G. Brown LaboratoryUniversity of MichiganAnn Arbor, MI 48109-2125T: 313-764-4303F: 313-764-4292

Dr. David S. YangSMW Seiko, Inc.2215 Dunn RoadHayward, CA 94545T: 510-783-4105F: 510-783-4323


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SoilImprovement and Foundation Remediation · 2009. 12. 3. · T. Tumay, Director. This supportis gratefully acknowledged. We especially appreciate the encouragement and ideas provided