Investigations and Load Tests in Silty Soil

LINKPING 1997

STATENS GEOTEKNISKA INSTITUTSWEDISH GEOTECHNICAL INSTITUTE

Repor

t 54

Investigations and load testsin silty soilsResults from a series of investigationsin silty soils in Sweden

ROLF LARSSON

Investigations and load testsin silty soilsResults from a series of investigationsin silty soils in Sweden

ROLF LARSSON

This project was partly financed jointly by the Swedish Council for BuildingResearch (BFR), Grant No. 930591-70 and the Swedish Geotechnical Institute.

STATENS GEOTEKNISKA INSTITUTSWEDISH GEOTECHNICAL INSTITUTE

RapportReport No 54

LINKPING 1997

SGI Report No 542

Swedish Geotechnical InstituteSE-581 93 Linkping

SGI Literature ServicePhone: 013-20 18 04Fax: 013-20 19 09E-mail: [email protected]: http://www.sgi.geotek.se

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Roland Offset AB, Linkping, June, 1997

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Investigations and load tets in silty soils 3

Preface

This report deals with the results of a research project concerning investigationsand evaluation of properties in silty soils. The investigations in this part of theproject have been preceded by a comprehensive literature survey reported in SGIReport 49 Silt - Geotechnical Properties and their Determination. Thereafter, aseries of investigations has been performed at different locations with silty soilsand the results from different types of sounding tests have been compared to eachother and to results from laboratory tests concerning properties and classificationof the soils. In three locations, a number of in situ and laboratory tests have alsobeen performed and compared to the results from large scale field loading tests.

The report contains comments and recommendations on the use of the varioustypes of investigation methods together with special precautions that should beobserved in silts because of the special properties and conditions encountered inthis type of soil. It also contains comments and recommendations regardingcommonly used methods for estimation of properties from the different types oftest results and the calculation of bearing capacity of shallow foundations andsettlements in silt. Most of these methods have been elaborated for use undernormal conditions in sands and the findings in this report do not necessarily reflecttheir usefulness in these conditions.

The report is intended for geotechnical engineers, both designers and researchers,and others who are involved in geotechnical investigations and design in silty soils.

The research project has been financed jointly by the Swedish Council for BuildingResearch (BFR), Grant No. 930591-0, and the Swedish Geotechnical Institute.

The author wishes to express his thanks to all those who have participated and co-operated in the project. Special thanks go to Per Lfling and his colleagues at theSwedish National Road Administration and to Mats Larsson and Lars-Gran Iversat KM-Geokonsult AB for their invaluable help in locating and establishing the testfields in the Borlnge area, and also to the owners, Mr. Lars-ke Mattsson and

SGI Report No 544

Vasakronan AB, for their kind permission to use the land. Special thanks also goto Anna-Lena berg and her colleagues at Chalmers University of Technology fortheir co-operation in the tests concerning the variability of the ground watersituation in Vatthammar. Many other colleagues, both at the Swedish Geotechni-cal Institute and other institutions, have also been involved and contributed to theproject to various extents. Finally, the successful execution of the project is largelya result of the skill and dedication of the staff at the Institutes division for Fieldand Measuring Techniques, often exerted under long working days and harshclimatic conditions.

Linkping, December 1996

Rolf Larsson


Readers guide to this report

Why this report is special

This book gives a broad review of the usefulness of different investigation andcalculation methods when applied to silty soils. It focuses on the practicalapplication and use of the most rational ways of solving the engineering problemsencountered. It describes the possible sources of error when using differentmethods and also the possible hazards in design arising through the extensiveinfluence of the ground water conditions, which during the lifetime of the structuremay be very different from the conditions at the time of site investigation. Thereport also describes problems encountered during the execution of the testprogramme, which are very common also in practical foundation works but whichare rarely reported.

The goal of this report

The goal of this report is to provide recommendations for the methods that shouldbe used in investigations in silts, the special precautions that should be observedwhen using the methods and the way in which the results should be interpreted. Thereport also contains recommendations for the way in which bearing capacity andsettlements for shallow foundations should be calculated in this type of soil.

Who should read this report and why

This report will give the reader an insight into the advantages and shortcomings ofthe investigation and calculation methods normally employed in silty soils.

The report will be useful for geotechnical engineers planning investigations forselecting the most appropriate methods, giving instructions on how they should becarried out, specifying what supplementary investigations and observations shouldbe made and stating how the results should be interpreted. It is useful for fieldengineers understanding how the methods work in this type of soil, whatprecautions have to be taken and why it is important to follow different special

SGI Report No 546

procedures. It also gives guidance to designers when selecting calculationmethods, parameters and critical conditions during the design life. It gives aninsight into the problems that may occur during execution of the foundation worksand the precautions that need to be taken in that context.

Finally, the report gives clients for geotechnical investigations and design aninsight into the relevance of different methods and procedures, which enables abetter understanding of the quality of different procedures and why certainprecautions have to be taken.

How this report is organised

The report starts with a summary setting out the main results and recommenda-tions. It then continues with a detailed description of the investigations and testsperformed and the results from these. The results of the investigations and thepractical experiences and observations in connection with these are summarisedin a special chapter, where also more detailed recommendations are given for theinvestigations that should be performed and the special precautions to be observed.A description of the various methods for calculation of settlements and bearingcapacity of shallow foundation then follows. Finally, the results obtained withthese methods are compared to the results obtained in the large scale loading testsin the field and more detailed recommendations are given for the methods andprecautions that should be employed.


Contents

PREFACE

READERS GUIDE TO THIS REPORT

SUMMARY ................................................................................................ 11 Effect of ground water conditions Site investigations Determination of shear strength Determination of compressibility Sampling and laboratory tests Calculation of settlements Calculation of bearing capacity

1. INTRODUCTION ............................................................................................ 17 Purpose and background of the investigation Scope of the investigation

2. INVESTIGATIONS ........................................................................................... 22

3. PRINCIPLE OF THE PLATE LOADING TESTS .................................................... 29

4. INVESTIGATIONS AND LOAD TESTS AT THE THREE MAIN TEST LOCATIONS .. 324.1 Mjrdevi, Linkping ............................................................................. 32

Test field First investigation Pressuremeter tests Raft foundation with pre-loading Dilatometer tests CPT tests and weight sounding tests

SGI Report No 548

4.2 Vgverket, Borlnge ............................................................................. 504.2.1 Test field ..................................................................................... 504.2.2 Investigations in the current porject ........................................... 52

- CPT tests Standard CPT tests Seismic tests and excess pore pressure dissipation tests

- Pore pressure measurements- Sampling and laboratory tests

Sampling Classification tests Oedometer tests Triaxial tests

- Field vane tests- Dilatometer tests- Pressuremeter tests

4.2.3 Preparations for load tests and problems with ground water ..... 72- First excavation- Ground water lowering

4.2.4 Plate load tests ............................................................................ 85- Installation of plates and instrumentation- Reaction system- Loading system- Measuring system- Loading procedure

4.2.5 Results of the load tests .............................................................. 93- 0.5 x 0.5 metre plate- 1 x 1 metre plate- 2 x 2 metre plate- Settlement distribution in the load tests

4.3 Vatthammar, Stora Tuna, Borlnge .................................................... 1104.3.1 Test field ................................................................................... 1104.3.2 Investigations in the current project ......................................... 112

- CPT tests- Seismic cone tests- Sampling and laboratory tests

Sampling Classification tests Oedometer tests Triaxial tests


- Pore pressure measurements- Field vane tests- Dynamic probing test- Dilatometer tests- Pressuremeter tests

4.3.3 Preparations for the load tests .................................................. 1264.3.4 Results of the load tests ............................................................ 134

- 0.5 x 0.5 metre plate- 1 x 1 metre plate- 2 x 2 metre plate- Distribution of settlements with depth

4.3.5 Supplementary study of the possible variation inpore pressure distribution ......................................................... 149

5. EXPERIENCE FROM THE INVESTIGATIONS .................................................... 1515.1 Sampling and laboratory tests ............................................................ 151

5.1.1 Sampling ................................................................................... 1515.1.2 Laboratory tests ........................................................................ 153

- Bulk density- Grain size distribution- Water content- Atterberg limits- Classification- Capillarity- Oedometer tests- Triaxial tests

5.2 Field tests ............................................................................................ 1575.2.1 Soundings.................................................................................. 157

- Weight sounding tests- Dynamic probing tests- CPT tests

5.2.2 In-situ tests ................................................................................ 166- Dilatometer tests- Pressuremeter tests- Field vane tests- Pore pressure measurements- Seismic cone tests- Pore pressure dissipation tests

5.2.3 Further experience from the field tests ..................................... 175- Ground water and ground water variations- Need for sealing test holes

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5.2.4 Comparison between different results obtained in field andlaboratory tests .......................................................................... 177

- Soil classification- Shear strength- Modulus

6. CALCULATION METHODS FOR PREDICTION OF SETTLEMENTSAND BEARING CAPACITY ............................................................................. 1946.1 General .............................................................................................. 1946.2 Prediction of settlements .................................................................... 195

Empirical methods Calculations based on stress distribution according to

theory of elasticity and moduli Calculation of settlements of footings based on

Menard type pressuremeter tests The Briaud method of calculating settlement and bearing capacity

from results of pressuremeter tests. Other approaches in order to take the variation in the moduli

into account6.3 Calculation of bearing capacity .......................................................... 211

The general bearing capacity equation Empirical methods based on sounding test results Calculation based on pressuremeter test results Calculation of bearing capacity with respect to maximum

settlement criteria

7. COMPARISON BETWEEN PREDICTED AND MEASURED SETTLEMENTS ........... 2237.1 Mjrdevi .............................................................................................. 2237.2 Vgverket ........................................................................................... 2267.3 Vatthammar ........................................................................................ 232

Effect of negative pore pressure Summing-up of the results of the settlement predictions at Vatthammar

7.4 Summary of the findings in the comparisons ofsettlement predictions ......................................................................... 239

8. COMPARISONS BETWEEN PREDICTED AND MEASUREDBEARING CAPACITY ..................................................................................... 2418.1 Vgverket ........................................................................................... 2418.2 Vatthammar ........................................................................................ 245

REFERENCES .............................................................................................. 251


Summary

Effect of ground water conditions

The results from the present investigation have illustrated the paramount impor-tance of the ground water conditions for the engineering properties of silt. Theground water conditions may vary from high free ground water levels and artesianground water pressures to low-lying free ground water levels with a considerablematrix suction in the soil above. In areas with deep silt deposits with alternatingplains and deep ravines because of erosion by streams and rivers, the conditionsmay be highly variable. The conditions may also be highly variable duringdifferent seasons, with significantly higher ground water levels and pore pres-sures, particularly during the thawing and snow melting season in spring. Becauseof the often high degree of saturation in silt also above the free ground water level,the ground water conditions can react rapidly on precipitation and even limitedamounts of rainfall can cause significant changes in the ground water conditionsalso during the other seasons.

The ground water conditions during the time for the field investigations have asignificant effect on the results and the evaluation should take this into consider-ation. Also the results of in situ loading tests, such as pressuremeter tests, screwplate tests and plate loading tests, are heavily dependent on the prevailing porepressure conditions in the soil during the tests and have to be evaluated according-ly. This entails that, at low-lying ground water tables, also the matrix suction in thesoil above has to be measured. Furthermore, the bearing capacity of foundationsin silt is also heavily dependent on the groundwater conditions prevailing through-out the lifetime of the construction. The pore pressure conditions therefore haveto be measured at the time for the field investigations and for a sufficient time toenable a prediction of the extreme maximum pore pressures during the lifetime ofthe construction. This should be done with a frequency adapted in such a way thatalso the effect of periods with heavy rainfall can be studied. Construction in siltoften entails excavation down to frost free depths and precautions should be taken

SGI Report No 5412

so that any ground water conditions occurring during this operation can be dealtwith in a satisfactory way.

Site Investigations

Site investigations in order to determine the statigraphy of the soil shouldpreferably be made by CPT tests with simultaneous measurement of the generatedpore pressure, (also called piezocone tests or CPTu tests). In very stiff soils, verydeep profiles and profiles with embedded coarse or stiff layers or larger objects,CPT tests may have to be supplemented by dynamic probing tests in which theprobe can be advanced by blows. On the other hand, the latter type of test has amuch poorer resolution and is not very useful in very loose silt. The results of theCPT tests are significantly affected by the in situ pore pressure conditions and thesehave to be taken into account. Also a matrix suction has a large effect on the resultsand unless this is taken into account the soil becomes classified as coarser andstiffer than is actually the case. When the pore pressure conditions are properlyconsidered, the classification proposed in SGI Information No.15 (Larsson 1992)appears generally to function well but the results need to be inspected carefully inorder to avoid mistakes.

The results of weight sounding tests in silt were found to be generally unreliable.This is probably mainly related to the effects of the ground water conditions, forwhich these results cannot be corrected.

In all site investigations in areas where artesian ground water pressures may exist,the need for sealing of test holes and sampling holes must be considered. Thisshould preferably be established very early in the investigation process bymeasuring the stabilised pore pressures in pore pressure dissipation tests in deeperand more permeable layers during the initial CPT tests.

Determination of shear strength

The drained shear strength properties in silt in terms of a friction angle can beestimated from CPT tests, provided that the test has been performed under drainedconditions. This can be controlled by the pore pressure readings which must thenregister no or only very small generated excess pore pressures. When significantexcess pore pressures are registered, the Senneset and Janbu (1984) method can beapplied. However, it should then be considered that the friction angle thusevaluated primarily relates to the friction angle at constant volume, which mayoften be used in evaluation of the stability of natural slopes but which is too high


for evaluation of bearing capacity of foundations on loose silt where the soil iscompressed. In the latter case, it is better to use empirical values based on soil typeand relative stiffness as presented by Bergdahl et al (1993) and the SwedishNational Road Administration (1994).

The undrained shear strength, when applicable, is best determined by triaxial testsin the laboratory. Field vane tests may also be used but the risk of significantdisturbance is great, particularly in varved and layered soils, and the tests may notbe relevant for undrained conditions. The relevance of undrained tests and the testmethod in the particular soil can be estimated by dissipation tests during the CPTsoundings.

Determination of compressibility

The compressibility of silt may be estimated from CPT tests, provided that checkshave been made to ensure that the tests have been drained, and in stiffer silt fromdynamic probing tests using the empirical relations presented by Bergdahl et al(1993) and the Swedish National Road Administration (1994). For CPT tests themodulus can be estimated from E = 4.3qT

0.93 and for dynamic probing tests therelation may be written E 2.8N20(HfA)

0.91. The modulus estimated in this way tendsto become too low at shallow depths in crusts and stiff soils.

A more reliable estimate of the compression modulus is obtained by dilatometertests. The dilatometer can be used in a large number of soil types and, with a specialprocedure described in the report, it is also possible to sort out and correct valuesin layers where the penetration causes excessive disturbance, such as varvedclayey silt and alternating thin layers of silt and clay. This reduces the need forsupplementary undisturbed sampling and oedometer tests to layers of normallyconsolidated or only slightly overconslidated clay. Similar to the CPT test, the soilclassification from the dilatometer tests requires the pore pressures and possiblematrix suction to be measured and accounted for in order to yield good results.When this requirement is fulfilled, the classification proposed in SGI InformationNo. 10 (Larsson 1990), appears to function well.

Pressuremeter tests may also be used to determine the compressibility of the soil.However, it is difficult to create a test cavity of good quality at levels below thefree ground water level by predrilling a hole. Above the ground water level, theresults appear to be heavily dependent on the matrix suction and their relevance hasto be examined.

SGI Report No 5414

The initial shear modulus G0, also called the dynamic shear modulus, can bedetermined by seismic cone tests or by the empirical relations presented by Hardin(1978).

Sampling and laboratory tests

It is possible to obtain relatively undisturbed samples in silt by Swedish standardpiston sampling. The samples then retain enough of their stucture to be tested forundrained shear strength in triaxial tests and for compressibility in oedometer tests.The latter should preferably be performed as CRS tests. However, the samplingoperations and the amount of oedometer testing required to obtain a good pictureof the compressibility in the soil profile are so extensive that for rational reasonsthe oedometer tests should be restricted to such layers where the results of the insitu tests are not relevant, i.e. mainly layers of normally consolidated or onlyslightly overconslidated clay.

Standard piston sampling is also preferred when the samples are taken forverification of the soil classification and there is less demand for high qualitysamples. In Sweden, the soil in silt profiles is often more or less varved and layeredand this and the related geotechnical implications are often largely missed outwhen coarser sampling methods are employed, in which the soil is more or lessremoulded.

Both the classification and the performance of classification tests in this oftenvarved and layered soil require certain consideration. It should also be observedthat in silt, the Swedish classification based on grain size distribution may differconsiderably from other classification systems which are based on results fromconsistency limit tests.

Calculation of settlements

Calculation of settlements can be made by the use of theory of elasticity andmoduli. The results of the present investigations indicate that the distribution ofsettlements with depth agrees fairly well with the distribution calculated in thisway. The only notable exception is that the measurements indicate that nosignificant settlements occur below a relative depth of 2b - 2.5b under a squareplate with width b, while the theory of elasticity indicates that some smallsettlements will occur below this level. The moduli are best determined bydilatometer tests, possibly supplemented by oedometer tests in more clayey layers.Moduli estimated from CPT tests and dynamic probing tests may also be used withcertain reservations. The settlements can also be calculated from results of


pressuremeter tests using the Menard method. All of these settlement calculationsnormally refer to 10-year settlements.

The settlements calculated in this way in principle yield a straight line relationbetween load and settlement. However, in reality the load-settlement relation iscontinuously curved with a continuously decreasing modulus with increasingload. The calculated settlements therefore mainly correspond to the real settle-ments at a certain relative settlement which was found to be about 0.014b. Forsmaller relative settlements, the aforementioned methods yield too large calculat-ed values and at larger settlements the calculated values become too small. This canbe rectified by using the proposed method which takes the curved load-settlementrelation into account.

Another way of calculating the continuously curved relation between load andsettlement is to use the method proposed by Briaud (1995) for results frompressuremeter tests. The method has been used for the results in the presentinvestigation with good results but a modified -factor for tranferring pressurem-eter pressure to footing pressure has to be applied, (Larsson 1997), and someuncertainty regarding the time function remains. Care must also be taken when amatrix suction plays a significant role for the test results and the bearing capacity.

The DeBeer and Schmertmann methods often employed for calculation of settle-ments in sand yield very conservative results if they are applied in silt, i.e. thecalculated setttlements become much too large.

Calculation of bearing capacity

The ultimate bearing capacity of foundations on silt at failure can be calculatedby the general bearing capacity equation. In these calculations, the effect of amatrix suction has a great importance. However, if it cannot be ascertained that amatrix suction will prevail throughout the lifetime of the construction, these effectsshould rather not be taken into account.

The ultimate bearing capacity can also be calculated from results from pressurem-eter tests but it should be observed that in case the pressuremeter tests wereconducted at levels where a matrix suction was prevailing at the time for the test,the results only apply for conditions with a similar matrix suction.

The bearing capacity at a given limiting settlement can be calculated using theproposed method of calculating settlements with stress distribution according to

SGI Report No 5416

theory of elasticity and moduli taking the curved load-settlement relation intoaccount. It can also be calculated from pressuremeter test results using the methodproposed by Briaud (1995) with the modifications proposed by Larsson (1997).However, also in this case the restrictions related to matrix suctions apply,particularly if a higher bearing capacity is utilized than that calculated by thegeneral bearing capacity equation without regard to the matrix suction.


Chapter 1.

Introduction

Purpose and background of the investigation

Silty soils occur frequently in Sweden and entail a considerable number of specialgeotechnical problems concerning field investigations, sampling and laboratorytesting, frost susceptibility, ground water conditions, excavations, handling andcompaction of soil masses, bearing capacity and settlements, among other things.In this project, questions regarding field investigations, sampling and laboratorytesting, ground water conditions, bearing capacity of shallow foundations andsettlements in natural soils below footings and fills are addressed.

In spite of being a common type of soil, the number of special investigations in siltsis limited and the number of special investigation and calculation methods few.Instead, methods originally intended for clays or sands are normally used, incombination or individually, depending on what is considered most appropriate inthe particular case. Empirical methods for estimation of parameters and themethods elaborated for calculation of bearing capacity and settlements on the basisof results from sounding tests in sands are often cautiously modified with respectto silt content in the sand. However, in most cases the originators have neverclaimed that they should be applicable to pure silts or even clayey silts, and therelevance of the results is very uncertain.

In Sweden, earlier practice when investigating silt deposits has to a great extentconsisted of performing weight sounding tests and/or dynamic probing tests toestimate the denseness of the soil and taking disturbed samples to verify the soiltype. From the results, design friction angles and a modulus of elasticity have beenestimated on the basis of empirical experience. In fine and clayey silts, vane testsin the field and oedometer tests on undisturbed samples in the laboratory have oftenbeen performed as supplements. Other sounding methods such as total pressuresounding, in situ tests such as screw plate tests and Menard-type pressuremetertests, and laboratory tests such as direct simple shear tests have also been employedto some extent.

SGI Report No 5418

Furthermore, deposits of silty soils are often inhomogeneous, with alternatinglayers of coarser soils and clayey silt or clay, which often necessiates differentinvestigation methods for the different layers.

In recent years, new investigation methods, such as the piezocone tests (CPT test)and the dilatometer test, have appeared. In particular, the dilatometer has alreadyproved to be a useful tool for investigating the compressibility of silty soils and hasgained a certain acceptance in the current practice. A certain development of theolder methods has also taken place and some new interpretation methods andcalculation methods have been presented. However, many of these interpretationand calculation methods have, like the older methods, been intended mainly forclean sands.

The present investigation has therefore been aimed at investigating the useful-ness of the different investigation methods in silty soil deposits and determiningthe possible modifications to the existing interpretation methods that should beperformed. The aim has also been to investigate the present methods ofcalculating bearing capacity of shallow foundations and settlements in this typeof soil. The final goal has been to find a recommendation as to whichinvestigation method or combination of methods should be used in differenttypes of deposits, the special precautions that should be observed in connectionwith investigations in this type of soil, how different soil properties should beevaluated and how bearing capacity and settlements for shallow foundationsshould be calculated.

All references to evaluated properties, soil classifications etc. from field andlaboratory tests in this report refer to established methods used in Swedish practiceunless otherwise stated.

The test and interpretation methods commonly used in Sweden are described in thefollowing publications:


Scope of the investigation

The project was started with a thorough review of the existing geotechnicalliterature on silty soils. The contents were synthesised and reported in SGI ReportNo. 49 Silt - Geotechnical Properties and their Determination, (Larsson 1995).

The following part of the project reported here should comprise extensiveinvestigations with all available and possibly relevant methods and large scaleloading tests in three locations covering a wide range of conditions in silty soils.

SGI Report No 5420

A series of investigations was then started in order to find suitable locations for testfields where different in situ tests as well as large scale loading tests could beperformed and compared. An inventory of available older investigations was madeand a number of possible locations were selected. One of these locations, with veryheterogeneous silty soils, is located in Linkping close to the Institute and full scaleloading tests had already been performed in connection with preloading of the soilbefore the construction of buildings. In this area, only supplementary investiga-tions with new test methods were required.

The new investigations in the other locations proved to illustrate the lack ofprecision of the older investigation methods. Site after site was eliminated becauseunexpected soil types or layers that had previously been undetected or misinter-preted were encountered. Although the results have been very useful in evaluatingthe different investigation methods employed, the search for suitable test sites hadto be performed in another way.

The problem was solved with help from the Swedish Road Administration inBorlnge and the consulting firm KM-Geokonsult AB, both of which had accessto more detailed investigations in the Borlnge area where it was ascertained thatthe soil conditions were suitable. Two locations with very different conditionswere selected, one with an approximately 15 m thick deposit of loose fine tomedium silt with some layers of clayey silt and a free ground water level about 2m below the ground level, and one with a deposit of dense medium to coarse siltat least 10 m thick with a free ground water level about 18 m below the groundsurface. Quite homogeneous profiles with silt are rare and also the latter profilecontains thin layers of more fine-grained soil. Both deposits overlie coarser soilextending to great depths, but these layers do not affect the results from the loadingtests.

The first profile later proved to have artesian water pressure in the bottom layersand this and especially the free ground water level varied strongly with season andrainfall. This created considerable problems at excavation and installation of theplates for the load tests, which were to be placed at the normal foundation depthbelow the frost limit and the weathered zone. The pore pressures have thereforebeen recorded for long periods together with meteorological and hydrologicalobservations, and special precautions had to be taken to control the ground watersituation at installation of the plates. The results from the investigations and theloading tests in the second profile proved to be heavily dependent on the prevailingnegative pore pressures in the ground. The measurements of the pore pressureprofile at the investigations and during the loading tests at this site were later


supplemented by a special study of the possible effects of thawing and very heavyrain. In this study, the ground was soaked by filling water into a shallow excavationcovering a large area. The pore pressure profile was then monitored by frequentautomatic readings of installed pore pressure gauges for a period of time. Thisstudy was conducted by Anna-Lena berg at Chalmers University of Technologyas part of her studies of the effect of negative pore pressures on stability conditionsin silty soils.

The large scale loading tests in Borlnge were conducted as two series of plateloading tests. At each location, three load tests were performed on square plateswith dimensions 0.5 x 0.5, 1.0 x 1.0 and 2.0 x 2.0 m.

The results of the tests have then been compared to bearing capacities andsettlements calculated with the currently available interpretation and calculationmethods. As stated before, many of these methods were not originally intended foruse in silts and the purpose was then to examine the possibility of extending theiruse also to this type of soil.

SGI Report No 5422

Chapter 2.

Investigations

Investigations have mainly been performed at five sites, Fig. 2.1. Two of thesewere discarded after the preliminary supplementary investigations, which wereaimed at confirming the suitability of the sites for comparative studies of full scaleloading tests and predictions based on ordinary geotechnical investigations. Twoother sites were also considered, based on results from available investigations, butwere discarded after a visual inspection on site. In one case, this was due to a veryuneven terrain combined with large trees and boulders on the surface, which wouldhave entailed a considerable cost to remove and excavate. In the other case, theintended test site was considered to be too close to a residential area. Extensiveprograms of investigations and load tests have been carried out in three main testlocations.

Kil

The preliminary supplementary investigations consisted of CPT tests and thetaking of undisturbed samples with a Swedish standard piston sampler in orderto verify the stratigraphy and the estimated type of soil in the various layers. In thefirst profile in Kil, which was found in connection to planning of a new road, theavailable investigations, consisting mainly of weight sounding tests and disturbedsampling, had indicated a fairly homogeneous deposit of dense to very dense silt.Supplementary dynamic probing tests had shown that the thickness of the depositwas about 50 m. The silt had been judged to be somewhat clayey close to theground surface and to contain alternating layers of coarse and medium silt furtherdown. However, the supplementary investigations showed that the upper 6.5 m inthe profile consisted of medium stiff partly silty clay. Below this level, alternatinglayers of medium dense sand and medium stiff clay were found down to 10 mdepth, where dense sand was encountered. The free ground water level was locatedat 12 m depth below the ground level and the influence of the resulting negativepore pressures in the upper part of the profile appears to be the main reason for theoriginal misinterpretation.


Fig. 2.1. Location of test sites.

! MJRDEVI

" KIL

# BRANS $ VGVERKET% VATTHAMMAR

SGI Report No 5424

Brans

In the second profile in Brans, which was close to a bridge over a river foundedon piles in silt, research had previously been carried out concerning the growth inbearing capacity of driven friction piles with time, (stedt et al. 1992), and it wasascertained that the profile contained silt to great depths. Results from varioussounding tests and dilatometer tests as well as certain laboratory triaxial tests werealso available. However, the supplementary investigations showed that the upper4 m of the profile consisted of sand which had not been pointed out and wasprobably considered insignificant in the previous investigations. The free groundwater level was regulated by the river and was about 1.5 m below the groundsurface. It would thus have been very difficult to perform plate loading tests at thesite in such a way that the results reflected the properties of the underlying silt.

Mjrdevi

At the third location, Mjrdevi, which is close to the Institute in Linkping,investigations have been performed at various times for a number of years inconnection with development of the area. The first investigations consisted ofstatic total pressure sounding, dynamic probing tests and disturbed sampling. Theresults showed very heterogeneous profiles with mainly silty soils containingalternating layers of silt and stiff clay and with infusions of lenses and coarserobjects. The infusions of coarser objects increased with depth and rotation oftenhad to be applied to advance the static total pressure sounding. In spite of this, thepenetration depth of the static total pressure soundings was often limited andnormally varied between 5 and 15 m. The dynamic probing tests were in most casesstopped above 20 m depth even if they could be advanced further since a numberof initial tests to greater depths had resulted in breakage and loss of the soundingrods.

An attempt was made to estimate the compressibility of the soil by Menard typepressuremeter tests. However, the ground water level was about 1 m below theground surface and the difficulties in obtaining proper pre-drilled holes proved tobe very great. The results were therefore considered unreliable. Because of this andthe heterogeneity of the soil in the area, which entailed a great risk for unevensettlements, it was decided to preload the ground where the buildings were to beconstructed. This was done by means of earth fills covering the whole area of theprojected buildings and high enough to provide a load that corresponded to that ofthe future buildings plus a surcharge. The fills were instrumented with settlementgauges and horizontal settlement hoses. The settlements were followed duringload application and for some time afterwards until the fills were removed and the


buildings constructed. The method proved to be very successful and has since thenbeen utilised for most of the area, even if the instrumentation is usually omitted.

The wisdom of this approach was later illustrated in a case where higher buildingswere constructed in the area and the foundations had to be made on piles. The siteinvestigation demanded dynamic probing tests to estimate the piling depth andresulted in heavy losses of drilling equipment. In the following piling operationwith precast concrete piles, a large number of the driven piles were lost.

In connection with the introduction of the dilatometer test in Sweden, a new seriesof investigations was performed in the area. It then proved possible to penetrateand test to approximately the same depths as for the static total pressure sounding,and it was also possible to use the results to select levels at which fairly undisturbedsamples could be taken with the standard piston sampler. The tests and measure-ments at Mjrdevi have been used in practical design and gathered in SGI files, buthave not been published before.

In connection with the present project, a series of CPT tests and weight soundingtests has been performed. Also these tests reached about the same levels as theprevious static total pressure soundings and the dilatometer tests.

Vgverket

The fourth test site is located just outside the head office of the Swedish NationalRoad Administration (Vgverket) in Borlnge between the buildings and a nearbyravine with a brook. The location of the test site originates from the investigationsat the construction of the buildings in the late 70s. Further investigations havebeen performed close to the present test site in connection to a research projectconcerning stress conditions and movements in and close to natural slopes,(Andersson et al. 1991). The previous investigations mainly consisted of weightsounding tests, dilatometer tests, pore pressure observations and undisturbedsampling. These investigations had shown that the soil in the profile consisted ofloose silt with a more significant layer of clayey silt/silty clay at about 5 m depthand some less pronounced clayey layers between 8 and 15 m below the groundsurface. Below about 15 m depth, there is a thick sand layer which is estimated toextend to at least 40 m below the ground surface. The pore pressures had beenobserved on 6 occasions during about half a year from December to July. Thesituation had then been found to be fairly stable with a free ground water levelabout 2.3 m below the ground surface and gradually increasing artesian waterpressures below this level down to the sand layer 15 m below the ground surface,

SGI Report No 5426

where the water pressure corresponded to a hydrostatic water pressure from theground surface. The most significant deviation had occurred in April, which wouldcoincide with the snow melting and thawing period in the spring, when the freeground water level was about 1 m higher. The corresponding pore pressureincrease further down in the profile decreased gradually to only about 2 kPa in thebottom sand layer. The results from the dilatometer tests indicated that the soil isslightly overconsolidated at the top and becomes normally consolidated withdepth.

The investigations in the current project comprised CPT tests at four points,additional dilatometer tests at two points, field vane tests at two points, Menardtype pressuremeter tests in two pre-drilled holes (plus a few tests in an additionalhole), one seismic CPT sounding, pore pressure dissipation tests in one CPT test,pore pressure observations at different levels at a number of points and undis-turbed piston sampling at two points down to 10 m depth. Dynamic probing wasnot considered to be relevant in this soft soil.

The samples have been investigated in the laboratory concerning classification,bulk density, water content, liquid limit, plastic limit and grain size distribution.The compressibility has been investigated in oedometer tests, both incrementallyloaded tests and automatic constant rate of strain tests. The shear strengthparameters have been tested in triaxial tests at SGI and also at the NorwegianGeotechnical Institute in connection with another research project concerning thedynamic behaviour of silt, using samples taken in the current investigation atVgverket.

Certain additional investigations and measurements, mainly concerning the var-iability of the ground water conditions, later became necessary because ofproblems encountered at the installation of the plates for the load tests.

Vatthammar

The last site, Vatthammar, is located at Stora Tuna about 5 km south-east ofBorlnge. This site had been investigated in connection with a proposed newrailway crossing for a local road. These previous investigations comprised weightsounding tests, CPT tests and disturbed sampling. They indicated that the soilconsisted of very stiff silt down to at least 10 m, that the soil profile was thickerthan 15 m and that the free ground water level was located below 11 m depth.


The new investigations comprised three CPT tests, one seismic CPT test, twodilatometer tests, one borehole with Menard-type pressuremeter tests, one dynam-ic probing test, pore pressure measurements with the CPT-equipment in the deepersoil layers and measurement of negative pore pressures by BAT-piezometers at anumber of levels in the upper part of the soil profile. Undisturbed samples weretaken in two holes down to 10 m depth and were investigated in the laboratoryconcerning classification, bulk density, water content, liquid limit, plastic limitand grain size distribution. The compressibility has been investigated in oedom-eter tests, both incrementally loaded tests and automatic constant rate of straintests. The shear strength parameters have been tested in triaxial tests at SGI.

After the following plate load tests, a special investigation was performed byAnna-Lena berg at Chalmers University of Technology concerning the possiblevariations in the pore pressure profile at soaking of the top of the profile. In theseinvestigations, the pore water pressures in the upper part were measured bycontinuously monitored BAT piezometers. Supplementary to these measure-ments, the water retention curves of the soil in the profile were determined in thelaboratory at Chalmers University of Technology and the capillarity of the soil wasmeasured in a new type of capillarity meter developed at SGI. Tests with the latterequipment were also performed on the soil in the first profile in Kil.

The two test fields in the Borlnge area cover a large part of the range of conditionsin natural fairly homogeneous silt deposits in Sweden, from a loose, only slightlyoverconsolidated deposit of partly clayey silt with a high free ground water leveland artesian pore water pressures to a medium stiff silt with a deep free groundwater level and negative pore water pressures in a large part of the profile. Thereare deposits with coarser silts, but these are closer to sand and may be expected tobehave in a similar way. There are also stiffer silts, mainly in the form of siltmoraine, but these are outside the scope of the present project which has beenrestricted to sedimentary silt deposits.

It would have been desirable to perform more types of tests, particularly in the lasttwo test fields, but this was not possible within the present project. One such testis the screw plate test, which has previously been found to be useful in silts.However, the previous models of this equipment have been too weak andvulnerable, and no operational equipment was available. Development of a new,more robust type of equipment is reported to be under way at the NorwegianInstitute of Technology in Trondheim, but this was not ready in time for the currentproject. Another type of equipment, which it would have been desirable to test, is

SGI Report No 5428

the self-boring pressuremeter. At present, there is only one operational unit of thistype in Sweden and unfortunately it was not possible to have it brought to Borlngeat the time for the investigations. A further type of pressuremeter, the TEXAM type,also allows more elaborate measurements and interpretations to be made. Howev-er, this type of pressuremeter was not introduced in Sweden before the investiga-tions in the present project were finished.


The plate loading tests were performed in order to check the applicability ofdifferent methods of estimating bearing capacity and settlements for shallowfoundations in silt. The tests were performed in series of tests on square plates withdimensions selected in such a way that ultimate bearing capacity failure wasexpected to be reached for at least the smallest plates and with such variations thatthe effect of the stresses reaching down to various depths could be studied. Thelargest plates also had dimensions similar to an ordinary foundation.

Foundations in Sweden are made so deep that they remain unaffected by frostaction, which normally means a foundation level somewhere between 1.1 and2.5 m below the ground surface, unless special precautions are taken. In the plateloading tests, it was also desired to lay the plates on top of fairly homogeneous soilbelow the desiccated and stiffer crust in order to facilitate the interpretation of theresults. In ordinary foundations, a part or all of the excavation for the foundationis often back-filled after the foundation work has been completed.

In the current case, load tests with basically the same equipment should beperformed at two sites with very different soil conditions. Much of the loadingequipment is standard equipment used for this kind of tests, which sets limits forpossible dimensions and maximum loads. The loading equipment consisted of asystem of loading beams, ground anchors and hydraulic jacks and pumps. Themain beams consisted of two 17 m long steel profiles, which could be boltedtogether. Such beams are kept in depots spread over the country by the SwedishNational Rail Administration to be at hand in case they should be required fortemporary emergency repairs of the railway lines and can often be made availablefor short term loading tests. The ends of these beams are placed on a support ofwooden rafts of the type normally used to support excavators on soft ground. In thiscase, they are used to provide a firm and level base and the number of rafts at thetwo ends can also be adjusted to place the beams in a horizontal position. Therequired reaction force can be provided by deadweights or by ground anchors. In

Chapter 3.

Principle of the plate loading tests

SGI Report No 5430

this case, it was considered unsuitable to use deadweights, especially at the sitewith the soft soil, and a system of four ground anchors was installed at each site.The ground anchors consisted of Swellex type expander bodies which werelowered in pre-drilled holes down to firm soil strata and then expanded by pumpingin cement grout under high pressure. The pre-drilled holes at the site with the highground water level were supported by a bentonite suspension. The ground anchorswere placed in pairs, one on each side of the pair of beams, and fitted with tie rodsextending well above the beams. Shorter beams were placed across the ends of thelong beams and these were tied down with a pre-stress in the tie rods in order tofix the system.

The dimensions of the reaction system made it possible to install three platesmeasuring 0.5 x 0.5 m, 1 x 1 m and 2 x 2 m in a row along the beam withoutsignificant interference in terms of the same soil mass being affected by thedifferent load tests. The distances between the plates and the loading sequencewere also adjusted in such a way that the influence from a preceding load test onthe results from the following load tests was minimised. In this way, the smallestplate was placed at one end of the row, the largest plate was placed about in middlebut somewhat closer to the smallest plate and the intermediate plate was placed atthe other end. The plates were then tested in a sequence with the smallest plate firstand the largest plate last. The same plate dimensions were used at both sites.However, since the preliminary calculations showed that it would be uncertain iffailure could be reached even for the smallest plate at the site with the stiffer soil,most of the back-filling had to be omitted at this site.

The load on the plates was provided by hydraulic pumps and jacks. According toSwedish practice, the load in tests on friction soils is normally applied in steps witha duration long enough to enable a study of the creep rate of the deformations(Bergdahl et al. 1993). For cohesive soils, a longer duration is required to allow forfull dissipation of excess pore pressures and related consolidation if the drainedproperties are to be studied. A minimum of ten steps is normally used to enableevaluation of both load-settlement curves and failure loads. In tests on friction soilsand in undrained tests on clays, a duration for each load step of about 8 minutes isnormally considered sufficient. In the present series of tests, the tests at the sitewith the stiffer and coarser silt and the very deep ground water level wereperformed in steps with 16 minutes duration. At the site with the softer and moreclayey silt and with a high ground water level, preliminary calculations showedthat longer duration was required in order to ensure full pore pressure dissipation.The durations of the load steps in these tests were therefore selected to be 2, 3 and


5 hours respectively for the three plates and the pore pressure in the soil below theplate was measured during the tests in order to verify that full excess pore pressuredissipation was achieved.

Different systems for application of both constant static loads, cyclic loads anddynamic loads by hydraulic jacks have been developed at the Institute. Because ofinterference with other ongoing load test projects, two different systems wereused, one manually regulated for the short duration tests and one electronicallyoperated in the more long term tests. The load was measured and regulated bymeans of an electronic load cell.

SGI Report No 5432

4.1 MJRDEVI, LINKPING

Test field

The test field is located in western Linkping, just across the road from the SwedishGeotechnical Institute and Linkping University. The area has been developed inthe late 80s and the 90s in connection with the construction of Mjrdevi SciencePark. The soil profile is dominated by silty soils but is very heterogeneous withalternating layers of silt and clay and many infusions of clay lenses and coarserobjects, the latter increasing with depth. The weathered crust in the upper part ofthe profile is about 2 m thick and this and a somewhat softer layer about 1 m thicklying just below consist of more clayey soil. The ground water level is locatedabout 1 m below the ground surface and may vary with season from the groundsurface to 1.8 m below. The pore water pressure is approximately hydrostatic fromthe ground water level. The silty soil profile is estimated to be 16 to 17 m thick andis followed by coarser soil. Only the upper parts of the profile down to about 20 mdepth have been investigated, the depth of the various investigations depending onthe ability of the various methods to penetrate this type of soil.

First investigation

The soil conditions at the test site, which is relatively large and comprises half ablock with five buildings, Fig. 4.1.1, were first investigated in 1988 beforeconstruction of the buildings, (Ottoson and Bergdahl 1988). The first investiga-tions were aimed at determining the type of soil, its stiffness and the possibilitiesfor foundation on spread footings, and also to investigate the depth of penetrationin dynamic probing and thereby the necessary length of driven piles if these wererequired. The investigations were performed with a local non-standard type ofstatic total pressure sounding, dynamic probing according to the Swedish HfAmethod, pore pressure measurements at two levels and disturbed sampling byscrew auger. The non-standard static total pressure sounding method basicallyuses the same principle as the ordinary Swedish static total pressure sounding

Chapter 4.

Investigations and load testsat the three main test locations


method, i. e. 22 mm rods, a total pushing force of 10 kN and rotation of the rodswith the maximum force applied when this force alone is insufficient to penetratethe soil. The difference is that the ordinary square tip with 1000 mm2 cross sectionand a slip coupling, which enables separation of tip resistance and rod friction, isreplaced by a twisted 25 mm screw-shaped tip normally used in the weightsounding test. The Swedish HfA dynamic probing test uses a 63.5 kg hammer witha free fall of 0.5 m and a 45 mm conical tip with apex angle 90o and a mantlelength of 90 mm. The penetration resistance is registered as the number of blowsrequired for every 0.2 m penetration. The rod friction is estimated by measuringthe torque required to rotate the rods and the net penetration resistance is thencalculated. From experience, this method yields approximately the same results astheSPT-test, N20 HfA, Net N30 SPT, (Bergdahl and Ottosson 1988).

Fig. 4.1.1 Plan of buildings in the test area.

SGI Report No 5434

According to the results from these penetration tests, the silty soil generallyappears to be very loose down to 3.5 m depth. It then becomes varyingly loose tomedium stiff down to about 10 m depth and then mainly loose at further depthsdown to stop in penetration. The variation in the results was considerable,particularly below 3.5 m depth, Figs. 4.1.2 - 3. Penetration stop was obtained atwidely varying levels. The static total pressure soundings stopped at depthsvarying from 5 to 20 m, in all cases but one at less than 15 m, and the cause wasgenerally judged to be a large object such as a cobble or boulder. The dynamicprobing tests generally penetrated somewhat further, but initial attempts topenetrate past what were believed to be large objects down to a firm bottom soon

Fig. 4.1.2 Variation in results from static total pressure soundings at Mjrdevi.

Investigations and load tets in silty soils35 Fig. 4.1.3 Results of dynamic probing tests at Mjrdevi.

SGI Report No 5436

resulted in breakage of the rods and loss of the tips together with a certain numberof rods. In all cases, the encountering of large objects was judged to be the reasonfor the stop in penetration.

The classification of the disturbed samples showed that the soil in the upper 7.5 mconsists mainly of silt but that it contains numerous thinner layers of clay. The drycrust on top, particularly its upper part, consists of clay. Also the softest layer justbelow the crust contained so much clay that it appeared possible to take relativelyundisturbed samples with the Swedish standard piston sampler. This was attempt-ed in three boreholes where samples were taken at 2.5 m depth and in two holesattempts were also made to take undisturbed samples at deeper levels where moreclayey soil had been found.

The latter samples provided more precise information on the stratification of thesoil and showed that also the soil in the more clayey parts of the profile below thedry crust consists of varved/layered partly clayey silt and alternating varves/layers of clay and silt. The clay also appears as small lenses in the silt. The unitweight of the soil is about 1.95 t/m3 and the natural water content varies between20 and 42 % depending on the clay content. A number of specimens were preparedfor oedometer tests on the most clayey parts of the samples and constant rate ofstrain oedometer tests and incrementally loaded tests were performed, both typesin accordance with the relevant Swedish standard. Indications of a preconsolida-tion pressure could be observed only from the results of two of the constant rateof strain tests. Both specimens were from 2.5 m depth and the indicated precon-solidation pressures were 110 and 204 kPa respectively. The preconsolidationpressure can vary rapidly with depth just below the dry crust and the penetrationtest results indicate that the minimum strength value may be found about half ametre further down. Nevertheless, the results from the oedometer tests indicate acertain overconsolidation in the soil. The evaluated moduli from the oedometertests show that the minimum value of the oedometer modulus would be around2.7 MPa.

Pressuremeter tests

An attempt was then made to obtain better values of the compressibility of the soil,which would also be more representative for a larger volume of the heterogeneousmass, by using pressuremeter tests. The pressuremeter tests were performed withMenard type equipment and the pre-drilled holes were made by use of a so-calledbentonite screw. The bentonite screw is a screw auger with a hollow stem andhollow rods through which a bentonite suspension is pumped down when the screw


is withdrawn. This technique had previously been found to work very well inpreparing pre-drilled holes for pressuremeter tests in sand, (Bergdahl et al. 1984).At Mjrdevi, however, pre-drilling proved to be very difficult because of theheterogeneity of the soil and the embedded coarse objects. The results of thefollowing pressuremeter tests also indicated that the soil had been heavilydisturbed. The measured volume-pressure-creep curves were erratic. In manytests, the limit pressures and yield pressures could not be evaluated because theinitial size of the cavity in the pre-drilled hole was too large in relation to thepossible expansion of the pressuremeter probe and the evaluated moduli were verylow. Several attempts were made to achieve good pre-drilled holes and tests wereperformed at four different points. A total of twelve tests were carried out but,according to the criteria for the relations between the pressuremeter modulus andthe net limit pressure presented by Baguelin et al. (1978), the soil in all tests butone at a shallow depth in the lower part of the crust was to be considered asremoulded.

Raft foundation with pre-loading

The results from these investigations showed that, even if the softness of the soilestimated from the results of the penetration tests and the very low measuredmoduli in the pressuremeter tests could both be considered exaggerated, afoundation on spread footings would require very large dimensions of the footingsand still involve possible problems because of the risk of relatively large andparticularly uneven settlements. The results of the investigations also showed thatthere would be a considerable risk of breaking and loosing driven pre cast pilesbecause of the embedded coarse objects in the soil. Furthermore, it was anticipatedthat a considerable amount of pile testing and re-driving would be required becauseof the risk of false stops in this silty soil. It was therefore considered moreprudent to use raft foundations for the buildings in the area. Also in this case, therewas a risk of uneven settlements and a scheme for pre-loading was designed.

The largest buildings were to be 4-storey buildings with a ground plan in the formof an H. The loads from the buildings were concentrated to the outer walls and toa row of pillars along the centre line of the connecting central part of the building.The contact pressure on the ground in these parts was calculated to be 50 kPa andthe pressure in the areas inside was estimated to be 25 kPa. The pre-loading wasintended to correspond to this load plus a possible ground water lowering of 1 mand a certain overload. The pre-loading consisted of earth fills with heights andshapes modulated to closely model this loading situation, Fig. 4.1.4. Also the otherbuildings with other shapes and heights were pre-loaded in a similar fashion.

SGI Report No 5438

Fig. 4.1.4 Layout of pre-loading fill for the H-shaped 4-storey buildingsand photo of a fill under construction.


The fills were constructed in a sequence in such a way that the site for one buildingwas first pre-loaded and the load was allowed to act until the primary settlementprocess had been finished, whereupon the masses in the fill were moved to the nextsite and so on. Before the fills were put in place, a number of settlement gauges andhorizontal settlement hoses were placed on the ground. The settlements of thesegauges and hoses were recorded throughout the period of pre-loading.

The settlements beneath the fills were roughly estimated to become about 0.1 m.It was difficult to predict the time for consolidation more accurately but this wasestimated to be a couple of months. The fills were to be rather high , about 5 m, andin order to assure that no stability problems would occur, piezometers wereinstalled below the fills and the filling operations were to be halted in caseexcessive pore pressures developed.

In the actual pre-loadings, in which the filling up was performed in about 10 days,most of the settlements occurred during the time for load application and thesettlements had evened out to consist only of long term creep settlements afterabout 20 days. No significant excess pore pressures developed during the construc-tion of the fills. In most cases, the piezometers showed maximum increasesbetween 0 and 5 kPa. In one case, an increase in pore pressure of 20 kPa wasrecorded, but this may be assumed to have been very local in a more clayey portionof the soil mass or a measuring error.

The fills were moved about 30 days after their construction was started. Thesettlements in the points located beneath the highest portions of the fills at this timeranged from 31 to 74 mm and were randomly distributed. The same patternappeared for all the fills and in four similar pre-loaded areas in the test field theaverage settlements ranged from 40 to 53, mm with an average of the total of 47mm. These values refer to settlements after 1 month and they may be extrapolatedto correspond to about 55 to 75 mm and 66 mm respectively after 10 years by usingthe Schmertmann (1970) time factor.

After the pre-loading, the buildings were constructed, Fig. 4.1.5. Only very smallsettlements occurred during construction and no settlement problems have beenreported afterwards.

The method of raft foundation with pre-loading has been adopted for most of thesurrounding area, both at Mjrdevi Science Park and in the adjacent universityarea, and is now used on a more routine basis. The same fill material is being movedabout in the area and placed well in advance on sites for planned buildings. This

SGI Report No 5440

pre-loading is normally performed without special settlement observations or porepressure measurements. At the same time, problems with loss of piles have beenreported whenever driving of pre-cast piles has been attempted. In one such case,more than 50 m of drilling rods and a large number of probe tips were lost in thedynamic probing to predetermine the depth to end bearing strata. In the followingpiling operation, also a large number of the piles were damaged.

Dilatometer tests

Shortly after the first series of investigations, and when the pre-loading operationswere already in progress, the Institute acquired its first flat dilatometer. Theequipment was tested in a number of test fields with different soil conditions,among them the site at Mjrdevi. Being the only available equipment, thedilatometer was handled very carefully and was pushed down with a hand operateddrill rig until it encountered hard resistance against penetration. In spite of this, thedilatometer was found to penetrate to about the same depths as the previous staticsoundings, i.e. until it hit a large enough object embedded in the soil. No attemptswere made to force it past such objects and tests were only made at a few pointsin order to gain experience of how the equipment worked in this type of soil.

The results at first appeared to be very erratic and the soil classification based onexisting charts in general yielded coarser material than had been established from

Fig 4.1.5 Building under construction.


the sampling operations, particularly in the upper levels of the profile. In the lightof what expired from the gathered results from the investigations in all the testfields, a new classification chart was developed based on a material indexcorrected for overconsolidation ratio, (Larsson 1990). Using this chart, the soilclassification for most of the profile agreed more closely with the actual soilconditions. However, at several levels the material index became very low andactually fell below the lower limits for the clay region in previous classificationcharts. The gathered experience shows that this is typical for the types of silty claysor clays with silt layers which become severely remoulded in all types of soundingsand at insertion of in situ test equipment, and in which it is often very difficult toobtain undisturbed samples. Very low values of the material index may also befound in organic soils, but in this particular case this possibility could be ruled outdirectly based on geological considerations, Figs. 4.1.6 - 7.

Because of the disturbance, very low values of most other parameters, such asundrained shear strength, overconsolidation ratio and particularly compressionmodulus, were evaluated in the zones where the soil could be assumed to have beenmore or less remoulded at the start of the test. However, since these zones are easilyidentified, it is possible to make a better estimate using the overall picture from theless affected zones, together with empirical relations. The measurements in allzones not obviously disturbed indicated a certain overconsolidation with anestimated overconsolidation ratio of 1.5 or higher. In overconsolidated cohesivesoils, the modulus in the overconsolidated stress range is often estimated as a directfunction of the undrained shear strength. From the dilatometer tests, a value of theundrained shear strength is estimated and this value has been found to be lessaffected by disturbance at insertion of the dilatometer than the other parameters.Rules for how the moduli in overconsolidated clayey soils can be estimated fromdilatometer tests in this way were presented by Larsson (1990). When these ruleswere applied to the results from Mjrdevi, they were found to yield approximatelythe same moduli as the ordinary interpretation in undisturbed layers and highervalues in the disturbed zones, Fig. 4.1.8. A check against the results from theoedometer tests also indicated that the empirically estimated values were in theright range. The procedure for evaluating the dilatometer tests in this and otherdifficult soil profiles is described in further detail in Chapter 6.

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Fig. 4.1.6 a. Results from a dilatometer test at Mjrdevi.

Investigations and load tets in silty soils43

Fig. 4.1.6 b. Results from a dilatometer test at Mjrdevi.

SGI Report No 5444

Probably remoulded soil Clay Silt Sand

Fig. 4.1.7 Identification of probably remoulded zones.

0

2

4

6

8

10

12

14

0.01 0.1 1 10

Corrected material index ID

Dep

th, m


Fig. 4.1.8 Estimated moduli from dilatometer tests at Mjrdevi.

0

2

4

6

8

10

12

0 10 20 30 40

Compression modulus, MPa

Dep

th, m

CPT tests and weight sounding tests.

Recently, in connection with the current project, supplementary CPT tests andweight sounding tests have been performed. The weight sounding tests wereperformed at two points and indicated a heterogeneous soil profile with a weakerlayer between 2 and 4 m depth. The ordinary tests both stopped at 8-9 m depth. Oneof the soundings could be advanced further after using blows to pass large objectsat this level and then again at 12 m depth until it finally had to be stopped at 14 mdepth, Fig. 4.1.9.

The CPT tests were also performed at two points and the cone penetrated down to9 and 11 m respectively until the tests had to be terminated, Fig. 4.1.10. For bothweight sounding tests and for CPT tests, the ability to penetrate in this type of soilwas thus about the same as for the other types of soundings and push-in equipment,

SGI Report No 5446

Fig. 4.1.9 Results of weight sounding tests at Mjrdevi.


Fig. 4.1.10 a. Results of a CPT test at Mjrdevi

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Fig. 4.1.10 b. Results of a CPT test at Mjrdevi


and only dynamic probing showed a significantly better ability to penetrate. Theresults of the CPT tests indicated crust effects down to 2.5 m depth, clayey siltysoil down to 4.5 m, a coarser soil classified as sand/silt down to about 8 m depthand then again clayey silty soil down to the level where a large object wasencountered. A modulus of elasticity or a compression modulus is not estimatedfrom CPT tests with the CONRAD programme commonly used in Sweden,(Larsson et al. 1995), except for sands. However, there is an older Swedishempirical relation between cone resistance and modulus of elasticity, (Bergdahl etal. 1993, Swedish National Road Administration 1994), which can be expressedas E = 4.3qT

0.93. If this relation is applied, the estimated moduli become roughlyequal to those estimated from the dilatometer test, Fig. 4.1.11.

Fig. 4.1.11 Empirically estimated modulus ofelasticity from CPT tests at Mjrdevi.

0

2

4

6

8

10

12

0 10 20 30 40

Modulus, MPa

Dep

th, m

SGI Report No 5450

4.2 VGVERKET, BORLNGE

4.2.1 Test field

The test field is located about 70 to 80 m south-west of the buildings of the headoffice of the Swedish National Road Administration (Vgverket) in the city ofBorlnge. The test field is located on a relatively flat green area, but some 30 metresfurther to the south-west the ground slopes down to an erosion ravine created bya small brook. The depth of the ravine is about 6 metres. The area was selected onthe basis of previous investigations in connection with the construction of thebuildings in the late 70s and also more recent investigations performed closer tothe test field in connection with research concerning stresses in natural slopes(Andersson et al. 1991).

The soil profile consists of a dry crust which is approximately 1.5 metres thick. Thecrust and the underlying soil consist mainly of medium silt. Between 4 and 5 metresdepth, there is a layer of silty clay, followed by medium silt down to about 9 metresdepth, where more clayey layers are found. Silt and layers of silty clay thenalternate down to about 15 metres depth where coarser silt/fine sand is found.Below 20 metres depth, there is coarser sand, which is estimated to reach down toat least 40 metres below the ground surface. The silty soils in the profile areclassified as loose or very loose, Fig. 4.2.1.

In these previous investigations, the ground water situation had been found to beartesian, with a water pressure in the coarser soil below 15 metres depth roughlycorresponding to a hydrostatic head at the ground surface. The free ground waterlevel in the upper soil layers had been found normally to be located about2.3 metres below the ground level but had also occasionally been found to be 1metre higher, which had been attributed to the coinciding period of snow meltingand thawing of the ground. During the period for the first series of fieldinvestigations, which was carried out in late autumn 1994, the free ground waterlevel in the upper soil layers was found to be about 1.5 metres below the groundsurface, which was then attributed to a normally higher ground water table afterthe autumn rains. The ground water situation later proved to be much morecomplicated and variable.


Fig. 4.2.1 Soil profile in the test field at Vgverket (Andersson et al. 1991)

SGI Report No 5452

4.2.2 Soil investigations in the current project

CPT tests

Standard CPT testsThe previous soil investigations had included weight sounding tests down to25 metres depth, dilatometer tests down to 18 metres depth, undisturbed samplingto 14 metres depth and pore pressure observations at three levels for about sevenmonths, with intervals between the readings of one to two months. The results hadgiven a good picture of the stratigraphy and soil conditions at the site, and the firsttests in the new investigation were two CPT tests performed in order to verify thatthe soil conditions were uniform over the intended test area. This was readilyestablished. The CPT test penetrated down to 28 metres depth with an ordinarydrill rig and the results were very similar and in agreement with the previousresults, Fig. 4.2.2.

The results of the CPT tests clearly identify the clayey layer between 4 and 5 metresdepth, even if the generated pore pressures remain relatively low and indicate afairly silty and/or overconsolidated soil. The more fine-grained layers between9 and 15 metres depth are also clearly identified, as well as the coarser silt/fine sandbetween 15 and 20 metres depth where only small excess pore pressures aregenerated and the transition at 20 metres depth, after which no excess porepressures are measured. The results indicate that the soil is loose to very loosedown to about 15 metres depth, where it becomes medium dense.

Seismic tests and excess pore pressure dissipation testsFurther CPT tests were then performed, one as a seismic cone test in order tomeasure the initial shear modulus at small strain, G0, and one CPT test with stopsat every metre of penetration for measurement of the excess pore pressuredissipation with time in order to estimate the drainage conditions in the soil profile.

The results of the seismic cone tests showed initial shear moduli increasing fairlylinearly with depth. In view of the almost constant bulk density and thereby fairlyconstant void ratio of the soil, this is in line with what would be expected fromHardins (1978) empirical relation. A direct comparison shows that the measuredvalues are in the same range but in general somewhat less than the empirical values,Fig. 4.2.3.


Fig. 4.2.2 a. Typical results of a CPT test in the test field at Vgverket.

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54

Fig. 4.2.2 b. Typical results of a CPT test in the test field at Vgverket.


The pore pressure dissipation tests showed that in general almost all generatedexcess pore pressures had dissipated within 5 minutes, Fig. 4.2.4. The only notableexception was the clayey layer between 4 and 5 metres depth, where about 25 %of the generated excess pore pressure remained after 5 minutes. According to theclassification proposed by Larsson and Mulabdic (1991) this entails that, fornormal loading conditions, the entire soil profile except for the clayey layer can beconsidered as free-draining. The clayey layer can in the same way be consideredas semi-draining bordering on free-draining.

The corresponding times for 50 % pore pressure equalisation and the coefficientsfor consolidation at horizontal pore water flow, ch, evaluated according toTorstensson (1977) are shown in Fig. 4.2.5. According to Torstensson, the valuesevaluated theoretically in this way should be empirically corrected to yieldrepresentative values for practical use.

Fig. 4.2.3 Initial shear moduli in the test field at Vgverket.

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Empirical relation,Hardin (1978)

SGI Report No 5456

Fig. 4.2.4Excess porepressure dissipa-tion curves meas-ured at stops inpenetration inCPT tests in thetest field atVgverket.

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3,0 m3, 7 m4,65 m5,65 m6,68 m7,67 m8,67 m9,66 m10,65 m11,65 m12,64 m13,64 m14,62 m15,62 m

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Fig. 4.2.5 Measured time for 50% excess pore pressure dissipation and values ofthe coefficient of consolidation, ch ,evaluated according to Torstensson1977.

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In most of the pore pressure dissipation tests, the pore pressures were allowed todissipate fully. An almost continuous profile of the in situ pore pressure wasthereby obtained, which was also checked against the readings in the stationarypiezometers, Fig. 4.2.6. The pore pressure profile measured at that time showedthat the pressure in the bottom layers below about 11 metres depth correspondedto a hydrostatic pressure from a level 1 metre above the ground surface. Thehydrostatic pressure head at higher levels started to decline at about 11 metresdepth. A maximum rate of decline, corresponding to the lowest permeability, wasobserved at about 5 metres depth and the free ground water level in the upper layerswas found at about 1.5 metres below the ground surface.

Owing to later problems with the ground water, the location of the plate load testshad to be moved about 15 metres and a new CPT test was then performed in orderto verify that the uniform soil conditions extended also over that area. The resultsfrom all CPT tests are unanimous and yield almost identical pictures of the soilconditions and stratification. The results of the four tests performed with thestandard equipment in this field are shown together in Fig. 4.2.7.

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SGI Report No 5458

Fig. 4.2.6 In situ pore pressure distribution with depth in the test field atVgverket measured in November 1994.

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Fig. 4.2.7 Results of four CPT tests in the test field at Vgverket in terms of totalcone resistance versus depth.

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SGI Report No 5460

Pore pressure measurements

The pore pressure situation in the test field was measured by observation of the freeground water level at shallow depths at pre-drilling through the dry crust, byrecording the in situ pore pressures in the coarser layers below 20 metres depth inthe CPT tests and by measurements in piezometers installed at 3, 5 and 7 metresdepth around the area intended for the plate load tests. These pore pressureobservations, together with the measurements in the pore pressure dissipation testsat the CPT-soundings, were used to establish the prevailing conditions during theinvestigation period and in evaluation of the test results, Fig. 4.2.6. The porepressures were then occasionally read in the piezometers during the followingspring and summer in order to check whether there were any significant changes.A certain increase in pore pressure was observed in May after snow melting andthawing, but for the rest of the summer the measured pore pressures weresignificantly lower than they had been during the investigation period and more orless corresponded to the situation found in the previous investigation.

Sampling and laboratory tests

SamplingUndisturbed samples were taken with a Swedish standard piston sampler typeSt II in two bore holes down to 10 metres depth. In order to minimise thedisturbance and to obtain a continuous soil profile, the samples were taken at every0.7 metre of depth. This distance corresponds to the stroke of the sampler and inthis way the sampling could be performed without pushing soil in front of thesampler during installation to the sampling level. The continuous soil profile wasobtained by taking care also of the soil in the cutting edge and protecting tubes,which is normally thrown away. The sampling was stopped at 10 metres depthbecause this was well below any zone that could possibly be affected by theplanned plate load tests.

Classification testsThe samples were classified in the laboratory. In this process, the samples wereinvestigated concerning bulk density, natural water content, fall cone liquid limitand plasticity index. A large number of sedimentation analyses were also per-formed for determination of the grain size distribution.

The bulk density of the soil varies mainly between 1.9 and 2.0 t/m3 and the naturalwater content is around 30 %, which corresponds to a fully saturated soil. Theliquid limit is close to the natural water content and the plasticity index was foundto vary between 6 and 12 %.


The visual inspection of the samples showed that the soil mainly consists of silt,but there are thin layers of clay and clayey silt embedded in the silt with a frequencythat varies with depth. The upper 3 metres thus contain only a few seams of clayeysoil. These seams then become more frequent and between 4 and 5 metres depththey dominate. The frequency then decreases again and between 6 and 9 metresdepth the clayey seams are sparse. At 10 m depth they are again somewhat morefrequent, Fig. 4.2.8.

Fig. 4.2.8 Soil profile in the test field at Vgverket determined by laboratory testsand visual inspections in the current project.

SGI Report No 5462

According to the Swedish classification system, (Karlsson and Hansbo 1984),which is based on grain size distribution, all the soil in the profile except between4 and 5 metres depth should be classified as silt. This is also in accordance with thebehaviour of the soil in various simple shaking tests used for classification. Sincethe clay content is less than 10 %, the soil should not even be given the sub-designation clayey. Between 4 and 5 me

Documents

Investigations and Load Tests in Silty Soil