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Backfill for Subsurface Structures

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UFC 3-220-04FA 16 January 2004

UNIFIED FACILITIES CRITERIA (UFC)

BACKFILL FOR SUBSURFACE

STRUCTURES

APPROVED FOR PUBLIC RELEASE; DISTRIBUTION UNLIMITED

UFC 3-220-04FA 16 January 2004

1

UNIFIED FACILITIES CRITERIA (UFC)

BACKFILL FOR SUBSURFACE STRUCTURES

Any copyrighted material included in this UFC is identified at its point of use. Use of the copyrighted material apart from this UFC must have the permission of the copyright holder. U.S. ARMY CORPS OF ENGINEERS (Preparing Activity) NAVAL FACILITIES ENGINEERING COMMAND AIR FORCE CIVIL ENGINEER SUPPORT AGENCY Record of Changes (changes are indicated by \1\ ... /1/) Change No. Date Location

This UFC supersedes TM 5-818-4, dated 1 June 1983. The format of this UFC does not conform to UFC 1-300-01; however, the format will be adjusted to conform at the next revision. The body of this UFC is the previous TM 5-818-4, dated 1 June 1983.

UFC 3-220-04FA 16 January 2004

2

FOREWORD \1\ The Unified Facilities Criteria (UFC) system is prescribed by MIL-STD 3007 and provides planning, design, construction, sustainment, restoration, and modernization criteria, and applies to the Military Departments, the Defense Agencies, and the DoD Field Activities in accordance with USD(AT&L) Memorandum dated 29 May 2002. UFC will be used for all DoD projects and work for other customers where appropriate. All construction outside of the United States is also governed by Status of forces Agreements (SOFA), Host Nation Funded Construction Agreements (HNFA), and in some instances, Bilateral Infrastructure Agreements (BIA.) Therefore, the acquisition team must ensure compliance with the more stringent of the UFC, the SOFA, the HNFA, and the BIA, as applicable. UFC are living documents and will be periodically reviewed, updated, and made available to users as part of the Services responsibility for providing technical criteria for military construction. Headquarters, U.S. Army Corps of Engineers (HQUSACE), Naval Facilities Engineering Command (NAVFAC), and Air Force Civil Engineer Support Agency (AFCESA) are responsible for administration of the UFC system. Defense agencies should contact the preparing service for document interpretation and improvements. Technical content of UFC is the responsibility of the cognizant DoD working group. Recommended changes with supporting rationale should be sent to the respective service proponent office by the following electronic form: Criteria Change Request (CCR). The form is also accessible from the Internet sites listed below. UFC are effective upon issuance and are distributed only in electronic media from the following source: Whole Building Design Guide web site http://dod.wbdg.org/. Hard copies of UFC printed from electronic media should be checked against the current electronic version prior to use to ensure that they are current. AUTHORIZED BY: ______________________________________ DONALD L. BASHAM, P.E. Chief, Engineering and Construction U.S. Army Corps of Engineers

______________________________________DR. JAMES W WRIGHT, P.E. Chief Engineer Naval Facilities Engineering Command

______________________________________ KATHLEEN I. FERGUSON, P.E. The Deputy Civil Engineer DCS/Installations & Logistics Department of the Air Force

______________________________________Dr. GET W. MOY, P.E. Director, Installations Requirements and Management Office of the Deputy Under Secretary of Defense (Installations and Environment)

ARMY TM 5-818-4AIR FORCE AFM 88-5, Chap. 5

BACKFILLFOR

SUBSURFACE STRUCTURES

D E P A R T M E N T S O F T H E A R M Y A N D T H E A I R F O R C EJUNE 1983

*TM 5-818-4/AFM 88-5, Chap. 5

TECHNICAL MANUALNo. 5-818-4AIR FORCE MANUALNO. 88-5, CHAPTER 5

Number2- l2-22-32-42-57-lB-lB-2

HEADQUARTERSDEPARTMENTS OF THE ARMY

AND THE AIR FORCEWASHINGTON, DC, 1 June 1983

BACKFILL FOR SUBSURFACE STRUCTURES

CHAPTER 1. INTRODUCTIONBackground . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..Purpose and scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2. PLANNING AND DESIGN OF STRUCTURES AND EX-CAVATIONS TO ACCOMMODATE BACKFILL OP-ERATIONS

General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..Effect of excavation and structuraI configuration on back-

fill operations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Backfill problem areas . . . . . . . . . . . . . . . . . . . . . . . . . . . ..Instrumentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Optimum cost construction . . . . . . . . . . . . . . . . . . . . . . . . .

3. EVALUATION, DESIGN, AND PROCESSING OF BACK-FILL MATERIALS

General.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . ..Evaluation of backfill materials.. . . . . . . . . . . . . . . . . . . . .Selection of backfill materials . . . . . . . . . . . . . . . . . . . . . . .Processing of backfill materials. . . . . . . . . . . . . . . . . . . . . .

4. EARTHWORK: EXCAVATION AND PREPARATIONFOR FOUNDATIONS

Excavation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Foundationpreparation . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5. BACKFILL OPERATIONSPlacement of backfill. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Installation of instruments.. . . . . . . . . . . . . . . . . . . . . . . . .Postconstruction distress. . . . . . . . . . . . . . . . . . . . . . . . . . ..

6. SPECIFICATION PROVISIONSGeneral.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..Excavation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Foundation preparation. . . . . . . . . . . . . . . . . . . . . . . . . . ..Backfill operations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..

7. CONSTRUCTION CONTROLGeneral.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..Corps acceptance control organization. . . . . . . . . . . . . . . . .Excavation control techniques.. . . . . . . . . . . . . . . . . . . . . .Foundation preparation control techniques . . . . . . . . . . . . .Backfill quality acceptance control . . . . . . . . . . . . . . . . . . .

Paragraph Page

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l - ll - l

2 - l 2 - l

2-2 2 - l2-3 2-42-4 2-92-5 2-9

3 - l 3 - l3-2 3 - l3-3 3-43-4 3-7

4 - l 4 - l4-2 4-2

5 - l5-25-3

5 - l5-45-5

6 - l 6 - l6-2 6 - l6-3 6-2

6-4 6-2

7 - l7-27-37-47-5

7 - l7 - l7-37-37-3A - lAPPENDIX A. REFERENCES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

B. FUNDAMENTALS OF COMPACTION, FIELD COMPACTION TESTMETHODS, AND FIELD MOISTUREDENSITY TEST METHODS . . .

C. BIBLIOGRAPHY. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

LIST OF FIGURES

Open backfill zonesTitle

Confined backfill zonesComplex structuresExcavation subject to bottom heaveExcess lateral pressure against vertical walls induced by compactionAcceptance rejection scheme for a backfill areaCompaction CurveTypical compaction test curves

B- lC- l

*This manual supersedes TM 5-818-4/AFM 88-5, Chapter 5, 7 June 1969.

i

TM 5-818-4/AFM 88-5, Chap. 5

Number TitleB-3 Molding Water content versus density-lean clay (laboratory impact compaction)B-4 Illustration of one-point compaction methodB-5 Illustration of possible error using one- and two-point compaction methodsB-6 Illustration of two-point compaction methodB-7 Correlation between dry density and hand cone resistance at a depth of 6 inches below the surfaceB-8 Power applied by the oven to dry moist soils

Number3- l5- l

LIST OF TABLESTitle

Typical engineering properties of compacted materialsSummary of compaction criteria

ii

TM 5-818-4/AFM 88-5, Chap. 5

CHAPTER 1

INTRODUCTION

1-1. Background. Military facilities for theArmy and Air Force have included construction ofbuildings and other structures partially below groundsurface. In more recent years, missile launching andsupport structures, fallout and blast-protection shel-ters, and command-control centers have been con-structed below ground surface. Many of these struc-tures were constructed using cut-and-cover proceduresand required backfilling within confined areas usingvarious types of soil.

a. Numerous deficiencies in backfilling operationsoccurred in some of the earlier missile-launch con-struction programs and caused conditions that jeop-ardized the proper functioning of those structures.Measures to correct deficiencies were both time con-suming and costly. It was recognized that critical areasmust be delineated and the causes of the deficienciesbe determined and corrected.

b. Measures were taken to alleviate the overall back-filling problems. These measures were progressivemodification of design and configuration of structures,more detailed instructions to the construction person-nel, and close control during construction to ensurethat proper construction practices were being fol-lowed.

(1) Some of the problem areas were eliminated bymodification of design and configuration of structuresto allow easier placement of backfill and to permit ac-cess of compaction equipment so that requireddensities could beachieved.

(2) Construction personnel were issued more de-tailed field directives covering some of the particularlydifficult phases of backfill placement.

(3) Inspector training programs were conducted topoint out critical areas and emphasize proper backfillprocedures and the need for continuous surveillanceand close control.

c. The advent of energy efficient structures, partial-ly embedded below ground level, had increased the useof backfill. In addition, the ever increasing need forfuel conservation requires maximum use of all exca-vated or onsite materials for backfill to reduce fuelneeded for hauling in better materials from offsite.Thus, innovative planning and design and good con-struction control using rapid check tests are impera-tive for all backfill operations.

1-2. Purpose and scope. This manual is for theguidance of designers, specification writers, and es-pecially field personnel engaged in designing, plan-ning, and conducting earthwork operations aroundmajor deep-seated or subsurface structures.

a. The greatest deficiencies in earthwork operationsaround deep-seated or subsurface structures occur be-cause of improper backfilling procedures and inade-quate construction control during this phase of thework. Therefore, primary emphasis in this manual ison backfilling procedures. Design and planning con-siderations, evaluation and selection of materials, andother phases of earthwork construction are discussedwhere pertinent to successful backfill operations.

b. Although the information in this manual is pri-marily applicable to backfilling around large and im-portant deep-seated or buried structures, it is also ap-plicable in varying degrees to backfilling operationsaround all structures, including conduits.

1-1

TM 5-818-4/AFM 88-5, Chap. 5

CHAPTER 2

PLANNING AND DESIGN OF STRUCTURES ANDEXCAVATIONS TO ACCOMMODATE BACKFILL OPERATIONS

2-1. General. Many earthwork construction prob-lems can be eliminated or minimized through properdesign, thorough planning, and recognition of problemareas effecting backfill operations. Recognition andconsideration must be given in planning to design fea-tures that will make backfilling operations less diffi-cult to accomplish. Examples of problem areas andhow forethought in design and planning can help toeliminate backfill deficiencies are presented in the fol-lowing paragraphs.

2-2. Effect of excavation and structuralconfiguration on backfill operations.Some of the problems encountered in earthwork con-struction are related to the excavation and the configu-ration of the structures around which backfill is to beplaced. It is the designers responsibility to recognizethese problems and to take the necessary measures tominimize their impact on the backfill operations.

a. Open zones. An open zone is defined as a backfillarea of sufficient dimensions to permit the operation

of heavy compaction equipment without endangeringthe integrity of adjacent structures around which com-pacted backfill operations are conducted. Figure 2-1shows examples of open zones. In these zones wherelarge compaction equipment, can operate, it is general-ly not too difficult to obtain the desired density if ap-propriate materials and proper backfill procedures areused. For areas that can be economically compacted byheavy equipment, the designer can avoid problems byincluding in the design provisions sufficient workingspace between structures or between excavation slopesand structures to permit access by the heavy compac-tion equipment. Generally, a working space of at least12 feet between structure walls and excavation slopesand at least 15 feet between structures is necessary forheavy equipment to maneuver. In addition to maneu-vering room, the designer must also consider any ad-verse loading caused by the operation of heavy equip-ment too close to structure walls, as discussed in para-graph 2-3d.

b. Confined zones. Confined zones are defined asareas where backfill operations are restricted to theuse of small mechanical compaction equipment (fig.2-2) either because the working room is limited or be-cause heavy equipment (fig. 2-1) would impose exces-sive soil pressures that could damage the structure.Most deficiencies in compacted backfill around subsur-

face structures have occurred in confined zones whererequired densities are difficult to achieve because ofrestricted working room and relatively low compactioneffort of equipment that is too lightweight. The use ofsmall equipment to achieve required compaction isalso more expensive than heavy equipment since thin-ner lifts are required. However, because small compac-tion equipment can operate in spaces as narrow as 2feet in width, such equipment is necessary to achievethe required densities in some areas of most backfillprojects. Therefore, the designer should plan structureand excavation areas to minimize the use of small com-paction equipment.

c. Structure configuration. The designer familiarwith backfilling operations can avoid many problemsassociated with difficult to reach confined zones,which are created by structural shapes obstructing theplacement and compaction of backfill, by consideringthe impact of structural shape on backfill operations.In most cases, structural shapes and configurationsthat facilitate backfill operations can be used withoutsignificantly affecting the intended use of the struc-ture.

(1) Curved bottom and wall structures. Areas be-low the spring line of circular, elliptical, and similarshaped structures are difficult to compact backfillagainst because compaction equipment cannot get un-der the spring line. If possible, structures should be de-signed with continuously curved walls and flat floorssuch as in an igloo-shaped structure. For structureswhere a curved bottom is required to satisfy the in-tended function, it may be advisable for the designerto specify that a template shaped like the bottom ofthe structure be used to guide the excavation below thespring line so that uniform foundation support will beprovided.

(2) Complex structures. Complex structures havevariable shaped walls and complex configurations inplan and number of levels. These structures can also besimple structures interconnected by access shafts, tun-nels, and utility conduits. Because of their irregularshapes and configurations the different types of struc-tures significantly increase excavation and backfillproblems.

(a) Typical examples of complex structures arestepped multilevel structures and multichamberedstructures with interconnecting corridors (fig. 2-3).Complex structures are generally more difficult to

2 - 1

TM 5-818-4/AFM 88-5, Chap. 5

Figure 2- 1. Open backfill zones.

compact backfill around and are more likely to havesettlement problems (para 2-3a). Although the multi-level step structure (fig. 2-3a) is not particularly diffi-cult to compact backfill around, at least for the firstlevel, the compaction of backfill over the offset struc-ture will generally require the use of small equipment.Small equipment will also be required for compactionof backfill around and over the access corridor and be-tween the two chambers (fig. 2-3b). Where possible,the design should accommodate intended functionsinto structures with uniformly shaped walls and a sim-ple configuration.

(b) Where structures of complex configurationsare necessary, construction of a three-dimensionalmodel during the design and planning phases will beextremely beneficial. From the model, designers canmore easily foresee and eliminate areas in which itwould be difficult to place and compact backfill.

d. Service conduits. Since compaction of backfill isdifficult around pipes and conduits, utility lines should

be grouped together or placed in a single large conduitwhere feasible rather than allowed to form a haphaz-ard maze of pipes and conduits in the backfill. Utilitylines should be run either horizontally or verticallywherever possible. Plans for horizontally run appur-tenances, such as utility lines, access tunnels, andblast-delay tubing, should be coordinated with the ex-cavation plans so that wherever feasible these appur-tenances can be supported by undisturbed soils ratherthan by compacted backfill.

e. Excavation plans. The excavation plans should bedeveloped with the backfill operations and the struc-ture configurations in mind. The excavation and allcompleted structures within the excavation should beconducive to good backfill construction procedures,and access should be provided to all areas so that com-paction equipment best suited to the size of the areacan be used. The plans for excavation should also pro-vide for adequate haul roads and ramps. Positive exca-vation slopes should be required in all types of soil de-

2-2

TM 5-818-4/AFM 88-5, Chap. 5

Figure 2-2. Confined backfield zones.

(a) TWO-STORY STRUCTURE

(b) CONNECTING STRUCTURES

Figure 2-3. Complex structures.

2-3

TM 5-818-4/AFM 88-5, Chap. 5

posits to facilitate compaction of backfill against theslope and to ensure good bond between the backfill andthe excavation slopes. Loose material should be re-moved from the excavation slopes; in some cases,benches may be required to provide a firm surface tocompact backfill against.

f. Lines and grades. Care should be exercised inplanning lines and grades for excavation to ensurethat uniform, adequate support is provided at thefoundation level of important structures. Generally,foundations consisting of part backfill and part undis-turbed materials do not provide uniform bearing andshould be avoided wherever possible. The foundationshould be overexcavated where necessary, and back-filled with compacted select material to provide unif-orm support for the depth required for the particularstructure. Where compacted backfill is required be-neath a structure, the minimum depth specified shouldbe at least 18 inches.

g. Thin- walled metal structures. Thin-walled, corru-gated metal structures are susceptible to deflections ofstructural walls when subjected to backfill loads. Ad-verse deflections can be minimized by planning back-fill operations so that compacted backfill is brought upevenly on both sides of the structure to ensure uniformstress distribution. Temporary surcharge loads appliedto the structure crown may also be required to preventvertical distortions and inward deflection at the sides.

2-3. Backfill problem areas. Other featuresthat have the potential to become problem areas arediscussed in the following paragraphs. These potentialproblem areas have to be considered during the plan-ning and design phases to minimize deficiencies instructure performance associated with backfill place-ment and to make backfilling operations less difficult.

a. Settlement and downdrag. In the construction ofunderground structures and particularly missile-launch-site facilities, tolerances to movement are oftenconsiderably less than those in normal construction.The design engineer must determine and specify allow-able tolerances in differential settlement and ensurethat differential settlement is minimized and/or ac-commodated. Settlement analysis procedures are out-lined in TM 5-818-l/AFM 88-3, Chapter 7. See ap-pendix A, References.

(1) Critical zones. Critical backfill zones are thoseimmediately beneath most structures. Consolidationand swelling characteristics of backfill materialsshould be thoroughly investigated so that materialshaving unfavorable characteristics will not be used inthose zones. Some settlement can be expected to takeplace, but it can be minimized by requiring a higherthan normal compacted density for the backfill. Cohe-sive backfill compacted at a water content as little as 3

2-4

to 4 percentage points below optimum may result inlarge settlements caused by collapse of nonswellingsoil material or heave of swelling materials upon sat-uration after construction. Compacting cohesive back-fill material at optimum water content or slightly onthe wet side of optimum generally will reduce theamount of settlement and swelling that would occur.The reduction should be confirmed by consolidationand swell tests on compacted specimens (para 3-2b(4)).

(2) Service conduits. Settlement within the back-fill around structures will also occur. A proper designwill allow for the estimated settlement as determinedfrom studies of consolidation characteristics of thecompacted backfill. Where service conduits, access cor-ridors, and similar facilities connect to the structureoversize sleeves, flexible connections and other protec-tive measures, as appropriate, may be used to preventdamage within the structure.

(3) Differential settlement. Complex structuresare more susceptible to differential settlement becauseof the potential for large variations in loads carried byeach component foundation. In the multilevel steppedstructure (fig. 2-3a), the foundation supporting thelower level offset component must also support thevolume of backfill over that part of the structure.Measures must be taken to ensure that the properfunctioning of all elements is not hampered by differ-ential settlement. The increased cost of proper designand construction where unusual or difficult construc-tion procedures are required is insignificant whencompared with the cost of the structure. The cost of re-medial measures to correct deficiencies caused by im-proper design and construction usually will be greaterthan the initial cost required to prevent the deficien-cies.

(4) Downdrag. In addition to conventional serviceloads, cut and cover subsurface structures are suscepti-ble to downdrag frictional forces between the struc-ture and the backfill that are caused by settlement ofthe backfill material adjacent to and around the struc-ture. Downdrag loads can be a significant proportionof the total vertical load acting on the structure andmust be considered in the structure settlement analy-sis. Structure-backfill friction forces may also generatesignificant shear forces along the outer surface ofstructures with curve-shaped roofs and walls. Themagnitude of the friction forces depends upon the typeof backfill, roughness of the structures surface, andmagnitude of earth pressures acting against the struc-ture. Techniques for minimizing downdrag frictionforces generally include methods that reduce the struc-ture surface roughness such as coating the structuresouter surface with asphalt or sandwiching a layer ofpolyethylene sheeting between the structures outersurface and fiberboard (blackboard) panels. Backfillsettlement and associated downdrag can also be mini-

TM 5-818-4/AFM 88-5, Chap. 5

Figure 2-4. Excavation subject to bottom heave.

mized by requiring higher backfill densities adjacentto the structure.

mized by adequate planning and implementation ofgroundwater investigations.

b. Groundwater. Groundwater is an importantconsideration in planning for construction of subsur-face structures. If seepage of groundwater into the ex-cavation is not adequately controlled, backfillingoperations will be extremely difficult. The ground-water level must be lowered sufficiently (at least 2 to 3feet for granular soils and as much as 5 to 10 feet forfine-grained soils below the lowest level of backfilling)so that a firm foundation for backfill can be estab-lished. If the level is not lowered, the movement ofhauling or compaction equipment may pump seepagewater through the backfill, or the initial backfill layersmay be difficult to compact because of an unstablefoundation. Since the proper water content of thebackfill is essential for achieving proper compaction,prevention of groundwater seepage into the excava-tion during backfilling operations is mandatory.Figure 3-14 of EM 1110-2-1911 shows a method fordewatering rock foundations.

(1) The contractor is generally responsible for thedesign, installation, and operation of dewateringequipment. The Corps of Engineers is responsible forspecifying the type of dewatering system and evaluat-ing the contractors proposed dewatering plan. Sincethe dual responsibility of the contractor and the Corpsrelies on a thorough understanding of groundwaterconditions, inadequate dewatering efforts can be mini-

(2) The possibility of hydraulic heave in cohesivematerial must also be investigated to ensure stabilityof the excavation floor. Hydraulic heave may occurwhere an excavation overlies a confined permeablestratum below the groundwater table (fig. 2-4). If theupward hydrostatic pressure acting at the bottom ofthe confining layer exceeds the weight of overburdenbetween the bottom of the excavation and the confin-ing layer, the bottom of the excavation will rise bodilyeven though the design of the dewatering system isadequate for control of groundwater into the excava-tion. To prevent heave, the hydrostatic pressure be-neath the confined stratum must be relieved.

(3) Subsurface structures located in part or whollybelow the groundwater table require permanent pro-tection against groundwater seepage. The type of pro-tection may range from simple impermeable barriersto complex permanent dewatering systems.

(4) Dewatering and groundwater control proce-dures are described in TM 5-818-5/NACFACP-418/AFM 88-5, Chapter 6.

c. Gradation and filter criteria for drainage materi-als. Groundwater control is often accomplished byditches positioned to intercept the flow of groundwa-ter and filled with permeable granular material. Thewater is generally collected in perforated pipes locatedat the bottom of the ditch and pumped to a suitable

2-5

TM 5-818-4/AFM 88-5, Chap. 5

discharge area. Such drainage systems are referred toas filter drains. The gradation of the granular filtermaterial is critical for the functioning of the system.Selection of the proper gradation for the filter materi-al is dependent upon the gradation of the material thatis being drained. Drainage of silts and clays usually re-quires a graded filter made up of several layers ofgranular material with each layer having specific re-quirements for maximum grain size and gradation. De-tails on the design of filter drains are presented in TM5-818-5/NAVFACP-418/AFM 88-5, Chapter 6.

(1) Selected material. If materials at the jobsite donot meet the designed filter requirements, select ma-terial must be purchased from commercial sources andshipped to the jobsite. Filter material must be stock-piled according to gradation. For graded filter sys-tems, the materials must be placed with care to mini-mize mixing of individual components.

(2) Filter cloths. Both woven and nonwoven filtercloths, which have been found satisfactory for use as afilter media for subsurface drains, are available. Whengranular filter materials are not economicallyavailable, a single wrap of filter cloth around a pipemay be used in lieu of a coarser backfill. Whenavailable granular filter material is too coarse to satis-fy filter criteria for the protected soil, a single layer offilter cloth may be used adjacent to the protected soil.To reduce the chance of clogging, no filter cloth shouldbe specified with an open area less than 4 percent andor equivalent opening size (EOS) of less than the No.100 sieve (0.0059 inch). A cloth with openings as largeas allowable should be specified to permit drainageand prevent clogging. Additional information on air-field drainage is contained in TM 5-820-2/AFM 88-5,Chapter 2.

(3) Other uses. Filter cloth can also provide pro-tection for excavated slopes and serve as a filter to pre-vent piping of fine-grained soils. In one project, sandwas not available for backfill behind a wall and coarsegravel had to be used to collect seepage. The filtercloth used to protect the excavated slope served as afilter against piping of the natural silty clay underseepage gradients out of the excavated slope after thecoarse gravel backfill was placed.

d. Earth pressures. The rationale design of anystructure requires the designer to consider all loadsacting on the structure. In addition to normal earthpressures associated with the effective pressure dis-tribution of the backfill materials, subsurface cut-and-cover structures may also be subjected to surchargeloads caused by heavy equipment operating close tothe structure and by increased permanent lateral earthpressures caused by compaction of backfill materialwith heavy equipment. Procedures for predicting nor-mal earth pressures associated with the effective pres-sure of backfill materials are discussed in TM

2-6

5-818-1/AFM 88-3, Chapter 7, EM 1110-2-2902,and EM 1110-2-2502.

(1) Exact solutions for surcharge earth pressuresgenerated by heavy equipment (or other surchargeloads) do not exist. However, approximations can bemade using appropriate theories of elasticity such asBoussinesqs equations for load areas of regular shapeor Newmarks charts for irregular shaped load areas asgiven in NAVFAC DM-7. As a conservative guide,heavy-equipment surcharge earth pressures may beminimized by specifying that heavy compaction equip-ment maintain a horizontal distance from the struc-ture equivalent to the height of the backfill above thestructures foundation.

(2) Compaction-induced earth pressures can causea significant increase in the permanent lateral earthpressures acting on a vertical wall of a structure (fig.2-5a). This diagram is based on the assumption thatthe equipment can operate to within 6 inches of thewall. Significant reductions in lateral pressures occuras the closest allowable distance to the wall is in-creased (fig. 2-5b). For an operating distance 5 feetfrom the wall, the induced horizontal earth pressure ismuch less than that caused by the backfill. The magni-tude of the increase in lateral pressure is dependent,among other factors, on the effective weight of thecompaction equipment and the weight, earth pressurecoefficient, and Poissons ratio of the backfill material.Compaction-induced earth pressures against walls arealso described in TM 5-818-l/AFM 88-3, Chapter 7,and EM 1110-2-2502.

(3) The designer must evaluate the economics ofthe extra cost of structures designed to withstand veryclose-in operation of heavy compaction equipmentversus the extra cost associated with obtaining re-quired compaction of backfill in thin lifts with smallercompaction equipment. A more economical alternativemight be to specify how close to the walls differentweights of compaction equipment can be operated.

(4) One method of reducing lateral earth pres-sures behind walls has been to use about 4 feet of un-compacted granular (sand or gravel) backfill above thebase of the wall. Soil backfill can then be compacted inlayers above the granular backfill. Compression of thegranular material prevents the buildup of excessivelateral pressures against the wall.

e. Structural backfill. Structural backfill is definedas the compacted backfill required over and around astructure to prevent damage from heavy equipmentoperating over or near the structure. This backfillmust be compacted using small compaction equip-ment, such as mechanical rammers or vibratory-platecompactors, or intermediate size equipment such aswalk-behind, dual-drum vibratory rollers. The hori-zontal and vertical distances from the structure forwhich structural backfill is required should be deter-

TM 5-818-4/AFM 88-5, Chap. 5

a. MAXIMUM INDUCED LATERAL PRESSURES

b. EFFECT OF DISTANCE FROM WALL

Figure 2-5. Excess lateral pressure against vertical walls induced by compaction.

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TM 5-818-4/AFM 88-5, Chap. 5

mined from estimates of loads acting on the structurecaused by heavy equipment and on the strength of theembedded structure members as discussed in d above.A 2-foot cover over small utility conduits and pipes isadequate protection where proper bedding proceduresare followed. The minimum cover requirements overlarger diameter (6 inches or greater), rapid and flexiblepipes are presented in appendix II of TM 5-820-4/AFM 88-5, Chapter 4.

f. Slopes and bracing. Where open excavation isplanned, consideration should be given to the slopes towhich the materials to be encountered can be cut andremain stable. The stability analysis should includethe strength of the materials, groundwater conditions,and any surcharge load that may be imposed as the re-sult of stockpiles being placed or equipment operatingnear the crest of the excavation. Slope stability evalua-tion procedures are described in TM 5-818-1/AFM88-3, Chapter 7. Shoring and bracing should be usedto support excavation slopes where it is not feasible toexcavate to stable slopes (TM 5-818-l/AFM 88-3,Chapter 7). Requirements for shoring and bracingsafety are presented in EM 385-l-l.

g. Bedding for curved-bottom structures. Founda-tions for pipes, conduits, access tunnels, fuel and wa-ter storage tanks, and other curved-bottom structuresconstructed within the backfill are considered criticalzones that require special attention. Any bedding ma-terial used should be free of stones or other large parti-cles that would lead to nonuniform bearing. One of themost important functions of any bedding procedure isto provide firm support along the full length of thestructure. For areas where it is difficult to performfield density control tests because of limited workingspace, a procedure to ensure that proper compaction isobtained must be employed. Several methods of ob-taining adequate bedding are discussed in paragraph5-1c (2).

h. Cold weather construction. Cold weather canhave a very adverse effect on backfilling operationsand can cause considerable delay. If possible, the proj-ect should be planned to complete backfilling opera-tions prior to any extended period of freezing tempera-tures. The contractor and the resident engineer mustkeep up to date with weather data so that the con-tractor can plan the equipment and construction forcerequired to meet the construction schedule and to pro-tect the work already accomplished.

(1) The designer must establish definite limita-tions and requirements regarding placement of back-fill when the ambient temperature is below freezing.Most inorganic soils, particularly silts and lean clays,containing 3 percent, by weight, or more of particlesfiner than 0.02 millimetre in diameter are frost sus-ceptible. Such soils, when frozen in the presence of an

2-8

available source of water, develop segregated ice in theform of lenses, layers, veins, or masses commonly, butnot always oriented normal to the direction of heatloss. The expansion of the soil mass resulting from icesegregation is called frost heave. Frost heave of soilunder and against structures can cause detrimental ef-fects, which can be compounded during subsequentthawing by differential movement, loss of density, andloss of shear strength. Soils of this type should not beplaced during or immediately prior to freezingtemperatures and must not be placed in critical areas.Nonfrost susceptible soils should be used at thefinished grade to the depth of frost penetration whenthe finished grade serves as a load-bearing surface.

(2) Additives, such as calcium chloride, can beused to lower the freezing temperature of soil water,but such additives will ordinarily also change the com-paction and water content requirements. Therefore,additives must not be used without prior investigationto determine their effect on compaction and water con-tent requirements. Dry sand or sand-gravel mixturescan be placed satisfactorily when temperatures are be-low freezing without serious effects.

(3) Protection must be provided for in-placepermanent backfill in critical areas, such as thosearound and under structures and embedded items al-ready placed. To preclude structural damage from pos-sible frost heave, backfill materials around such struc-tures should be insulated with a protective covering ofmulch, hay, or straw. In some instances, loose lifts ofsoil can be used for insulation. However, rock or sandis too porous to provide sufficient insulation and toopermeable to resist water penetration. If soil is to beused as an insulating material, a material completelyforeign to the permanent fill, such as straw or buildingpaper, should be laid down prior to placement of theinsulation fill so that there will be a marked distinc-tion between the permanent and the temporary insula-tion fills. In this way, when the insulation fill is re-moved, the stripping limits can be readily discerned.

(4) Flooding of the excavation has also been usedsuccessfully to prevent frost penetration of the in-place permanent backfill. However, considerationmust be given to possible detrimental effects ofsaturating in-place backfill and the delay of removingthe water at the beginning of the next constructionseason if it freezes into a solid mass of ice.

(5) Concrete walls and floors of completed struc-tures provide poor insulation for the fill around andbeneath these structures. Therefore, these structuresshould be enclosed as much as possible and kept closedduring the winter when construction is halted becauseof adverse freezing weather. Reinforcing steel protrud-ing from a partially completed structure will conductcold through the concrete and increase the rate anddepth of frost penetration beneath the structure.

Every effort should be made to schedule constructionso that this condition will be kept to a minimum, andprotection must be required where necessary.

i. Seismic zones. The design considerations for sub-surface structures subjected to dynamic loads causedby seismic activity or explosive devices are beyond thescope of this manual. Design details are provided inTM 5-818-1/AFM 88-3, Chapter 7, and ER1110-2-1806. Specific problems relating to backfilloperations are primarily limited to possible potentialfor dynamically induced liquefaction. Certain materi-als are particularly susceptible to liquefaction; theseinclude saturated gravels, sands, silts, and clayeysands and gravel. Where these materials are used asbackfill, the potential for liquefaction can be mini-mized by requiring a high degree of compaction,particularly in critical areas such as beneath footingsand under the spring line of curved wall structures.The requirements for materials susceptible to liquefac-tion are discussed in paragraph 3 - 3d.

2-4. Instrumentation. For important struc-tures of unique design or for structures where the po-tential for postconstruction distress exists, instrumen-tation of the structure should be considered. The in-strumentation program may include monitoring theamount and rate of settlement, movement of retainingwalls and other structural elements, development ofstresses within the structure, and development of hy-drostatic and earth pressures against the structure.Analysis of the data will furnish a check on design as-sumptions and indicate what measures must be takento relieve or correct undesirable conditions beforedistress develops. Information of this nature can alsobe of significant value in future design and construc-tion.

a. Requirements. Specific requirements for instru-ments are ruggedness, reliability over the projectedservice life, and simplicity of construction, installa-tion, and observation. Other important considerationsin selecting the type of instruments are cost andavailability. Manufacturers of devices considered forinstallation should be asked to provide a list of projectson which their devices have been installed, and previ-ous users of new equipment should be contacted to as-certain their operating experiences.

b. Installation and observation of instrumentation.A rational instrumentation program must use theproper type of instruments and have the instrumentsinstalled properly at critical locations. Valid readingsoften depend on techniques and procedures used in in-stalling and observing the instrumentation.

(1) Schedules for observations are generally estab-lished by the design office. Initial observations shouldbe checked to assure their validity and accuracy, since

TM 5-818-4/AFM 88-5, Chap. 5

these readings usually form the basis to which subse-quent observations are related. Observations should beplotted immediately after each set of readings is takenand evaluated for reasonableness against previous setsof readings. In this way, it is often possible to detecterrors in readings and to obtain check readings beforesignificant changes in field conditions occur.

(2) EM 1110-2-1908 discusses in detail varioustypes of instrumentation devices; procedures for in-stallation, observation, and maintenance; collection,recording, analysis, and reporting of data; and possiblesource of error and causes of malfunctions.

2-5. Optimum cost construction. The de-signer should consider all details of the constructionprocess to ensure a safe and operational facility at thelowest possible cost.

a. Energy requirements. The consideration of ener-gy requirements is important not only for economicalreasons but also for the critical need to conserve ener-gy wherever possible. It should not be the intent of thedesign engineer to unduly restrict the competitive na-ture of current contractural procedures. Nevertheless,there are certain alternatives that the designer mayspecify that potentially could lead to more energy effi-cient construction with cost saving being reflected inbid prices. Some of the possible alternatives thatshould be considered are discussed below.

(1) Sources of suitable select backfill materialshould be located as close to the project site as possi-ble. The source may be either a borrow area or a com-mercial vendor.

(2) Hauling routes to and from the source of back-fill and the project site should follow the most directroute.

(3) Only compaction equipment that will compactthe specific backfill to the required density in an effi-cient manner should be approved for use. For largeprojects, the designer may require that the contractordemonstrate the capabilities of the equipment he in-tends to use prior to construction.

(4) If possible, material from excavations or with-in the immediate vicinity of the project site should beused as backfill, even though such material may bemarginally suitable. The engineering characteristics ofmarginal material may be enhanced by the use of addi-tives (para 3-3d).

(5) The energy requirements for adequate coldweather protection of construction personnel andstructures can be considerable. For project sites sub-ject to seasonal cold weather, construction should not,if possible, be scheduled during extreme cold weatherperiods.

b. Value engineering. Potential cost savings may berealized by encouraging the contractor to participatein value engineering, whereby the contractor shares

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TM 5-818-4/AFM 88-5, Chap. 5

any project saving derived from realistic cost-savingsuggestions submitted.

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TM 5-818-4/AFM 88-5, Chap. 5

CHAPTER 3

EVALUATION, DESIGN, AND PROCESSINGOF BACKFILL MATERIALS

3-1. General. The evaluation, design, and properprocessing of backfill materials are extremely impor-tant phases of the preconstruction operations. Thepurpose of the evaluation phase is to determine the en-gineering characteristics of potential backfill materi-als. The design phase must take into account the engi-neering characteristics required of the backfill andspecify materials that, when compacted properly, willhave these characteristics. Proper processing of thebackfill material will ensure that desirable engineering.characteristics will be obtained as the material isplaced.

3-2. Evaluation of backfill materlals.Evaluation of backfill materials consists of explora-tion, sampling, and laboratory testing to determinethe engineering characteristics of potential backfillmaterials. Detailed instructions for exploration, sam-pling, laboratory testing, and foundation design arepresented in TM 5-818-1/AFM 88-3, Chapter 7. How-ever, to emphasize the need for an adequate investiga-tion, some aspects of planning and investigation thatshould be considered are discussed in the followingparagraphs.

a. Field exploration and sampling. Field explorationand sampling are extremely important to the design offoundations, selection of backfill, and planning forconstruction. A great amount of material will be avail-able from required excavations, and the investigationfor foundation conditions should include the samplingand evaluation of these materials for possible use asbackfill. Where an adequate volume of suitable back-fill cannot be obtained from the construction excava-tion, the exploration and sampling program must beexpanded to find other sources of suitable materialwhether from nearby borrow areas or commercialsources.

(1) The purpose of the investigation is to delineatecritical conditions and provide detailed information onthe subsurface deposits so that proper design and con-struction, including backfilling operations, can be ac-complished with minimum difficulty. Thus carefulplanning is required prior to the field exploration andsampling phase of the investigation. Available geo-logic and soil data should be studied, and if possible,preliminary borings should be made. Once a site hasbeen tentatively selected, orientation of the structureto the site should be established. The engineer who

plans the detailed field exploration program musthave knowledge of the structure, i.e., its configurationand foundation requirements for design loads and set-tlement tolerances. The planning engineer should alsoknow the type and quantity of backfill required. Theimportance of employing qualified field explorationpersonnel cannot be overemphasized. The explorationcrews should be supervised in the field by a soils engi-neer or geologist familiar with the foundation andbackfill requirements so that changes can be made inthe exploration program where necessary to provideadequate information on subsurface conditions.

(2) The field engineer should also know the loca-tion of significant features of the structure so thatsampling can be concentrated at these locations. In ad-dition, he should have an understanding of the engi-neering characteristics of subsurface soil and rock de-posits that are important to the design of the structureand a general knowledge of the testing program sothat the proper type and quantity of samples will beobtained for testing.

(3) From the samples, the subsurface deposits canbe classified and boring logs prepared. The more con-tinuous the sampling operation, the more accurate willbe the boring logs. All borings should be logged withthe description of the various strata encountered asdiscussed in TM 5-818-1/AFM 88-3, Chapter 7. Accu-rate logging and correct evaluation of all pertinent in-formation are essential for a true concept of subsur-face conditions.

(4) When the exploratory borings at the construc-tion site have been completed, the samples and logs ofborings should be examined to determine if the materi-al to be excavated will be satisfactory and in sufficientquantity to meet backfill requirements. Every effortshould be made to use the excavated materials; how-ever, if the excavated materials are not satisfactory orare of insufficient quantity, additional explorationshould be initiated to locate suitable borrow areas. Ifborrow areas are not available, convenient commercialsources of suitable material should be found. Backfillsources, whether excavation, borrow, or commercial,should contain several times the required volume ofcompacted backfill.

(5) Groundwater studies prior to construction ofsubsurface structures are of the utmost importance,since groundwater control is necessary to provide adry excavation in which construction and backfilling

3-1

TM 5-818-4/AFM 88-5, Chap. 5

operations can be properly conducted. Data on ground-water conditions are also essential for forecasting con-struction dewatering requirements and stability prob-lems. Groundwater studies must consist of investiga-tions to determine: groundwater levels to include anyseasonal variations and artesian conditions; the loca-tion of any water-bearing strata; and the permeabilityand flow characteristics of water-bearing strata. Meth-ods for investigating groundwater conditions are de-scribed in TM 5-818-1/AFM 88-3, Chapter 7, and TM5-818-5/NAVFACP-418/AFM 88-5, Chapter 6.

b. Laboratory testing. The design of any foundationis dependent on the engineering characteristics of thesupporting media, which may be soil or rock in eitherits natural state or as compacted backfill. The labora-tory testing program will furnish the engineer infor-mation for planning, designing, and constructing subsurface structures. Laboratory testing programs usual-ly follow a general pattern and to some extent can bestandardized, but they should be adapted to particularproblems and soil conditions. Special tests and re-search should be utilized when necessary to developneeded information. The testing program should bewell planned with the engineering features of thestructure and backfill in mind; testing should be con-centrated on samples from areas where significant fea-tures will be located but should still present a completepicture of the soil and rock properties. The laboratorytest procedures and equipment are described in TM5-818-1/AFM 88-3, Chapter 7, EM 1110-2-1906,and MIL-STD-621.

(1) Identification and classification of soils. TheUnified Soil Classification System used for classifyingsoils for military projects (MIL-STD-619 and TM5-818-1/AFM 88-3, Chap. 7) is a means of identifyinga soil and placing it in a category of distinctive engi-neering properties. Table 3-1 shows the properties ofsoil groups pertinent to backfill and foundations.Using these characteristics, the engineer can preparepreliminary designs based on classification and planthe laboratory testing program intelligently and eco-nomically.

(a) The Unified Soil Classification System clas-sifies soils according to their grain-size distributionand plasticity characteristics and groups them with re-spect to their engineering behavior. With experience,the plasticity and gradation properties can be esti-mated using simple, expedient tests (see table 2-2 and2-3 of TM 5-818-1/AFM 88-3, Chap. 7 or AFM 89-3,Chap. 2) and these estimates can be confirmed usingsimple laboratory tests. The principal laboratory testsperformed for classification are grain-size analysesand Atterberg limits.

(b) The engineering properties in table 3-1 arebased on Standard Proctor (CE 25) maximum

3-2

density except that the California Bearing Ratio (CBR)and the subgrade modulus are based on CE 55 maxi-mum density. This information can be used for initialdesign studies. However, for final design of importantstructures, laboratory tests are required to determineactual performance characteristics, such as CE 55compaction properties, shear strength, permeability,compressibility, swelling characteristics, and frost sus-ceptibility where applicable, under expected construc-tion conditions.

(c) The Unified Soil Classification System isparticularly useful in evaluating, by visual examina-tion, the suitability of potential borrow materials foruse as compacted backfill. Proficiency in visual clas-sification can be developed through practice by com-paring estimated soil properties with results of labora-tory classification tests.

(2) Compaction testing. Compaction test proce-dures are described in detail in MIL-STD-621 andASTM D 1557 (app. A). It is important that the de-signer and field inspection personnel understand thebasic principles and fundamentals of soil compaction.The principles of soil compaction are discussed in ap-pendix B of this manual.

(a) The purpose of the laboratory compactiontests are to determine the compaction characteristicsof available backfill materials. Also, anticipated fielddensity and water content can be approximated in laboratory-compacted samples in order that other engi-neering properties, such as shear strength, compressi-bility, consolidation, and swelling, can be studied. Formost soils there is an optimum water content at whicha maximum density is obtained with a particular com-paction effort. A standard five-point compaction curverelating density and water content (fig. B-1, app. B)can be developed by the procedures outlined in MIL-STD-621.

(b) The impact compaction test results normallyconstitute the basis on which field compaction controlcriteria are developed for inclusion in the specifica-tions. However, for some cohesionless soils, higherdensities can be obtained by the vibratory compactionmethod (commonly referred to as maximum relativedensity), described in appendix XII of EM 1110-2-1906. The required field compaction is generally speci-fied as a percentage of laboratory maximum dry densi-ty and referred to as percent CE 55 maximum density.Water content is an important controlling factor in ob-taining proper compaction. The required percentage ofmaximum dry density and the compaction water con-tent should be selected on the basis of the engineeringcharacteristics, such as compression moduli, settle-ment, and shear strength, desired in the compactedbackfill. It should be noted that these characteristicscould be adversely effected by subsequent increases in

TM 5-818-4/AFM 88-5, Chap. 5

(3) Shear strength testing. When backfill is to beplaced behind structure walls or bulkheads or asfoundation support for a structure, and when fills areto be placed with unrestrained slopes, shear. testsshould be performed on representative samples of thebackfill materials compacted to expected field densi-ties and water contents to estimate as-constructedshear strengths. The appropriate type of test requiredfor the conditions to be analyzed is presented in TM5-818-1/AFM 88-3, Chapter 7. Procedures for shearstrength testing are described in EM 1110-2-1906.

water content after placement. This situation could re-sult from an increase in the groundwater level afterconstruction.

(c) Density control of placed backfill in the fieldcan be facilitated by the use of rapid compaction checktests (para 7-5c). A direct rapid test is the one-pointimpact compaction test. Rapid indirect tests, such asthe Proctor needle penetration for cohesive soils or thecone resistance load for cohesionless soils, can also beused when correlations with CE 55 maximum densityhave been established.

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TM 5-818-4/AFM 88-5, Chap. 5

(4) Consolidation and swell testing. The rate andmagnitude of consolidation under a given load are in-fluenced primarily by the density and type of soil andthe conditions of saturation and drainage. Fine-grained soils generally consolidate more and at aslower rate than coarse-grained soils. However, poorlygraded, granular soils and granular soils composed ofrounded particles will often consolidate significantlyunder load but usually at a relatively fast rate.

(a) The procedure for the consolidation test isoutlined in EM 1110-2-1906. The information ob-tained in this test can be used in settlement analyses todetermine the total settlement, the time rate of settle-ment, and the differential settlement under varyingloading conditions. Consolidation characteristics areimportant considerations in selection of backfill mate-rials. The results of consolidation tests performed onlaboratory compacted specimens of backfill materialcan be used in determining the percent compaction tobe required in the specifications.

(b) Swelling characteristics can be determinedby a modified consolidation test procedure. The degreeof swelling and swelling pressure should be deter-mined on all backfill and foundation materials sus-pected of having swelling characteristics. This fact isparticularly important when a considerable overbur-den load is removed by excavation or when the com-pacted backfill with swelling tendencies may becomesaturated upon removal of the dewatering system andsubsequent rise of the groundwater level. The resultsof swelling tests can be used to determine the suitabil-ity of material as backfill. When it is necessary to usebackfill materials that have a tendency to swell uponsaturation because more suitable materials are un-available, the placement water content and densitythat will minimize swelling can be determined from aseries of tests. TM 5-818-1/AFM 88-3, Chapter 7,and FHWA-RD-79-51 (app. A) provide further infor-mation applicable to compacted backfills.

(5) Permeability tests. Permeability tests to deter-mine the rate of flow of water through a material canbe conducted in the laboratory by procedures describedin EM 1110-2-1906. Permeability characteristics offine-grained materials at various densities can also bedetermined from consolidation tests.

(a) Permeability characteristics for the designof permanent drainage systems for structures foundedbelow the groundwater level must be obtained fromlaboratory tests. The tests should be performed on rep-resentative specimens of backfill materials compactedin the laboratory to densities expected in the field.

(b) In situ material permeability characteristicsfor the design of construction excavation dewateringsystems can also be approximated from laboratorytests on representative undisturbed samples. Labora-tory permeability tests on undisturbed samples are

3-4

less expensive than in situ pumping tests performed inthe field; however, laboratory tests are less accurate inpredicting flow characteristics.

(6) Slake durability of shales. Some clay shalestend to slake when exposed to air and water and mustbe protected immediately after they are exposed. Theextent of slaking also governs the manner in whichthey are treated as a backfill material (para 3-3c).Slaking characteristics can be evaluated by laboratoryjar-slake tests or slake-durability tests.

(a) The jar-slake test is qualitative with six de-scriptive degrees of slaking determined from visualobservation of ovendried samples soaked in tap waterfor as long as 24 hours. The jar-slake test is not astandardized test. One version of the jar-slake test isdiscussed in FHWA-RD-78-141. Six suggested valuesof the jar-slake index are listed below:

Behavior

1 Degrades into pile of flakes or mud2 Breaks rapidly and forms many chips3 Breaks rapidly and forms few chips4 Breaks slowly and forms several fractures5 Breaks slowly and develops few fractures6 No change

Shales with values of 1 to 3 should be protectedwhen occurring in excavated slopes and compacted assoil if used for backfill.

(b) The slake-durability test is a standardizedtest that gives a quantitative description in percent byweight of material remaining intact at the conclusionof the test. Details of the test are presented inFHWA-RD-78-141.

(7) Dynamic tests for special projects. The dynam-ic analysis of projects subject to seismic or blast in-duced loading conditions requires special dynamictests on both in situ and backfill materials. Tests re-quired for dynamic analysis include: cyclic triaxialtests; in situ density measurements; and tests to deter-mine shear wave velocities, shear modulus, and damping (ER 1110-2-1806).

(8) In situ water content. The in situ water con-tent, including any seasonal variation, must be deter-mined prior to construction for materials selected foruse as backfill. Natural in situ water contents will de-termine the need for wetting or drying the backfillmaterial before placement to obtain near optimum wa-ter contents for placement and compaction. ASTM D2216 discusses the test method for determining watercontent.

3-3. Selection of backfill materials. Selec-tion of backfill materials should be based upon theengineering properties and compaction characteristicsof the materials available. The results of the field ex-ploration and laboratory test programs should provideadequate information for this purpose. The materials

may come from required excavation, adjacent borrowpits, or commercial sources. In selecting materials tobe used, first consideration should be given to themaximum use of materials from required excavation.If the excavated materials are deficient in quality orquantity, other sources should be considered. Commonbackfill having the desired properties may be found inborrow areas convenient to the site, but it may be nec-essary to obtain select backfill materials having par-ticular gradation requirements, such as filter sandsand gravels and pipe or conduit bedding materialsfrom commercial sources.

a. Primary considerations. Primary considerationsfor borrow material sources are suitability and quan-tity. Accessibility and proximity of the borrow area tothe jobsite should also be considered. The water con-tents of the borrow area material should be deter-mined seasonally, and a source of water should be lo-cated if the natural water contents are considerablyless than the required placement water content. If sev-eral sources of suitable backfill are available, other fac-tors to be considered in selecting the borrow materialsare ease of loading and spreading and the means foradding or reducing water. The need for separating ormixing soil strata from excavation or borrow sourcesshould be considered if necessary to provide reason-ably uniform engineering properties throughout thecompacted backfill.

b. Compaction characteristics. If compaction char-acteristics of the major portion of the backfill are rela-tively uniform, problems of controlling placement ofbackfill will be significantly reduced since the in-spector will be able to develop more rapidly the abilityto recognize the adequacy of the compaction proce-dures. In addition, the frequency of testing for com-paction control could be reduced. When available back-fill materials are unusual, test sections of compactedbackfill are sometimes justified to develop placementprocedures and to determine the engineering char-acteristics to be expected in field-compacted materials.

c. Workability. An important factor in choosingbackfill materials is the workability or ease with whichthe soil can be placed and compacted. Material charac-teristics that effect workability include: the ease ofadjusting water contents in the field by wetting oraeration; the sensitivity to the compaction water con-tent with respect to optimum; and the amount of com-paction effort required to achieve specified densities.

d. Types of backfill material. A discussion of themany types of backfill and their compaction character-istics is beyond the scope of this manual since soiltypes will vary on each project. However, the compac-tion characteristics of several rather broad categoriesof backfill (table 3-1) are discussed briefly. MIL-

TM 5-818-4/AFM 88-5, Chap. 5

STD-619 should be studied for more detailed informa-tion.

(1) Coarse-grained soils. Coarse-grained soilsinclude gravelly and sandy soils and range from clayeysands (SC) through the well-graded gravels of gravel-sand mixtures (GW) with little or no fines (table 3-l).They will exhibit slight to no plasticity. All of the well-graded soils falling in this category have fairly goodcompaction characteristics and when adequately com-pacted provide good backfill and foundation support.

(a) One difficulty that might arise with soils inthis category would be in obtaining good compactionof the poorly graded sands and gravels. These poorlygraded materials may require saturation with down-ward drainage and compaction with greater compac-tion effort to achieve sufficiently high densities. Also,close control of water content is required where silt ispresent in substantial amounts. Coarse-grained mate-rials compacted to a low relative density are suscepti-ble upon saturation to liquefaction under dynamicloads.

(b) For sands and gravelly sands with little or nofines, good compaction can be achieved in either theair-dried or saturated condition. Downward drainageis required to maintain seepage forces in a downwarddirection if saturation is used to aid in compaction.Consideration may be given to the economy of addingcement to stabilize moist clean sands that are particu-larly difficult to compact in narrow confined areas.However, the addition of cement may produce zoneswith greater rigidity than untreated adjacent backfilland form hard spots resulting in nonuniformstresses and deformations in the structure.

(c) Cohesionless materials are well suited forplacement in confined areas adjacent to and aroundstructures where heavy equipment is not permittedand beneath and around irregulary shaped structures,such as tunnels, culverts, utilities, and tanks. Clean,granular, well-graded materials having a maximumsize of 1 inch with 95 percent passing the No. 4 sieveand 5 percent or less passing the No. 200 sieve are ex-cellent for use in these zones. However, a danger existsof creating zones where seepage water may accumulateand saturate adjacent cohesive soils resulting in unde-sirable consolidation or swelling. In such cases, provi-sions for draining the granular backfill, sealing thesurface, and draining surface water away from thestructure are necessary.

(2) Fine-grained soils of low to medium plasticity.Inorganic clays (CL) of low to medium plasticity (grav-elly, sandy, or silty clays and lean clays) and inorganicsilts and very fine sands (ML) of low plasticity (silty orclayey fine sands and clayey silts) are included in thiscategory. The inorganic clays are relatively imperviousand can be compacted fairly easily with heavy compac-

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TM 5-818-4/AFM 88-5, Chap. 5

tion equipment to provide a good stable backfill. Soilsin the CL group can be compacted in confined areas toa fairly high degree of compaction with proper watercontent and lift thickness control. The clayey sands ofthe SC group and clayey silts of the ML group can becompacted to fairly high densities, but close control ofwater content is essential and sometimes critical, par-ticularly on the wet side of optimum water content.Some ML soils, if compacted on the dry side of opti-mum, may lose considerable strength upon saturationafter compaction. Considerable settlement may occur.Caution must therefore be exercised in the use of suchsoils as backfill, particularly below the groundwaterlevel. Also, saturated ML soils are likely to be highlysusceptible to liquefaction when dynamically loaded.Where such soils are used as backfill in seismic proneareas, laboratory tests should be conducted to deter-mine their liquefaction potential (see para. 17-5 and17-6, TM 5-818-1/AFM 88-3, Chap. 7).

(3) Rock. The suitability of rock as backfill mate-rial is highly dependent upon the gradation and hard-ness of the rock particles. The quantity of hard rockexcavated at most subsurface structure sites is rela-tively small, but select cohesionless materials may bedifficult to find or may be expensive. Therefore, exca-vated hard rock may be specified for crusher process-ing and used as select cohesionless material.

(4) Shale. Although shale is commonly referred toas rock, the tendency of some shales to breakdownunder heavy compaction equipment and slake whenexposed to air or water after placement warrants spe-cial consideration.

(a) Some soft shales break down under heavycompaction equipment causing the material to haveentirely different properties after compaction than ithad before compaction. This fact should be recognizedbefore this type of material is used for backfill. Estab-lishing the proper compaction criteria may requirethat the contractor construct a test fill and vary thewater content, lift thickness, and number of coverageswith the equipment proposed for use in the backfilloperation. This type of backfill can be used only inunrestricted open zones where heavy towed or self-pro-pelled equipment can operate.

(b) Some shales have a tendency to break downor slake when exposed to air. Other shales that appearrock-like when excavated will soften or slake and dete-riorate upon wetting after placement as rockfill. Alter-nate cycles of wetting and drying increases the slakingprocess. The extent of material breakdown determinesthe manner in which it is treated as a backfill material.If the material completely degrades into constituentparticles or small chips and flakes, it must be treatedas a soil-like material with property characteristicssimilar to ML, CL, or CH materials, depending uponthe intact composition of the parent material. Com-

3-6

plete degradation can be facilitated by alternately wet-ting, drying, and disking the material before compac-tion A detailed discussion on the treatment of shalesas a fill material is given in FHWA-RD-78-141.

(5) Marginal materials. Marginal materials arethese materials that because of either their poor com-paction, consolidation, or swelling characteristicswould not normally be used as backfill if sources ofsuitable material were available. Material consideredto be marginal include fine-grained soils of high plas-ticity and expansive clays. The decision to use mar-ginal materials should be based on economical andenergy conservation considerations to include the costof obtaining suitable material whether from a distantborrow area or commercial sources, possible distressrepair costs caused by use of marginal material, andthe extra costs involved in processing, placing, andadequately compacting marginal material.

(a) The fine-grained, highly plastic materialsmake poor backfill because of the difficulty in han-dling, exercising water-content control, and com-pacting. The water content of highly plastic fine-grained soils is critical to proper compaction and isvery difficult to control in the field by aeration or wet-ting. Furthermore, such soils are much more compres-sible than less-plastic and coarse-grained soils; shearstrength and thus earth pressures may fluctuate be-tween wide limits with changes in water content; andin cold climates, frost action will occur in fine-grainedsoils that are not properly drained. The only soil typein this category that might be considered suitable asbackfill is inorganic clay (CH). Use of CH soils shouldbe avoided in confined areas if a high degree of com-paction is needed to minimize backfill settlement or toprovide a high compression modulus.

(b) The swelling (and shrinking) characteristicsof expansive clay vary with the type of clay mineralpresent in the soil, the percentage of that clay mineral,and the change in water content. The active clay min-erals include montmorillonite, mixed-layer combina-tions of montmorillonite and other clay minerals, andunder some conditions chlorites and vermiculites.Problems may occur from the rise of groundwater,seepage, leakage, or elimination of surface evaporationthat may increase or decrease the water content ofcompacted soil and lead to the tendency to expand orshrink. If the swelling pressure developed is greaterthan the restraining pressure, heave will occur andmay cause structural distress. Compaction on the wetside of optimum moisture content will produce lowermagnitudes of swelling and swell pressure. Expansiveclays that exhibit significant volume increases shouldnot be used as backfill where the potential for struc-tural damage might exist. Suitability should be basedupon laboratory swell tests (TM 5-818-1/AFM 88-3,Chapter 7).

(c) Additives, such as hydrated lime, quicklime,and fly ash, can be mixed with some highly plasticclays to improve their engineering characteristics andpermit the use of some materials that would otherwisebe unacceptable. Hydrated lime can also be mixed withsome expansive clays to reduce their swelling char-acteristics (TM 5-818-1/AFM 88-3, Chapter 7). Laboratory tests should be performed to determine theamount of the additive that should be used and thecharacteristics of the backfill material as a result ofusing the additive. Because of the complexity of soil-additive systems and the almost complete empiricalnature of the current state of the art, trial mixes mustbe varified in the field by test fills.

(6) Commercial by-products. The use of commer-cial by-products, such as furnace slag or fly ash asbackfill material, may be advantageous where suchproducts are locally available and where suitable nat-ural materials cannot be found. Fly ash has been usedas a lightweight backfill behind a 25-foot-high walland as an additive to highly plastic clay. The suitabil-ity of these materials will depend upon the desirablecharacteristics of the backfill and the engineeringcharacteristics of the products.3-4. Processing of backfill materials. Theconstruction of subsurface structures often requiresthe construction of elements of the structure within orupon large masses of backfill. The proper functioningof these elements are often critically affected by ad-verse behavioral characteristics of the backfill. Be-havioral characteristics are related to material type,water content during compaction, gradation, and com-paction effort. While compaction effort may be easily

TM 5-818-4/AFM 88-5, Chap. 5

controlled during compaction, it is difficult to controlmaterial type, water content, and gradation of thematerial as it is being placed in the backfill; controlcriteria must be established prior to placement.

a. Material type. Backfill material should consist ofa homogeneous material of consistent and desirablecharacteristics. The field engineer must ensure thatonly the approved backfill material is used and thatthe material is uniform in nature and free of anyanomalous material such as organic matter or claypockets. Stratified material should be mixed prior toplacing to obtain a uniform blend. Excavated materialto be used as backfill should be stockpiled according toclass or type of material.

b. Water content. While water content can be ad-justed to some extent after placing (but before com-pacting), it is generally more advantageous to adjustthe water content to optimum compaction conditionsbefore placing. Adjustment of water content can be ac-complished by aeriation (disking or turning) or sprin-kling the material in 12- to 18- inch layers prior toplacing or stockpiling. If the material is stockpiled,provisions should be made to maintain a constantmoisture content during wet or dry seasons.

c. Ensuring gradation. Some backfill materials con-sisting of crushed rock, gravel, or sand require limita-tions on maximum and minimum particle-size orgradation distributions. Where materials cannot be lo-cated that meet gradation criteria, it may be advanta-geous to require processing of available material bysieving to obtain the desired gradation.

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TM 5-818-4/AFM 88-5, Chap. 5

CHAPTER 4

EARTHWORK: EXCAVATION AND PREPARATION FOR FOUNDATIONS

4-1. Excavation.a. General. In general, excavation for subsurface

structures will consist of open excavation and shaftand tunnel excavation. Where excavation to greatdepths is required, a variety of soils and rock may beencountered at a single site. Soils may range through awide spectrum of textures and water contents. Rockencountered may vary from soft rock, very similar to afirm soil in its excavation requirements, to extremelyhard rock requiring extensive blasting operations forremoval. Groundwater may or may not be present. Thegroundwater conditions and the adequacy of ground-water control measures are important factors in exca-vation, in maintaining a stable foundation, and inbackfilling operations. The extent to which ground-water can be controlled also influences the slopes towhich the open excavation can be cut, the bracing re-quired to support shaft and tunnel excavation, and thehandling of the excavated material.

b. Good construction practices, and problems. Amajority of the problems encountered during excava-tion are related to groundwater conditions, slope sta-bility, and adverse weather conditions. Many of theproblems can be anticipated and avoided by precon-struction planning and by following sound construc-tion practices.

(1) Groundwater. Probably the greatest source ofproblems in excavation operations is groundwater. Ifthe seepage of groundwater into an excavation is ade-quately controlled, other problems will generally beminor and can be easily handled. Several points shouldbe recognized that, if kept in mind, will help to reduceproblems attributable to groundwater. In some in-stances, groundwater conditions can be more severethan indicated by the original field exploration investi-gation since field explorations provide informationonly for selected locations and may not provide a truepicture of the overall conditions.

(a) If groundwater seepage begins to exceed thecapacity of the dewatering system, conditions shouldnot be expected to improve unless the increased flow isknown to be caused by a short-term condition such asheavy rain in the area. If seepage into the excavationbecomes excessive, excavation operations should behalted until the necessary corrective measures are de-termined and effected. The design and evaluation ofdewatering systems require considerable experience

that the contractor or the contracting office often donot possess, and the assistance of specialists in thisfield should be obtained.

(b) Groundwater without significant seepageflow can also be a problem since excess hydrostaticpressures can develop below relatively imperviousstrata and cause uplift and subsequent foundation orslope instability. Excess hydrostatic pressures can alsooccur behind sheet pile retaining walls and shoringand bracing in shaft and tunnel excavations. Visual ob-servations should be made for indications of trouble,such as uncontrolled seepage flow, piping of materialfrom the foundation or slope, development of soft wetareas, uplift of ground surface, or lateral movements.

(c) Accurate daily records should be kept of thequantity of water removed by the dewatering systemand of the piezometric levels in the foundation and be-neath excavation slopes. Separate records should bekept of the flow pumped by any sump-pump system re-quired to augment the regular dewatering system tonote any increase of flow into the excavation. Flow-meters or other measuring devices should be installedon the discharge of these systems for measurementpurposes (TM 5-818-5/NAVFAC P-418/AFM 88-5,Chap. 6). These records can be invaluable in evaluatingChanged Condition claims submitted by the contrac-tor. The contractor should be required to have stand-by equipment in case the original equipment breaksdown.

(2) Surface water. Sources of water problemsother than groundwater are surface runoff into the ex-cavation and snow drifting into the excavation. A pe-ripheral, surface-drainage system, such as a ditch andberm, should be required to collect surface water anddivert it from the excavation, In good weather there isa tendency for the contractor to become lax in main-taining this system and for the inspection personnel tobecome lax in enforcing maintenance. The result canbe a sudden filling of the excavation with water duringa heavy rain and consequent delay in construction. Thesurface drainage system must be constantly main-tained until the backfill is complete. Drifting snow is aseasonal and regional problem, which can best be con-trolled by snow fences placed at strategic locationsaround the excavation,

(3) Slope integrity. Another area of concern dur-ing excavation is the integrity of the excavation

4 - l

TM 5-818-4/AFM 88-5, Chap. 5

slopes. The slopes may be either unsupported or sup-ported by shoring and bracing. The lines and grades in-dicated in the plans should be strictly adhered to. Thecontractor may attempt to gain additional workingroom in the bottom of the excavation by steepeningthe slopes; this change in the plans must not be al-lowed.

(a) Where shoring and bracing are necessary toprovide a stable excavation, and the plans and specifi-cations do not provide details of these requirements,the contractor should be required to submit the plansin sufficient detail so that they can be easily followedand their adequacy checked. The first principle of ex-cavation stabilization, using shoring and bracing, isthat the placing of supports should proceed with exca-vation. The excavation cut should not be allowed toyield prior to placing of shoring and bracing since thelateral pressures to be supported would generally beconsiderably greater after yield of the unshored cutface than if no movement had occurred prior to place-ment of the shoring. Excavation support systems arediscussed in TM 5-818-1/AFM 88-3, Chapter 7. Allsafety requirements for shoring and bracing as con-tained in EM 385-1-1 should be strictly enforced.

(b) The inspector must be familiar with stockpil-ing requirements regarding the distance from thecrest of the excavation at which stockpiles can be es-tablished and heavy equipment operated without en-dangering the stability of the excavation slopes. Hemust also know the maximum height of stockpile orweight of equipment that can be allowed at this dis-tance.

(c) Excessive erosion of the excavation slopesmust not be permitted. In areas subject to heavy rain-fall, it may be necessary to protect excavation slopeswith polyethylene sheeting, straw, silt fences, or byother means to prevent erosion. Excavation slopes forlarge projects that will be exposed for several seasonsshould be vegetated and maintained to prevent ero-sion.

(4) Stockpiling excavated material. Generally,procedures for stockpiling are left to the discretion ofthe contractor. Prior to construction, the contractormust submit his plans for stockpiling to the contract-ing officer for approval. In certain cases, such as wherethere are different contractors for the excavation andthe backfill phases, it may be necessary to include thedetails for stockpiling operations in the specifications.In either case, it is important that the stockpiling pro-cedures be conducive to the most advantageous use ofthe excavated materials.

(a) As the materials are excavated, they shouldbe separated into classes of backfill and stockpiled ac-cordingly. Thus the inspection personnel controllingthe excavation should be qualified to classify the mate-rial and should be thoroughly familiar with backfill re-

4-2

quirements. Also, as the materials are placed in stock-piles, water should be added or the materials should beaerated as required to approximate optimum watercontent for compaction. Field laboratory personnel canassist in determining the extent to which this is neces-sary. The requirements of shaping the stockpile todrain and sealing it against the entrance of undesir-able water by rolling with spreading equipment orcovering with polyethylene sheeting should be en-forced. This step is particularly important for cohesivesoils that exhibit poor draining characteristics andtend to remain wet if once saturated by rains. Stock-piles must be located over an area that is large enoughto permit processing and where they will not interferewith peripheral drainage around the excavation andwill not overload the slopes of the excavation.

(b) In cases where significant energy and costsaving can be realized, special stockpiling require-ments should be implemented. An example would be alarge project consisting of a number of excavation andbackfilling operations. The excavation material fromthe first excavation could be stockpiled for use as back-fill in the last excavation. The material from the inter-mediate excavations could in turn be immediately usedas backfill for the first, second, etc., phases of the proj-ect and thereby eliminate double handing of excavatedbackfill for all but the first-phase excavation.

(5) Protection of exposed material. If materialsthat are exposed in areas, such as walls of a silo shaft,foundation support, or any other area against whichconcrete will be placed, are susceptible to deteriorationor swell when exposed to the weather, they should beproperly protected as soon after exposure as possible.Depending on the material and protection require-ments, this protection may be pneumatic concrete, as-phalt spray, or plastic membrane (TM 5-818-1/AFM88-33, Chap. 7). In the case of a foundation area, thecontractor is required to underexcavate leaving a coverfor protection, as required, until immediately prior toplacement of the structure foundation. Any frost-sus-ceptible materials encountered during excavationshould be protected (para 2-3h (3) and (4)) if the exca-vation is to be left open during an extended period offreezing weather.

(6) Excavation record. As the excavation pro-gresses, the project engineer should keep a daily recordof the type of material excavated and the progressmade. This record would be of value if subsequentclaims of Changed Conditions are made by the con-tractor.

4-2. Foundation preparation.a. General. In this manual, preparation applies to

foundations for backfill as well as those for structuresto be placed in the excavation. Generally, if proper ex-cavation procedures have been followed, very little ad-

ditional preparation will be required prior to backfillplacement.

b. Good construction practices, and problems. Asmentioned previously, the problems associated withfoundation preparation are greatly reduced by follow-ing such proper excavation procedures as maintaininga dry excavation and planning ahead. The principles ofgood foundation preparation are simple, but enforcingthe provisions of the specifications concerning thework is more difficult. Inspection personnel mustrecognize the importance of this phase of the worksince, if not properly controlled, problems can result.

(1) It is most important that a stable foundationbe provided. Thus it may be necessary, particularly inthe case of sensitive fine-g-rained materials, to requirethat the final excavation for footings be carefully donewith hand tools and that no equipment be allowed tooperate on the final cut surface. To provide a workingplatform on which to begin backfill placement on thesesensitive materials, it may be necessary to place aninitial layer of granular material.

(2) If the foundation is to be supported on rock,the soundness of the exposed rock should be checkedby a slaking test (soaking a piece of the rock in waterto determine the resulting degree of deterioration(para 3-2b (6)) and visual observation to determine ifthe rock is in a solid and unshattered condition. If re-moval of rock below the foundation level is required,the space should be filled with concrete. A qualifiedgeological or soils en