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 5-8184/AFM 85, Chap. 5TECHNICAL MANUAL HEADQUARTERSNo. 5-818-4 DEPARTMENTS OF THE ARMYAIR FORCE MANUAL AND THE AIR FORCENo. 88-5, CHAPTER 5 WASHINGTON, DC, 1 June 1983

BACKFILL FOR SUBSURFACE STRUCTURESParagraph Page

CHAPTER 1. INTRODUCTIONBackground ..................................... .............................................. 1-1 1-1Purpose and scope ................................ ....................................... 1-2 1-1

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

General ....................................................................................... 2-1 2-1Effect of excavation and structural configuration on back-

fill operations .......................................................................... 2-2 2-1Backfill problem areas.................................................................... 2-3 2-4Instrumentation............................................................................... 2-4 2-9Optimum cost construction............................................................. 2-5 2-9

3. EVALUATION, DESIGN, AND PROCESSING OF BACKFILLMATERIALS

General ....................................................................................... 3-1 3-1Evaluation of backfill materials ....................................................... 3-2 3-1Selection of backfill materials ......................................................... 3-3 3-4Processing of backfill materials ...................................................... 3-4 3-7

4. EARTHWORK: EXCAVATION AND PREPARATIONFOR FOUNDATIONS

Excavation...................................................................................... 4-1 4-1Foundation preparation .................................................................. 4-2 4-2

5. BACKFILL OPERATIONSPlacement of backfill ...................................................................... 5-1 5-1Installation of instruments .............................................................. 5-2 5-4Postconstruction distress ............................................................... 5-3 5-5

6. SPECIFICATION PROVISIONSGeneral ....................................................................................... 6-1 6-1Excavation...................................................................................... 6-2 6-1Foundation preparation .................................................................. 6-3 6-2Backfill operations .......................................................................... 6-4 6-2

7. CONSTRUCTION CONTROLGeneral ....................................................................................... 7-1 7-1Corps acceptance control organization.......................................... 7-2 7-1Excavation control techniques........................................................ 7-3 7-3Foundation preparation control techniques.................................... 7-4 7-3Backfill quality acceptance control ................................................. 7-5 7-3

APPENDIX A. REFERENCES............................................................................... A-1B. FUNDAMENTALS OF COMPACTION, FIELD COMPACTION TEST

METHODS, AND FIELD MOISTURE-DENSITY TEST METHODS B-1C. BIBLIOGRAPHY............................................................................. C-1

LIST OF FIGURESNumber Title2-1 Open backfill zones2-2 Confined backfill zones2-3 Complex structures2-4 Excavation subject to bottom heave2-5 Excess lateral pressure against vertical walls induced by compaction7-1 Acceptance rejection scheme for a backfill areaB-1 Compaction CurveB-2 Typical compaction test curves

*This manual supersedes TM 5-818-4/AFM 88-5, Chapter 5, 7 June 1968.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

LIST OF TABLES

Number Title3-1 Typical engineering properties of compacted materials5-1 Summary of compaction criteria

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TM 5-818-41AFM 88-5, Chap. 5

CHAPTER 1

INTRODUCTION

1-1. Background. Military facilities for the Army andAir Force have included construction of buildings andother structures partially below ground surface. In morerecent years, missile launching and support structures,fallout and blast-protection shelters, and command-control centers have been constructed below groundsurface. Many of these structures were constructedusing cut-and-cover procedures and required backfillingwithin confined areas using various types of soil.

a. Numerous deficiencies in backfilling operationsoccurred in some of the earlier missile-launchconstruction programs and caused conditions thatjeopardized the proper functioning of those structures.Measures to correct deficiencies were both timeconsuming and costly. It was recognized that criticalareas must be delineated and the causes of thedeficiencies be determined and corrected.

b. Measures were taken to alleviate the overallback- filling problems. These measures wereprogressive modification of design and configuration ofstructures, more detailed instructions to the constructionperson- nel, and close control during construction toensure that proper construction practices were being fol-lowed.

(1) Some of the problem areas were eliminatedby modification of design and configuration of structuresto allow easier placement of backfill and to permit accessof compaction equipment so that required densities couldbe achieved.

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

(3) Inspector training programs were conductedto point out critical areas and emphasize proper backfillprocedures and the need for continuous surveillance andclose control.

c. The advent of energy efficient structures, partiallyembedded below ground level, had increased the use ofbackfill. In addition, the ever increasing need for fuelconservation requires maximum use of all excavated oronsite materials for backfill to reduce fuel needed forhauling in better materials from offsite. Thus, innovativeplanning and design and good construction control usingrapid check tests are imperative for all backfilloperations.

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

a. The greatest deficiencies in earthwork operationsaround deep-seated or subsurface structures occurbecause of improper backfilling procedures andinadequate construction control during this phase of thework. Therefore, primary emphasis in this manual is onbackfilling procedures. Design and planningconsiderations, evaluation and selection of materials,and other phases of earthwork construction arediscussed where pertinent to successful backfilloperations.

b. Although the information in this manual isprimarily applicable to backfilling around large andimportant deep-seated or buried structures, it is alsoapplicable in varying degrees to backfilling operationsaround all structures, including conduits.

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

CHAPTER 2PLANNING AND DESIGN OF STRUCTURES AND

EXCAVATIONS TO ACCOMMODATE BACKFILL OPERATIONS

2-1. General. Many earthwork constructionproblems can be eliminated or minimized through properdesign, thorough planning, and recognition of problemareas effecting backfill operations. Recognition andconsideration must be given in planning to designfeatures that will make backfilling operations less difficultto accomplish. Examples of problem areas and howforethought in design and planning can help to eliminatebackfill deficiencies are presented in the followingparagraphs.

2-2. Effect of excavation and structuralconfiguration on backfill operations. Some of theproblems encountered in earthwork construction arerelated to the excavation and the configuration of thestructures around which backfill is to be placed. It is thedesigner’s responsibility to recognize these problemsand to take the necessary measures to minimize theirimpact on the backfill operations.

a. Open zones. An open zone is defined as abackfill area of sufficient dimensions to permit theoperation of heavy compaction equipment withoutendangering the integrity of adjacent structures aroundwhich compacted backfill operations are conducted.Figure 2-1 shows examples of open zones. In thesezones where large compaction equipment can operate, itis generally not too difficult to obtain the desired density ifappropriate 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 compactionequipment. Generally, a working space of at least 12feet between structure walls and excavation slopes andat least 15 feet between structures is necessary forheavy equipment to maneuver. In addition tomaneuvering room, the designer must also consider anyadverse loading caused by the operation of heavyequipment too close to structure walls, as discussed inparagraph 2-3d.

b. Confined zones. Confined zones are defined asareas where backfill operations are restricted to the useof small mechanical compaction equipment (fig.2-2) either because the working room is limited orbecause heavy equipment (fig. 2-1) would imposeexcessive soil pressures that could damage thestructure. Most deficiencies in compacted backfillaround subsurface structures have occurred

in confined zones where required densities are difficult toachieve because of restricted working room andrelatively low compaction effort of equipment that is toolightweight. The use of small equipment to achieverequired compaction is also more expensive than heavyequipment since thinner lifts are required. However,because small compaction equipment can operate inspaces as narrow as 2 feet in width, such equipment isnecessary to achieve the required densities in someareas of most backfill projects. Therefore, the designershould plan structure and excavation areas to minimizethe use of small compaction equipment.

c. Structure configuration. The designer familiarwith backfilling operations can avoid many problemsassociated with difficult to reach confined zones, whichare created by structural shapes obstructing theplacement and compaction of backfill, by considering theimpact of structural shape on backfill operations. In mostcases, structural shapes and configurations that facilitatebackfill operations can be used without significantlyaffecting the intended use of the structure.

(1) Curved bottom and wall structures. Areasbelow the spring line of circular, elliptical, and similarshaped structures are difficult to compact backfill againstbecause compaction equipment cannot get under thespring line. If possible, structures should be designedwith continuously curved walls and flat floors such as inan igloo-shaped structure. For structures where acurved bottom is required to satisfy the intendedfunction, it may be advisable for the designer to specifythat a template shaped like the bottom of the structurebe used to guide the excavation below the spring line sothat uniform foundation support will be provided.

(2) Complex structures. Complex structureshave variable shaped walls and complex configurationsin plan and number of levels. These structures can alsobe simple structures interconnected by access shafts,tunnels, and utility conduits. Because of their irregularshapes and configurations the different types ofstructures 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

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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 multilevelstep structure (fig. 2-3a) is not particularly difficult tocompact backfill around, at least for the first level, thecompaction of backfill over the offset structure willgenerally require the use of small equipment. Smallequipment will also be required for compaction of backfillaround and over the access corridor and between thetwo chambers (fig. 2-3b). Where possible, the designshould accommodate intended functions into structureswith uniformly shaped walls and a simple 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 it wouldbe 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 haphazardmaze of pipes and conduits in the backfill. Utility linesshould be run either horizontally or vertically whereverpossible. Plans for horizontally run appurtenances, suchas utility lines, access tunnels, and blast-delay tubing,should be coordinated with the excavation plans so thatwherever feasible these appurtenances can besupported by undisturbed soils rather than by compactedbackfill.

e. Excavation plans. The excavation plans shouldbe developed with the backfill operations and thestructure configurations in mind. The excavation and allcompleted structures within the excavation should beconducive to good backfill construction procedures, andaccess should be provided to all areas so thatcompaction equipment best suited to the size of the areacan be used. The plans for excavation should alsoprovide for adequate haul roads and ramps. Positiveexcavation slopes should be required in all types of soil

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TM 5-818-4/AFM 85, Chap. 5

Figure 2-2. Confined backfield zones.

Figure 2-3. Complex structures.

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

deposits to facilitate compaction of backfill against theslope and to ensure good bond between the backfill andthe excavation slopes. Loose material should beremoved 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 ensure thatuniform, adequate support is provided at the foundationlevel of important structures. Generally, foundationsconsisting of part backfill and part undisturbed materialsdo not provide uniform bearing and should be avoidedwherever possible. The foundation should beoverexcavated where necessary, and backfilled withcompacted select material to provide uniform support forthe depth required for the particular structure. Wherecompacted backfill is required beneath a structure, theminimum depth specified should be at least 18 inches.

g. Thin-walled metal structures. Thin-walled,corrugated metal structures are susceptible todeflections of structural walls when subjected to backfillloads. Adverse deflections can be minimized byplanning backfill operations so that compacted backfill isbrought up evenly on both sides of the structure toensure uniform stress distribution. Temporary surchargeloads applied to the structure crown may also berequired to prevent vertical distortions and inwarddeflection at the sides.

2-3. Backfill problem areas. Other features thathave the potential to become problem areas arediscussed in the following paragraphs. These potentialproblem areas have to be considered during the planningand design phases to minimize deficiencies in structureperformance associated with backfill placement and tomake 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 specifyallowable tolerances in differential settlement and ensurethat differential settlement is minimized and/oraccommodated. Settlement analysis procedures areoutlined in TM 5-818-1/AFM 88-3, Chapter 7. Seeappendix A, References.

(1) Critical zones. Critical backfill zones arethose immediately beneath most structures.Consolidation and swelling characteristics of backfillmaterials should be thoroughly investigated so thatmaterials having unfavorable characteristics will not beused in those zones. Some settlement can be expectedto take place, but it can be minimized by requiring ahigher than normal compacted density for the backfill.Cohesive backfill compacted at a water content as littleas 3 to 4 percentage points below optimum may result inlarge settlements caused by collapse of nonswelling soil

material or heave of swelling materials upon saturationafter construction. Compacting cohesive backfill materialat optimum water content or slightly on the wet side ofoptimum generally will reduce the amount of settlementand swelling that would occur. The reduction should beconfirmed by consolidation and swell tests on compactedspecimens (para 3-2b(4)).

(2) Service conduits. Settlement within thebackfill around structures will also occur. A properdesign will allow for the estimated settlement asdetermined from studies of consolidation characteristicsof the compacted backfill. Where service conduits,access corridors, and similar facilities connect to thestructure oversize sleeves, flexible connections andother protective measures, as appropriate, may be usedto prevent damage 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 the lowerlevel offset component must also support the volume ofbackfill over that part of the structure. Measures must betaken to ensure that the proper functioning of allelements is not hampered by differential settlement. Theincreased cost of proper design and construction whereunusual or difficult construction procedures are requiredis insignificant when compared with the cost of thestructure. The cost of remedial measures to correctdeficiencies caused by improper design and constructionusually will be greater than the initial cost required toprevent the deficiencies.

(4) Downdrag. In addition to conventional serviceloads, cut and cover subsurface structures aresusceptible to downdrag frictional forces between thestructure and the backfill that are caused by settlementof the backfill material adjacent to and around thestructure. Downdrag loads can be a significantproportion of the total vertical load acting on the structureand must be considered in the structure settlementanalysis. Structure-backfill friction forces may alsogenerate significant shear forces along the outer surfaceof structures with curve-shaped roofs and walls. Themagnitude of the friction forces depends upon the type ofbackfill, roughness of the structure’s surface, andmagnitude of earth pressures acting against thestructure. Techniques for minimizing downdrag frictionforces generally include methods that reduce thestructure surface roughness such as coating thestructure’s outer surface with asphalt or sandwiching alayer of polyethylene sheeting between the structure’souter surface and fiberboard (blackboard) panels.Backfill settlement and associated downdrag can also beminimized

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

Figure 2-4. Excavation subject to bottom heave.

by requiring higher backfill densities adjacent to thestructure.

b. Groundwater. Groundwater is an importantconsideration in planning for construction of subsurfacestructures. If seepage of groundwater into theexcavation is not adequately controlled, backfillingoperations will be extremely difficult. The groundwaterlevel must be lowered sufficiently (at least 2 to 3 feet forgranular soils and as much as 5 to 10 feet for fine-grained soils below the lowest level of backfilling) so thata firm foundation for backfill can be established. If thelevel is not lowered, the movement of hauling orcompaction equipment may pump seepage waterthrough the backfill, or the initial backfill layers may bedifficult to compact because of an unstable foundation.Since the proper water content of the backfill is essentialfor achieving proper compaction, prevention ofgroundwater seepage into the excavation duringbackfilling operations is mandatory. Figure 3-14 of EM1110-2-1911 shows a method for dewatering rockfoundations.

(1) The contractor is generally responsible forthe design, installation, and operation of dewateringequipment. The Corps of Engineers is responsible forspecifying the type of dewatering system and evaluatingthe contractor’s proposed dewatering plan. Since thedual responsibility of the contractor and the Corps relieson a thorough understanding of groundwater conditions,inadequate dewatering efforts can be mini

minimized by adequate planning and implementation ofgroundwater investigations.

(2) The possibility of hydraulic heave in cohesivematerial must also be investigated to ensure stability ofthe excavation floor. Hydraulic heave may occur wherean excavation overlies a confined permeable stratumbelow the groundwater table (fig. 2-4). If the upwardhydrostatic pressure acting at the bottom of the confininglayer exceeds the weight of overburden between thebottom of the excavation and the confining layer, thebottom of the excavation will rise bodily even though thedesign of the dewatering system is adequate for controlof groundwater into the excavation. To prevent heave,the hydrostatic pressure beneath the confined stratummust be relieved.

(3) Subsurface structures located in part orwholly below the groundwater table require permanentprotection against groundwater seepage. The type ofprotection may range from simple impermeable barriersto complex permanent dewatering systems.

(4) Dewatering and groundwater controlprocedures are described in TM 5-818-5/NACFAC P-418/AFM 88-5, Chapter 6.

c. Gradation and filter criteria for drainage materials.Groundwater control is often accomplished by ditchespositioned to intercept the flow of groundwater and filledwith permeable granular material. The water is generallycollected in perforated pipes located at the bottom of theditch and pumped to a suitable

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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 material isdependent upon the gradation of the material that isbeing drained. Drainage of silts and clays usuallyrequires a graded filter made up of several layers ofgranular material with each layer having specificrequirements for maximum grain size and gradation.Details on the design of filter drains are presented in TM5-818-5/NAVFAC P-418/AFM 88-5, Chapter 6.

(1) Selected material. If materials at the jobsitedo not meet the designed filter requirements, selectmaterial must be purchased from commercial sourcesand shipped to the jobsite. Filter material must bestockpiled according to gradation. For graded filtersystems, the materials must be placed with care tominimize 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 economically available, asingle wrap of filter cloth around a pipe may be used inlieu of a coarser backfill. When available granular filtermaterial is too coarse to satisfy filter criteria for theprotected soil, a single layer of filter cloth may be usedadjacent to the protected soil. To reduce the chance ofclogging, no filter cloth should be specified with an openarea less than 4 percent and or equivalent opening size(EOS) of less than the No.100 sieve (0.0059 inch). Acloth with openings as large as allowable should bespecified to permit drainage and prevent clogging.Additional information on airfield drainage is contained inTM 5-820-2/AFM 88-5, Chapter 2.

(3) Other uses. Filter cloth can also provideprotection for excavated slopes and serve as a filter toprevent 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 filter clothused to protect the excavated slope served as a filteragainst piping of the natural silty clay under seepagegradients out of the excavated slope after the coarsegravel 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 presdsuredistribution of the backfill materials, subsurface cut-andcover structures may also be subjected to surchargeloads caused by heavy equipment operating close to thestructure and by increased permanent lateral earthpressures caused by compaction of backfill material withheavy equipment. Procedures for predicting normalearth pressures associated with the effective pressure ofbackfill materials are discussed in TM

5-818-1/lAFM 88-3, Chapter 7, EM 1110-2-2902, and EM1110-2-2502.

(1) Exact solutions for surcharge earth pressuresgenerated by heavy equipment (or other surcharge loads)do not exist. However, approximations can be made usingappropriate theories of elasticity such as Boussinesq’sequations for load areas of regular shape or Newmark’scharts for irregular shaped load areas as given in NAVFACDM-7. As a conservative guide, heavy-equipmentsurcharge earth pressures may be minimized by specifyingthat heavy compaction equipment maintain a horizontaldistance from the structure equivalent to the height of thebackfill above the structure’s foundation.

(2) Compaction-induced earth pressures can cause asignificant increase in the permanent lateral earth pressuresacting on a vertical wall of a structure (fig. 2-5a). Thisdiagram is based on the assumption that the equipment canoperate to within 6 inches of the wall. Significant reductionsin lateral pressures occur as the closest allowable distanceto the wall is increased (fig. 2-5b). For an operatingdistance 5 feet from the wall, the induced horizontal earthpressure is much less than that caused by the backfill. Themagnitude of the increase in lateral pressure is dependent,among other factors, on the effective weight of thecompaction equipment and the weight, earth pressurecoefficient, and Poisson’s ratio of the backfill material.Compaction-induced earth pressures against walls are alsodescribed in TM 5-818-1/AFM 88-3, Chapter 7, and EM1110-2-2502.

(3) The designer must evaluate the economics ofthe extra cost of structures designed to withstand veryclose-in operation of heavy compaction equipment versusthe extra cost associated with obtaining requiredcompaction of backfill in thin lifts with smaller compactionequipment. A more economical alternative might be tospecify how close to the walls different weights ofcompaction equipment can be operated.

(4) One method of reducing lateral earth pressuresbehind walls has been to use about 4 feet of uncompactedgranular (sand or gravel) backfill above the base of the wall.Soil backfill can then be compacted in layers above thegranular backfill. Compression of the granular materialprevents the buildup of excessive lateral pressures againstthe wall.

e. Structural backfill. Structural backfill is defined asthe compacted backfill required over and around a structureto prevent damage from heavy equipment operating over ornear the structure. This backfill must be compacted usingsmall compaction equipment, such as mechanical rammersor vibratory-plate compactors, or intermediate sizeequipment such as walk-behind, dual-drum vibratory rollers.The horizontal and vertical distances from the structure forwhich structural backfill is required should be deter

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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. A2-foot cover over small utility conduits and pipes isadequate protection where proper bedding procedures arefollowed. The minimum cover requirements over largerdiameter (6 inches or greater), rapid and flexible pipes arepresented in appendix II of TM 5-8204/AFM 88-5, Chapter4.

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 include thestrength of the materials, groundwater conditions, and anysurcharge load that may be imposed as the result ofstockpiles being placed or equipment operating near thecrest of the excavation. Slope stability evaluationprocedures are described in TM 5-818-1/AFM 88-3,Chapter 7. Shoring and bracing should be used to supportexcavation slopes where it is not feasible to excavate tostable slopes (TM 5-818-1/AFM 88-3, Chapter 7).Requirements for shoring and bracing safety are presentedin EM 385-1-1.

g. Bedding for curved-bottom structures. Foundationsfor pipes, conduits, access tunnels, fuel and water storagetanks, and other curved-bottom structures constructedwithin the backfill are considered critical zones that requirespecial attention. Any bedding material used should befree of stones or other large particles that would lead tononuniform bearing. One of the most important functionsof any bedding procedure is to provide firm support alongthe full length of the structure. For areas where it is difficultto perform field density control tests because of limitedworking space, a procedure to ensure that propercompaction is obtained must be employed. Severalmethods of obtaining adequate bedding are discussed inparagraph 5- 1c (2).

h. Cold weather construction. Cold weather can havea very adverse effect on backfilling operations and cancause considerable delay. If possible, the project shouldbe planned to complete backfilling operations prior to anyextended period of freezing temperatures. The contractorand the resident engineer must keep up to date withweather data so that the contractor can plan the equipmentand construction force required to meet the constructionschedule and to protect the work already accomplished.

(1) The designer must establish definite limitationsand requirements regarding placement of backfill when theambient temperature is below freezing. Most inorganicsoils, particularly silts and lean clays, containing 3 percent,by weight, or more of particles finer than 0.02 millimetre indiameter are frost susceptible. Such soils, when frozen inthe presence of an

available source of water, develop segregated ice in theform of lenses, layers, veins, or masses commonly, but notalways oriented normal to the direction of heat loss. Theexpansion of the soil mass resulting from ice segregation iscalled frost heave. Frost heave of soil under and againststructures can cause detrimental effects, which can becompounded during subsequent thawing by differentialmovement, loss of density, and loss of shear strength.Soils of this type should not be placed during orimmediately prior to freezing temperatures and must not beplaced in critical areas. Nonfrost susceptible soils shouldbe used at the finished grade to the depth of frostpenetration when the finished grade serves as a load-bearing surface.

(2) Additives, such as calcium chloride, can beused to lower the freezing temperature of soil water, butsuch additives will ordinarily also change the compactionand water content requirements. Therefore, additives mustnot be used without prior investigation to determine theireffect on compaction and water content requirements. Drysand or sand-gravel mixtures can be placed satisfactorilywhen temperatures are below freezing without seriouseffects.

(3) Protection must be provided for in-placepermanent backfill in critical areas, such as those aroundand under structures and embedded items already placed.To preclude structural damage from possible frost heave,backfill materials around such structures should beinsulated with a protective covering of mulch, hay, or straw.In some instances, loose lifts of soil can be used forinsulation. However, rock or sand is too porous to providesufficient insulation and too permeable to resist waterpenetration. If soil is to be used as an insulating material, amaterial completely foreign to the permanent fill, such asstraw or building paper, should be laid down prior toplacement of the insulation fill so that there will be amarked distinction between the permanent and thetemporary insulation fills. In this way, when the insulationfill is removed, the stripping limits can be readily discerned.

(4) Flooding of the excavation has also been usedsuccessfully to prevent frost penetration of the inplacepermanent backfill. However, consideration must be givento possible detrimental effects of saturating in-place backfilland the delay of removing the water at the beginning of thenext construction season if it freezes into a solid mass ofice.

(5) Concrete walls and floors of completedstructures 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 because ofadverse freezing weather. Reinforcing steel protrudingfrom a partially completed structure will conduct coldthrough the concrete and increase the rate and depth offrost penetration beneath the structure.

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

Every effort should be made to schedule construction sothat this condition will be kept to a minimum, and protectionmust be required where necessary.

i. Seismic zones. The design considerations forsubsurface structures subjected to dynamic loads causedby seismic activity or explosive devices are beyond thescope of this manual. Design details are provided in TM 5-818-1/AFM 88-3, Chapter 7, and ER 1110-2-1806. Specificproblems relating to backfill operations are primarily limitedto possible potential for dynamically induced liquefaction.Certain materials are particularly susceptible to liquefaction;these include saturated gravels, sands, silts, and clayeysands and gravel. Where these materials are used asbackfill, the potential for liquefaction can be minimized byrequiring a high degree of compaction, particularly in criticalareas such as beneath footings and under the spring line ofcurved wall structures. The requirements for materialssusceptible to liquefaction are discussed in paragraph 3-3d.

2-4. Instrumentation. For important structures ofunique design or for structures where the potential forpostconstruction distress exists, instrumentation of thestructure should be considered. The instrumentationprogram may include monitoring the amount and rate ofsettlement, movement of retaining walls and otherstructural elements, development of stresses within thestructure, and development of hydrostatic and earthpressures against the structure. Analysis of the data willfurnish a check on design assumptions and indicate whatmeasures must be taken to relieve or correct undesirableconditions before distress develops. Information of thisnature can also be of significant value in future design andconstruction.

a. Requirements. Specific requirements forinstruments are ruggedness, reliability over the projectedservice life, and simplicity of construction, installation, andobservation. Other important considerations in selectingthe type of instruments are cost and availability.Manufacturers of devices considered for installation shouldbe asked to provide a list of projects on which their deviceshave been installed, and previous users of new equipmentshould be contacted to ascertain their operatingexperiences.

b. Installation and observation of instrumentation. Arational instrumentation program must use the proper typeof instruments and have the instruments installed properlyat critical locations. Valid readings often depend ontechniques and procedures used in installing and observingthe instrumentation.

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

these readings usually form the basis to which subsequentobservations are related. Observations should be plottedimmediately after each set of readings is taken andevaluated for reasonableness against previous sets ofreadings. In this way, it is often possible to detect errors inreadings and to obtain check readings before significantchanges in field conditions occur.

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

2-5. Optimum cost construction. The designershould consider all details of the construction process toensure a safe and operational facility at the lowest possiblecost.

a. Energy requirements. The consideration of energyrequirements is important not only for economical reasonsbut also for the critical need to conserve energy whereverpossible. It should not be the intent of the design engineerto unduly restrict the competitive nature of currentcontractural procedures. Nevertheless, there are certainalternatives that the designer may specify that potentiallycould lead to more energy efficient construction with costsaving being reflected in bid prices. Some of the possiblealternatives that should be considered are discussedbelow.

(1) Sources of suitable select backfill materialshould be located as close to the project site as possible.The source may be either a borrow area or a commercialvendor.

(2) Hauling routes to and from the source of backfilland the project site should follow the most direct route.

(3) Only compaction equipment that will compactthe specific backfill to the required density in an efficientmanner should be approved for use. For large projects, thedesigner may require that the contractor demonstrate thecapabilities of the equipment he intends to use prior toconstruction.

(4) If possible, material from excavations or withinthe immediate vicinity of the project site should be used asbackfill, even though such material may be marginallysuitable. The engineering characteristics of marginalmaterial may be enhanced by the use of additives (para 3-3d).

(5) The energy requirements for adequate coldweather protection of construction personnel and structurescan be considerable. For project sites subject to seasonalcold weather, construction should not, if possible, bescheduled during extreme cold weather periods.

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

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any project saving derived from realistic cost-savingsuggestions submitted.

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CHAPTER 3

EVALUATION, DESIGN, AND PROCESSINGOF BACKFILL MATERIALS

3-1. General. The evaluation, design, and properprocessing of backfill materials are extremely importantphases of the preconstruction operations. The purposeof the evaluation phase is to determine the engineeringcharacteristics of potential backfill materials. The designphase must take into account the engineeringcharacteristics required of the backfill and specifymaterials that, when compacted properly, will have thesecharacteristics. Proper processing of the backfillmaterial will ensure that desirable engineeringcharacteristics will be obtained as the material is placed.

3-2. Evaluation of backfill materials. Evaluation ofbackfill materials consists of exploration, sampling, andlaboratory testing to determine the engineeringcharacteristics of potential backfill materials. Detailedinstructions for exploration, sampling, laboratory testing,and foundation design are presented in TM 5-818-1/AFM88-3, Chapter 7. However, to emphasize the need for anadequate investigation, some aspects of planning andinvestigation that should be considered are discussed inthe following paragraphs.

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 availablefrom required excavations, and the investigation forfoundation conditions should include the sampling andevaluation of these materials for possible use as backfill.Where an adequate volume of suitable backfill cannot beobtained from the construction excavation, theexploration and sampling program must be expanded tofind other sources of suitable material whether fromnearby borrow areas or commercial sources.

(1) The purpose of the investigation is todelineate critical conditions and provide detailedinformation on the subsurface deposits so that properdesign and construction, including backfilling operations,can be accomplished with minimum difficulty. Thuscareful planning is required prior to the field explorationand sampling phase of the investigation. Availablegeologic and soil data should be studied, and if possible,preliminary borings should be made. Once a site hasbeen tentatively selected, orientation of the structure tothe site should be established. The engineer who

plans the detailed field exploration program must haveknowledge of the structure, i.e., its configuration andfoundation requirements for design loads and settlementtolerances. The planning engineer should also know thetype and quantity of backfill required. The importance ofemploying qualified field exploration personnel cannot beoveremphasized. The exploration crews should besupervised in the field by a soils engineer or geologistfamiliar with the foundation and backfill requirements sothat changes can be made in the exploration programwhere necessary to provide adequate information onsubsurface conditions.

(2) The field engineer should also know thelocation of significant features of the structure so thatsampling can be concentrated at these locations. Inaddition, he should have an understanding of theengineering characteristics of subsurface soil and rockdeposits that are important to the design of the structureand a general knowledge of the testing program so thatthe proper type and quantity of samples will be obtainedfor testing.

(3) From the samples, the subsurface depositscan be classified and boring logs prepared. The morecontinuous the sampling operation, the more accuratewill be 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.Accurate logging and correct evaluation of all pertinentinformation are essential for a true concept of subsurfaceconditions.

(4) When the exploratory borings at theconstruction site have been completed, the samples andlogs of borings should be examined to determine if thematerial to be excavated will be satisfactory and insufficient quantity to meet backfill requirements. Everyeffort should be made to use the excavated materials;however, if the excavated materials are not satisfactoryor are 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 a dryexcavation in which construction and backfilling

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operations can be properly conducted. Data ongroundwater conditions are also essential for forecastingconstruction dewatering requirements and stabilityproblems. Groundwater studies must consist ofinvestigations to determine: groundwater levels toinclude any seasonal variations and artesian conditions;the location of any water-bearing strata; and thepermeability and flow characteristics of water-bearingstrata. Methods for investigating groundwater conditionsare described in TM 5-818-1/AFM 88-3, Chapter 7, andTM 5-818-5/NAVFAC P-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 either itsnatural state or as compacted backfill. The laboratorytesting program will furnish the engineer information forplanning, designing, and constructing subsurfacestructures. Laboratory testing programs usually follow ageneral pattern and to some extent can be standardized,but they should be adapted to particular problems andsoil conditions. Special tests and research should beutilized when necessary to develop needed information.The testing program should be well planned with theengineering features of the structure and backfill in mind;testing should be concentrated on samples from areaswhere significant features will be located but should stillpresent a complete picture of the soil and rockproperties. The laboratory test procedures andequipment are described in TM 5-818-1/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 TM 5-818-1/AFM 88-3, Chap. 7) is a means of identifying a soiland placing it in a category of distinctive engineeringproperties. Table 3-1 shows the properties of soil groupspertinent to backfill and foundations. Using thesecharacteristics, the engineer can prepare preliminarydesigns based on classification and plan the laboratorytesting program intelligently and economically.

(a) The Unified Soil Classification System classifiessoils according to their grain-size distribution andplasticity characteristics and groups them with respect totheir engineering behavior. With experience, theplasticity and gradation properties can be estimatedusing simple, expedient tests (see table 2-2 and 2-3 ofTM 5-818-1/AFM 88-3, Chap. 7 or AFM 89-3, Chap. 2)and these estimates can be confirmed using simplelaboratory tests. The principal laboratory testsperformed for classification are grain-size analyses andAtterberg limits.

(b) The engineering properties in table 3-1 are basedon "Standard Proctor" (CE 25) maximum

density except that the California Bearing Ratio (CBR)and the subgrade modulus are based on CE 55maximum density. This information can be used forinitial design studies. However, for final design ofimportant structures, laboratory tests are required todetermine actual performance characteristics, such asCE 55 compaction properties, shear strength,permeability, compressibility, swelling characteristics,and frost susceptibility where applicable, under expectedconstruction conditions.

(c) The Unified Soil Classification System isparticularly useful in evaluating, by visual examination,the suitability of potential borrow materials for use ascompacted backfill. Proficiency in visual classificationcan be developed through practice by comparingestimated soil properties with results of laboratoryclassification tests.

(2) Compaction testing. Compaction test proceduresare described in detail in MIL-STD-621 and ASTM D1557 (app. A). It is important that the designer and fieldinspection personnel understand the basic principles andfundamentals of soil compaction. The principles of soilcompaction are discussed in appendix B of this manual.

(a) The purpose of the laboratory compactiontests are to determine the compaction characteristics ofavailable backfill materials. Also, anticipated field densityand water content can be approximated in laboratory-compacted samples in order that other engineeringproperties, such as shear strength, compressibility,consolidation, and swelling, can be studied. For mostsoils there is an optimum water content at which amaximum density is obtained with a particularcompaction effort. A standard five-point compactioncurve relating density and water content (fig. B-1, app.B) can be developed by the procedures outlined inMILSTD-621.

(b) The impact compaction test results normallyconstitute the basis on which field compaction controlcriteria are developed for inclusion in the specifications.However, for some cohesionless soils, higher densitiescan be obtained by the vibratory compaction method(commonly referred to as maximum relative density),described in appendix XII of EM 1110-21906. Therequired field compaction is generally specified as apercentage of laboratory maximum dry density andreferred to as percent CE 55 maximum density. Watercontent is an important controlling factor in obtainingproper compaction. The required percentage ofmaximum dry density and the compaction water contentshould be selected on the basis of the engineeringcharacteristics, such as compression moduli, settlement,and shear strength, desired in the compacted backfill. Itshould be noted that these characteristics could beadversely effected by subsequent increases in

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water content after placement. This situation could resultfrom 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 as theProctor needle penetration for cohesive soils or the toneresistance load for cohesionless soils, can also be usedwhen correlations with CE 55 maximum density havebeen established.

(3) Shear strength testing. When backfill is to be placedbehind structure walls or bulkheads or as foundation supportfor a structure, and when fills are to be placed withunrestrained slopes, shear tests should be performed onrepresentative samples of the backfill materials compacted toexpected field densities and water contents to estimate as-constructed shear strengths. The appropriate type of testrequired for the conditions to be analyzed is presented in TM5-818-1/AFM 88-3, Chapter 7. Procedures for shear strengthtesting are described in EM 1110-2-1906.

Table 3-1. Typical Engineering Properties of Compacted Materials a

Notes: 1. All properties are for condition of "standard Proctor" maximum density, except values of k and CBR whichare for CE55 maximum density.

2. Typical strength characteristics are for effective strength envelopes and are obtained from USER data.3. Compression values are for vertical loading with complete lateral confinement.4. ( >) indicates that typical property is greater then the value shown. (-) indicates insufficient data available

for an estimate.a After DM-7.b From TM 5-18-2/AP 88-6. Chapter 4.

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(4) Consolidation and swell testing. The rate andmagnitude of consolidation under a given load areinfluenced primarily by the density and type of soil and theconditions of saturation and drainage. Fine grained soilsgenerally consolidate more and at a slower rate thancoarse-grained soils. However, poorly graded, granularsoils and granular soils composed of rounded particles willoften consolidate significantly under load but usually at arelatively fast rate.

(a) The procedure for the consolidation test isoutlined in EM 1110-2-1906. The information obtained inthis test can be used in settlement analyses to determinethe total settlement, the time rate of settlement, and thedifferential settlement under varying loading conditions.Consolidation characteristics are important considerationsin selection of backfill materials. The results ofconsolidation tests performed on laboratory compactedspecimens of backfill material can be used in determiningthe percent compaction to be required in thespecifications.

(b) Swelling characteristics can be determined by amodified consolidation test procedure. The degree ofswelling and swelling pressure should be determined on allbackfill and foundation materials suspected of havingswelling characteristics. This fact is particularly importantwhen a considerable overburden load is removed byexcavation or when the compacted backfill with swellingtendencies may become saturated upon removal of thedewatering system and subsequent rise of thegroundwater level. The results of swelling tests can beused to determine the suitability of material as backfill.When it is necessary to use backfill materials that have atendency to swell upon saturation because more suitablematerials are unavailable, the placement water contentand density that will minimize swelling can be determinedfrom a series of tests. TM 5-818-1/AFM 88-3, Chapter 7,and FHWA-RD-79-51 (app. A) provide further informationapplicable to compacted backfills.

(5) Permeability tests. Permeability tests to determinethe rate of flow of water through a material can beconducted in the laboratory by procedures described inEM 1110-2-1906. Permeability characteristics of fine-grained materials at various densities can also bedetermined from consolidation tests.

(a) Permeability characteristics for the design ofpermanent drainage systems for structures founded belowthe groundwater level must be obtained from laboratorytests. The tests should be performed on representativespecimens of backfill materials compacted in thelaboratory to densities expected in the field.

(b) In situ material permeability characteristics forthe design of construction excavation dewatering systemscan also be approximated from laboratory tests onrepresentative undisturbed samples. Laboratorypermeability tests on undisturbed samples are lessexpensive than in situ pumping tests performed in the

field; however, laboratory tests are less accurate inpredicting flow characteristics.

(6) Slake durability of shales. Some clay shales tendto slake when exposed to air and water and must beprotected immediately after they are exposed. The extentof slaking also governs the manner in which they aretreated as a backfill material (para 3-3c). Slakingcharacteristics can be evaluated by laboratory jar-slaketests or slake-durability tests.

(a) The jar-slake test is qualitative with sixdescriptive degrees of slaking determined from visualobservation of ovendried samples soaked in tap water foras 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 values ofthe jar-slake index Ij, are listed below:IJ 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 LJ values of 1 to 3 should be protected when

occurring in excavated slopes and compacted as soil ifused for backfill.

(b) The slake-durability test is a standardized testthat gives a quantitative description in percent by weight ofmaterial remaining intact at the conclusion of the test.Details of the test are presented in FHWA-RD-78-141.

(7) Dynamic tests for special projects. The dynamicanalysis of projects subject to seismic or blast inducedloading conditions requires special dynamic tests on bothin situ and backfill materials. Tests required for dynamicanalysis include: cyclic triaxial tests; in situ densitymeasurements; and tests to determine shear wavevelocities, shear modulus, and damping (ER 1110-2-1806).

(8) In situ water content. The in situ water content,including any seasonal variation, must be determined priorto construction for materials selected for use as backfill.Natural in situ water contents will determine the need forwetting or drying the backfill material before placement toobtain near optimum water contents for placement andcompaction. ASTM D 2216 discusses the test method fordetermining water content.3-3. Selection of backfill materials. Selection ofbackfill materials should be based upon the engineeringproperties and compaction characteristics of the materialsavailable. The results of the field exploration andlaboratory test programs should provide adequateinformation for this purpose. The materials

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may come from required excavation, adjacent borrow pits,or commercial sources. In selecting materials to be used,first consideration should be given to the maximum use ofmaterials from required excavation. If the excavatedmaterials are deficient in quality or quantity, other sourcesshould be considered. Common backfill having the desiredproperties may be found in borrow areas convenient to thesite, but it may be necessary to obtain select backfillmaterials having particular gradation requirements, such asfilter sands and gravels and pipe or conduit beddingmaterials from commercial sources.

a. Primary considerations. Primary considerations forborrow material sources are suitability and quantity.Accessibility and proximity of the borrow area to the jobsiteshould also be considered. The water contents of theborrow area material should be determined seasonally, anda source of water should be located if the natural watercontents are considerably less than the required placementwater content. If several sources of suitable backfill areavailable, other factors to be considered in selecting theborrow materials are ease of loading and spreading and themeans for adding or reducing water. The need forseparating or mixing soil strata from excavation or borrowsources should be considered if necessary to providereasonably uniform engineering properties throughout thecompacted backfill.

b. Compaction characteristics. If compactioncharacteristics of the major portion of the backfill arerelatively uniform, problems of controlling placement ofbackfill will be significantly reduced since the inspector willbe able to develop more rapidly the ability to recognize theadequacy of the compaction procedures. In addition, thefrequency of testing for compaction control could bereduced. When available backfill materials are unusual, testsections of compacted backfill are sometimes justified todevelop placement procedures and to determine theengineering characteristics to be expected in field-compacted materials.

c. Workability. An important factor in choosing backfillmaterials is the workability or ease with which the soil canbe placed and compacted. Material characteristics thateffect workability include: the ease of adjusting watercontents in the field by wetting or aeration; the sensitivity tothe compaction water content with respect to optimum; andthe amount of compaction effort required to achievespecified densities.

d. Types of backfill material. A discussion of the manytypes of backfill and their compaction characteristics isbeyond the scope of this manual since soil types will vary oneach project. However, the compaction characteristics ofseveral rather broad categories of backfill (table 3-1) arediscussed briefly. MIL STD-619 should be studied

for more detailed information.(1) Coarse-grained soils. Coarse-grained soils

include gravelly and sandy soils and range from clayeysands (SC) through the well-graded gravels ofgravelsand mixtues (GW) with little or no fines (table 3-1). They will exhibit slight to no plasticity. All of thewellgraded soils falling in this category have fairly goodcompaction characteristics and when adequatelycompacted provide good backfill and foundation support.

(a) One difficulty that might arise with soils in thiscategory would be in obtaining good compaction of thepoorly graded sands and gravels. These poorly gradedmaterials may require saturation with downward drainageand compaction with greater compaction effort toachieve sufficiently high densities. Also, close control ofwater content is required where silt is present insubstantial amounts. Coarse-grained materialscompacted to a low relative density are susceptible uponsaturation to liquefaction under dynamic loads.

(b) For sands and gravelly sands with little or nofines, good compaction can be achieved in either the air-dried or saturated condition. Downward drainage isrequired 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 particularlydifficult to compact in narrow confined areas.However, the addition of cement may produce zoneswith greater rigidity than untreated adjacent backfill andform "hard spots" resulting in nonuniform stresses anddeformations in the structure.

(c) Cohesionless materials are well suited forplacement in confined areas adjacent to and aroundstructures where heavy equipment is not permitted andbeneath and around irregulary shaped structures, suchas tunnels, culverts, utilities, and tanks. Clean, granular,well-graded materials having a maximum size of 1 inchwith 95 percent passing the No. 4 sieve and 5 percent orless passing the No. 200 sieve are excellent for use inthese zones. However, a danger exists of creatingzones where seepage water may accumulate andsaturate adjacent cohesive soils resulting in undesirableconsolidation or swelling. In such cases, provisions fordraining the granular backfill, sealing the surface, anddraining surface water away from the structure arenecessary.

(2) Fine-grained soils of low to medium plasticity. Inorganic clays (CL) of low to medium plasticity (gravelly,sandy, or silty clays and lean clays) and inorganic siltsand very fine sands (ML) of low plasticity (silty or clayeyfine sands and clayey silts) are included in this category.The inorganic clays are relatively impervious and can becompacted fairly easily with heavy compaction

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

equipment to provide a good stable backfill. Soils in the CLgroup can be compacted in confined areas to a fairly highdegree of compaction with proper water content and liftthickness control. The clayey sands of the SC group andclayey silts of the ML group can be compacted to fairly highdensities, but close control of water content is essential andsometimes critical, particularly on the wet side of optimumwater content. Some ML soils, if compacted on the dry sideof optimum, may lose considerable strength upon saturationafter compaction. Considerable settlement may occur.Caution must therefore be exercised in the use of such soilsas backfill, particularly below the groundwater level. Also,saturated ML soils are likely to be highly susceptible toliquefaction when dynamically loaded. Where such soils areused as backfill in seismic prone areas, laboratory testsshould be conducted to determine their liquefaction potential(see para. 17-5 and 17-6, TM 5-818-1/AFM 88-3, Chap. 7).

(3) Rock. The suitability of rock as backfill material ishighly dependent upon the gradation and hardness of therock particles. The quantity of hard rock excavated at mostsubsurface structure sites is relatively small, but selectcohesionless materials may be difficult to find or may beexpensive. Therefore, excavated hard rock may bespecified for crusher processing and used as selectcohesionless material.

(4) Shale. Although shale is commonly referred to asrock, the tendency of some shales to breakdown underheavy compaction equipment and slake when exposed toair or water after placement warrants special consideration.

(a) Some soft shales break down under heavycompaction equipment causing the material to have entirelydifferent properties after compaction than it had beforecompaction. This fact should be recognized before this typeof material is used for backfill. Establishing the propercompaction criteria may require that the contractor constructa test fill and vary the water content, lift thickness, andnumber of coverages with the equipment proposed for usein the backfill operation. This type of backfill can be usedonly in unrestricted open zones where heavy towed or self-propelled equipment can operate.

(b) Some shales have a tendency to break down orslake when exposed to air. Other shales that appear rock-like when excavated will soften or slake and deteriorateupon wetting after placement as rockfill. Alternate cycles ofwetting and drying increases the slaking process. Theextent of material breakdown determines the manner inwhich it is treated as a backfill material. If the materialcompletely degrades into constituent particles or small chipsand flakes, it must be treated as a soil-like material withproperty characteristics similar to ML, CL, or CH materials,depending upon the intact composition of the parentmaterial. Complete degradation can be facilitated byalternately wetting, drying, and disking the material before

compaction. A detailed discussion on the treatment ofshales as a fill material is given in FHWA-RD-78-141.

(5) Marginal materials. Marginal materials are thesematerials that because of either their poor compaction,consolidation, or swelling characteristics would notnormally be used as backfill if sources of suitablematerial were available. Material considered to bemarginal include fine-grained soils of high plasticity andexpansive clays. The decision to use marginal materialsshould be based on economical and energy conservationconsiderations to include the cost of obtaining suitablematerial whether from a distant borrow area orcommercial sources, possible distress repair costscaused by use of marginal material, and the extra costsinvolved in processing, placing, and adequatelycompacting marginal material.

(a) The fine-grained, highly plastic materialsmake poor backfill because of the difficulty in handling,exercising water-content control, and compacting. Thewater content of highly plastic finegrained soils is criticalto proper compaction and is very difficult to control in thefield by aeration or wetting. Furthermore, such soils aremuch more compressible than less-plastic and coarse-grained soils; shear strength and thus earth pressuresmay fluctuate between wide limits with changes in watercontent; and in cold climates, frost action will occur infine-grained soils that are not properly drained. The onlysoil type in this category that might be consideredsuitable as backfill is inorganic clay (CH). Use of CHsoils should be avoided in confined areas if a highdegree of compaction is needed to minimize backfillsettlement or to provide 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 clayminerals include montmorillonite, mixed-layercombinations of montmorillonite and other clay minerals,and under 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 greater thanthe restraining pressure, heave will occur and may causestructural distress. Compaction on the wet side ofoptimum moisture content will produce lower magnitudesof swelling and swell pressure. Expansive clays thatexhibit significant volume increases should not be usedas backfill where the potential for structural damagemight exist. Suitability should be based upon laboratoryswell tests (TM 5-818-1/AFM 88-3, Chapter 7).

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(c) Additives, such as hydrated lime, quicklime,and fly ash, can be mixed with some highly plastic claysto improve their engineering characteristics and permitthe use of some materials that would otherwise beunacceptable. Hydrated lime can also be mixed withsome expansive clays to reduce their swellingcharacteristics (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 of usingthe additive. Because of the complexity of soil-additivesystems and the almost complete empirical nature of thecurrent state of the art, trial mixes must be varified in thefield by test fills.

(6) Commercial by-products. The use of commercialby-products, such as furnace slag or fly ash as backfillmaterial, may be advantageous where such products arelocally available and where suitable natural materialscannot be found. Fly ash has been used as a lightweightbackfill behind a 25-foot-high wall and as an additive tohighly plastic clay. The suitability of these materials willdepend upon the desirable characteristics of the backfilland the engineering characteristics of the products.3-4. Processing of backfill materials. Theconstruction of subsurface structures often requires theconstruction of elements of the structure within or uponlarge masses of backfill. The proper functioning of theseelements are often critically affected by adversebehavioral characteristics of the backfill. Behavioralcharacteristics are related to material type, water contentduring compaction, gradation, and compaction effort.While compaction effort may be easily controlled

during compaction, it is difficult to control material type,water content, and gradation of the material as it is beingplaced in the backfill; control criteria must be establishedprior to placement.

a. Material type. Backfill material should consist ofa homogeneous material of consistent and desirablecharacteristics. The field engineer must ensure that onlythe approved backfill material is used and that thematerial is uniform in nature and free of any anomalousmaterial such as organic matter or clay pockets.Stratified material should be mixed prior to placing toobtain a uniform blend. Excavated material to be usedas backfill should be stockpiled according to class ortype of material.

b. Water content. While water content can beadjusted to some extent after placing (but beforecompacting), it is generally more advantageous to adjustthe water content to optimum compaction conditionsbefore placing. Adjustment of water content can beaccomplished by aeriation (disking or turning) orsprinkling 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 materialsconsisting of crushed rock, gravel, or sand requirelimitations on maximum and minimum particle-size orgradation distributions. Where materials cannot belocated that meet gradation criteria, it may beadvantageous to require processing of available materialby sieving to obtain the desired gradation.

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CHAPTER 4

EARTHWORK: EXCAVATION AND PREPARATION FOR FOUNDATIONS

4-1. Excavation.

a. General. In general, excavation for subsurfacestructures will consist of open excavation and shaft andtunnel excavation. Where excavation to great depths isrequired, a variety of soils and rock may be encounteredat a single site. Soils may range through a widespectrum of textures and water contents. Rockencountered may vary from soft rock, very similar to afirm soil in its excavation requirements, to extremely hardrock requiring extensive blasting operations for removal.Groundwater may or may not be present. Thegroundwater conditions and the adequacy ofgroundwater control measures are important factors inexcavation, in maintaining a stable foundation, and inbackfilling operations. The extent to which groundwatercan be controlled also influences the slopes to which theopen excavation can be cut, the bracing required tosupport shaft and tunnel excavation, and the handling ofthe excavated material.

b. Good construction practices, and problems.A majority of the problems encountered duringexcavation are related to groundwater conditions, slopestability, and adverse weather conditions. Many of theproblems can be anticipated and avoided bypreconstruction planning and by following soundconstruction practices.

(1) Groundwater. Probably the greatest sourceof problems in excavation operations is groundwater. Ifthe seepage of groundwater into an excavation isadequately 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 someinstances, groundwater conditions can be more severethan indicated by the original field explorationinvestigation 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 should notbe 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 aredetermined and effected. The design and evaluation ofdewatering systems require considerable experience

that the contractor or the contracting office often do notpossess, and the assistance of specialists in this fieldshould be obtained.

(b) Groundwater without significant seepage flowcan also be a problem since excess hydrostaticpressures can develop below relatively impervious strataand cause uplift and subsequent foundation or slopeinstability. Excess hydrostatic pressures can also occurbehind sheet pile retaining walls and shoring and bracingin shaft and tunnel excavations. Visual observationsshould be made for indications of trouble, such asuncontrolled seepage flow, piping of material from thefoundation or slope, development of soft wet areas, upliftof ground surface, or lateral movements.

(c) Accurate daily records should be kept of thequantity of water removed by the dewatering system andof the piezometric levels in the foundation and beneathexcavation slopes. Separate records should be kept ofthe flow pumped by any sump-pump system required toaugment the regular dewatering system to note anyincrease of flow into the excavation. Flowmeters or othermeasuring devices should be installed on the dischargeof these systems for measurement purposes (TM 5-818-5/NAVFAC P-418/AFM 88-5, Chap. 6). These recordscan be invaluable in evaluating "Changed Condition"claims submitted by the contractor. The contractorshould be required to have "standby" equipment in casethe original equipment breaks down.

(2) Surface water. Sources of water problems otherthan groundwater are surface runoff into the excavationand snow drifting into the excavation. A peripheral,surface-drainage system, such as a ditch and berm,should be required to collect surface water and divert itfrom the excavation. In good weather there is atendency for the contractor to become lax in maintainingthis system and for the inspection personnel to becomelax in enforcing maintenance. The result can be asudden filling of the excavation with water during a heavyrain and consequent delay in construction. The surfacedrainage system must be constantly maintained until thebackfill is complete. Drifting snow is a seasonal andregional problem, which can best be controlled by snowfences placed at strategic locations around theexcavation.

(3) Slope integrity. Another area of concern duringexcavation is the integrity of the excavation

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slopes. The slopes may be either unsupported orsupported by shoring and bracing. The lines and gradesindicated in the plans should be strictly adhered to. Thecontractor may attempt to gain additional working room inthe bottom of the excavation by steepening the slopes; thischange in the plans must not be allowed.

(a) Where shoring and bracing are necessary toprovide a stable excavation, and the plans andspecifications do not provide details of theserequirements, the contractor should be required to submitthe plans in sufficient detail so that they can be easilyfollowed and their adequacy checked. The first principleof excavation stabilization, using shoring and bracing, isthat the placing of supports should proceed withexcavation. 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 cut facethan if no movement had occurred prior to placement ofthe shoring. Excavation support systems are discussed inTM 5-818-1/AFM 88-3, Chapter 7. All safety requirementsfor shoring and bracing as contained in EM 385-1-1 shouldbe strictly enforced.

(b) The inspector must be familiar with stockpilingrequirements regarding the distance from the crest of theexcavation at which stockpiles can be established andheavy equipment operated without endangering thestability of the excavation slopes. He must also know themaximum height of stockpile or weight of equipment thatcan be allowed at this distance.

(c) Excessive erosion of the excavation slopesmust not be permitted. In areas subject to heavy rainfall, itmay be necessary to protect excavation slopes withpolyethylene sheeting, straw, silt fences, or by othermeans to prevent erosion. Excavation slopes for largeprojects that will be exposed for several seasons shouldbe vegetated and maintained to prevent erosion.

(4) Stockpiling excavated material. Generally,procedures for stockpiling are left to the discretion of thecontractor. Prior to construction, the contractor mustsubmit his plans for stockpiling to the contracting officerfor approval. In certain cases, such as where there aredifferent contractors for the excavation and the backfillphases, it may be necessary to include the details forstockpiling operations in the specifications. In either case,it is important that the stockpiling procedures be conduciveto the most advantageous use of the excavated materials.

(a) As the materials are excavated, they should beseparated into classes of backfill and stockpiledaccordingly. Thus the inspection personnel controlling theexcavation should be qualified to classify the material andshould be thoroughly familiar with backfill re

requirements. Also, as the materials are placed instockpiles, water should be added or the materials shouldbe aerated as required to approximate optimum watercontent for compaction. Field laboratory personnel canassist in determining the extent to which this is necessary.The requirements of shaping the stockpile to drain andsealing it against the entrance of undesirable water byrolling with spreading equipment or covering withpolyethylene sheeting should be enforced. This step isparticularly important for cohesive soils that exhibit poordraining characteristics and tend to remain wet if oncesaturated by rains. Stockpiles must be located over an areathat is large enough to permit processing and where theywill not interfere with peripheral drainage around theexcavation and will not overload the slopes of theexcavation.

(b) In cases where significant energy and costsaving can be realized, special stockpiling requirementsshould be implemented. An example would be a largeproject consisting of a number of excavation and backfillingoperations. The excavation material from the firstexcavation could be stockpiled for use as backfill in the lastexcavation. The material from the intermediate excavationscould in turn be immediately used as backfill for the first,second, etc., phases of the project and thereby eliminatedouble handing of excavated backfill for all but the first-phase excavation.

(5) Protection of exposed material. If materials that areexposed in areas, such as walls of a silo shaft, foundationsupport, or any other area against which concrete will beplaced, are susceptible to deterioration or swell whenexposed to the weather, they should be properly protectedas soon after exposure as possible.Depending on the material and protection requirements, thisprotection may be pneumatic concrete, asphalt spray, orplastic membrane (TM 5-818-1/AFM 88-33, Chap. 7). Inthe case of a foundation area, the contractor is required tounderexcavate leaving a cover for protection, as required,until immediately prior to placement of the structurefoundation. Any frost-susceptible materials encounteredduring excavation should be protected (para 2-3h (3) and(4)) if the excavation is to be left open during an extendedperiod of freezing weather.

(6) Excavation record. As the excavation progresses,the project engineer should keep a daily record of the typeof material excavated and the progress made. This recordwould be of value if subsequent claims of "ChangedConditions" are made by the contractor.4-2. Foundation preparation.a. General. In this manual, preparation applies tofoundations for backfill as well as those for structures to beplaced in the excavation. Generally, if proper excavationprocedures have been followed, very little

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additional preparation will be required prior to backfillplacement.

b. Good construction practices, and problems. Asmentioned previously, the problems associated withfoundation preparation are greatly reduced by followingsuch proper excavation procedures as maintaining a dryexcavation and planning ahead. The principles of goodfoundation preparation are simple, but enforcing theprovisions of the specifications concerning the work ismore difficult. Inspection personnel must recognize theimportance of this phase of the work since, if not properlycontrolled, problems can result.

(1) It is most important that a stable foundationbe provided. Thus it may be necessary, particularly inthe case of sensitive fine-grained 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 water todetermine the resulting degree of deterioration (para 3-2b (6)) and visual observation to determine if the rock isin a solid and unshattered condition. If removal of rockbelow the foundation level is required, the space shouldbe filled with concrete. A qualified geological or soilsengineer should inspect the area if it is suspected thatthe material will deteriorate or swell when exposed to theweather. If necessary, the materials must be protectedfrom exposure using the methods previously discussedin paragraph 4-1b (5).

(3) Before placement of any structure foundationis begun, the plans should be rechecked to ensure thatall required utilities and conduits under or adjacent to thefoundation have been placed, so that excavating underor undermining the foundation to place utilities andconduits will not become necessary later.

(4) Occasionally, it may be found uponcompletion of the excavation that if a structure wereplaced as shown on the plans, it would be supported ontwo materials with drastically different consolidationcharactertistics, such as rock and soil, rock and backfill,or undisturbed soil and backfill. This situation could

occur because the predesign subsurface informationwas inadequate, because the structure was relocated orreoriented by a subsequent change in the plans,because of an oversight of the design engineer, orbecause of the excavation procedures followed by thecontractor. Regardless of the reason, measures such asoverexcavation and placement of subsequent backfillshould be taken, where possible, and in coordination withthe design office to provide a foundation of uniformmaterial. Otherwise, the design office should evaluatethe differences in foundation conditions for possiblechanges to the structural foundation elements.

(5) Preparing the area to receive the backfillconsists of cleaning, leveling, and compacting the bottomof the excavation if the foundation is in soil. All debrisand foreign material, such as trash, broken concrete androck, boulders, and forming lumber, should be removedfrom the excavation. All holes, depressions, andtrenches should be filled with the same material as thatspecified to be placed immediately above such adepression, unless otherwise designated, andcompacted to the density specified for the particularmaterial used. If the depression is large enough toaccommodate heavy compacting equipment, the sides ofthe depression should have a positive slope and be flatenough for proper operation of compaction equipment.After the area is brought to a generally level condition bycompacting in lifts in accordance with specifications, theentire area to receive backfill should be sacrificed to thedepth specified, the water content adjusted if necessary,and the area compacted as specified. If the foundationis in rock, the area should be leveled as much aspossible and all loose material removed.

(6) All work in the excavation should beaccomplished in the dry; therefore, the dewateringsystem should be operated for the duration of this work.Under no circumstances should the contractor beallowed to dry an area by dumping a thick layer of drymaterial over it to blot the excess water. If soil exists atthe foundation level and becomes saturated, it cannot becompacted. The saturated soil will have to be removedand replaced or drained sufficiently so that it can becompacted. Any frozen material in the foundation shouldbe removed before placement of concrete footings orcompacted backfill.

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CHAPTER 5

BACKFILL OPERATIONS

5-1. Placement of backfill.

a. General. Backfill construction is the refilling ofpreviously excavated space with properly compactedmaterial. The areas may be quite large, in which casethe backfilling operation will be similar to embankmentconstruction. On the other hand, the areas may be quitelimited, such as confined areas around or between andbeneath concrete or steel structures and areas intrenches excavated for utility lines. Prior to constructionof the backfill, the inspection personnel should becomethoroughly familiar with the various classes of backfill tobe used. They should be able to readily identify thematerials on sight, know where the various types ofmaterial should be placed, and be familiar with thecompaction characteristics of the soil types. Compactioncharacteristics of various soil types are discussed inappendix B.

b. Good construction practices, and problems.Problems with placement of backfill will vary from oneconstruction project to another. The magnitude of theproblems will depend on the type of materials availablesuch as backfill, density requirements, and theconfiguration of the areas in which compaction is to beaccomplished. Problems should be expected during theinitial stages of backfill compaction unless the contractoris familiar with compaction characteristics of backfillmaterials. The inspector can be of great assistance tothe contractor during this period by performing frequentwater content and density checks. The information fromthese checks will show the contractor the effects of thecompaction procedures being used and point out anychanges that should be made.

(1) Backfilling procedures. Problems associatedwith the compaction of backfill can be minimized byfollowing good backfilling procedures. Good backfillingprocedures include: processing the material (para. 3-4)before it is placed in the excavation; placing the materialin a uniformly spread loose lift of the proper thicknesssuited to the compaction equipment and the type ofmaterial to be used; applying the necessary compactioneffort to obtain the required densities; and ensuring thatthese operations are not performed during adverseweather. Proper bond should be provided between eachlift and also between the backfill and the sides of theexcavation.

(2) Compaction equipment, backfill material, andzones. The type of compaction equipment used to

achieve the required densities will usually depend uponthe type of backfill material being compacted and thetype of zone in which the material is placed.

(a) In open zones, coarse-grained soils thatexhibit slight plasticity (clayey sands, silty sands, clayeygravels, and silty gravels) should be compacted witheither sheepsfoot or rubber-tired rollers; close control ofwater content is required where silt is present insubstantial amounts. For sands and gravelly sands withlittle or no fines, good compaction results are obtainedwith tractor compaction. Good compaction can also beachieved in gravels and gravel-sand mixtures with eithera crawler tractor or rubber-tired and steelwheeled rollers.The addition of vibration to any of the means ofcompaction mentioned above will usually improve thecompaction of soils in this category. In confined zones,adequate compaction of cohesionless soils in either theair-dried or saturated condition can be achieved byvibratory-plate compactors with a static weight of at least100 pounds. If the material is compacted in thesaturated condition, good compaction can be achievedby internal vibration (for example, by using concretevibrators). Downward drainage is required to maintainseepage forces in a downward direction if the placedmaterial is saturated to aid in compaction.

(b) Inorganic clays, inorganic silts, and very finesands of low to medium plasticity are fairly easilycompacted in open zones with sheepsfoot or rubbertiredrollers in the 15,000-pound and above wheel-load class.Some inorganic clays can be adequately compacted inconfined zones using rammer or impact compactors witha static weight of at least 100 pounds provided closecontrol of lift thickness and water content is maintained.

(c) Fine-grained, highly plastic materials, thoughnot good backfill materials, can best be compacted inopen zones with sheepsfoot rollers. Sheepsfoot rollersleave the surface of the backfill in a rough condition,which provides an excellent bond between lifts. Inconfined areas the best results, which are not consideredgood, are obtained with rammer or impact compactors.

(3) Lift thickness. The loose-lift thickness willdepend on the type of backfill material and thecompaction equipment to be used.

(a) As a general rule, a loose-lift thickness thatwill result in a 6-inch lift when compacted can be

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allowed for most sheepsfoot and pneumatic-tired rollers.Cohesive soils placed inapproximately 10-inch loose lifts willcompact to approximately 6 inches, and cohesionless soilsplaced in approximately 8-inch base lifts will compact to 6inches. Adequate compaction can be achieved incohesionless materials of about 12- to 15inch loose-liftthickness if heavy vibratory equipment is used. The additionof vibration to rolling equipment used for compactingcohesive soils generally has little effect on the lift thicknessthat can be compacted, although compaction to the desireddensity can sometimes be obtained by fewer coverages ofthe equipment.

(b) In confined zones where clean cohesionlessbackfill material is used, a loose-lift thickness of 4 to 6 inchesand a vibratory plate or walk-behind, dual-drum vibratoryroller for compaction is recommended. Where cohesivesoils are used as backfill in confined zones, use of rammercompactors and a loose-lift thickness of not more than 4inches should be specified. Experience has shown that "two-by-four" wood rammers, or single air tampers (commonlyreferred to as "powder puffs" or "pogo sticks’) do not producesufficient compaction.

(4) Density requirements. In open areas of backfillwhere structures will not be constructed, compaction can beless than that required in more critical zones. Compaction to90 percent of CE 55 maximum dry density as obtained byMIL-STD-621 should be adequate in these areas. Ifstructures are to be constructed on or within the backfill,compaction of cohesionless soils to within 95 to 100 percentof CE 55 maximum dry density and of cohesive soils to atleast 95 percent of CE 55 should be required for the fulldepth of backfill beneath these structures. The specifieddegree of compaction should be commensurate with thetolerable amount of settlement, and the compactionequipment used should be commensurate with the allowablelateral pressure on the structure. Drainage blankets andfilters having special gradation requirements should becompacted to within 95 to 100 percent of CE 55 maximumdry density. Table 5-1 gives a summary of type ofcompaction equipment, number of coverages, and liftthickness for the specified degree of compaction of varioussoil types (TM 5-818-1/AFM 88-3, Chap. 7).

(5) Cold weather. In areas where freezing temperatureseither hamper or halt construction during the winter, certainprecautions can and should be taken to prevent damagefrom frost penetration and subsequent thaw. Some of theseprecautions are presented below.

(a) Placement of permanent backfill should bedeferred until favorable weather conditions prevail.However, if placement is an absolute necessity duringfreezing temperatures, either dry, cohesionless, non

frost-susceptible materials or material containingadditives, such as calcium chloride, to lower the freezingtemperature of the soil water should be used. Each liftshould be checked for frozen material after compactionand before construction of the next lift is begun. If frozenmaterial is found, it should be removed; it should not bedisked in place. Additives should not be usedindiscriminately since they will ordinarily changecompaction and water content requirements. Priorlaboratory investigation should be conducted to determineadditive requirements and the effect on the compactioncharacteristics of the backfill material.

(b) Under no circumstances should frozenmaterial, from stockpile or borrow pit, be placed in backfillthat is to be compacted to a specified density.

(c) Prior to halting construction during the winter,the peripheral surface drainage system should bechecked and reworked where necessary to providepositive drainage of surface water away from theexcavation.

(d) Foundations beneath structures and backfillaround structures should not be allowed to freeze,because structural damage will invariably develop.Structures should be enclosed as much as possible andheated if necessary. Construction should be scheduledso as to minimize the amount of reinforcing steelprotruding from a partially completed structure since steelwill conduct freezing temperatures into the foundation.

(e) Permanent backfill should be protected fromfreezing as discussed in paragraphs 2-3h (3) and (4).Records should be made of all temporary coverings thatmust be removed before backfilling operations areresumed. A checklist should be maintained to ensure thatall temporary coverings are removed at the beginning ofthe next construction season.

(f) During freezing weather, records should bekept of the elevation of all critical structures to which thereis the remotest possibility of damage or movement due tofrost heave and subsequent thaw. It is important thatfrost-free bench marks be established to which movementof any structure can be referenced.Bench marks also should be established on the structuresat strategic locations prior to freezing weather.

(g) At the beginning of the following constructionseason and after the temporary insulating coverings areremoved, the backfill should be checked for frozenmaterial and ice lenses, and the density of the compactedmaterial should be checked carefully before backfillingoperations are resumed. If any backfill has lost itsspecified density because of freezing, it should beremoved.

(6) Zones having particular gradation requirements.Zones that have particular gradation requirements includethose needed to conduct and control ’ seepage, such asdrainage blankets, filters, and zones

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Table 5-1. Summary of Compaction Criteriaa

Note: The above requirements will be adequate in relation to most construction. In special cases where tolerable settlements are unusually small, it my benecessary to employ additional compaction equivalent to 95 to 100 of compaction effort. A coverage consists of one application of the wheel of arubber-tired roller or the threads of a crawler-type tractor over each point in the area being compacted. For a sheepafoot roller, one pass consists ofone movement of a sheepsfoot roller drum over the area being compacted.

a Prom TM 5-818-1.b Rubber-tired rollers having a wheel load between 18,000 and 25.000 lb and a tire pressure between 80 and 100 psi.c Crawler-type tractors weighing not less than 20,000 lb and exerting a foot pressure not less than 6-1/2 psi.d Power hand tampers weighing more than 100 lb; pneumatic or operated by gasoline engine.e Sheepsfoot rollers having a foot pressure between 250 and 500 psi and tamping feet 7 to 10 in. in length with a face area between 7 and 16 sq in.

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susceptible to frost penetration. Drainage zones are oftenextremely important to the satisfactory construction andsubsequent performance of the structure. To maintain theproper functioning of these zones, care must be taken toensure that the material placed has the correct gradationand is compacted according to specifications.

c. Special problems. In open zones, compaction ofbackfill will not generally present any particular problems ifproper compaction procedures normally associated with thecompaction of soils are exercised and the materialsavailable for use, such as backfill, are not unusually difficultto compact. The majority of the problems associated withbackfill will occur in confined zones where only smallcompaction equipment producing a low compaction effortcan be used or where because of the confined nature of thebackfill zone even small compaction equipment cannot beoperated effectively.

(1) Considerable latitude exists in the various types ofsmall compaction equipment available. Unfortunately, verylittle reliable information is available on the capabilities ofthe various pieces of equipment. Depending upon the soiltype and working room, it may be necessary to establish liftthickness and compaction effort based essentially on trialand error in the field. For this reason, close control must bemaintained particularly during the initial stages of thebackfill until adequate compaction procedures areestablished.

(2) Circular, elliptical and arched walled structures areparticularly difficult to adequately compact backfill beneaththe under side of haunches because of limited workingspace. Generally, the smaller the structure the moredifficult it is to achieve required densities. Rock, whereencountered, must be removed to a depth of at least 6inches below the bottom of the structure and the overdepthbackfilled with suitable material before foundation beddingfor the structure is placed. Some alternate bedding andbackfill placement methods are discussed below.

(a) One method is to bring the backfill to theplanned elevation of the spring line using conventionalheavy compaction equipment and methods. A template inthe shape of the structure to be bedded is then used toreexcavate to conform to the bottom contours of thestructure. If the structure is made of corrugated metal,allowance should be made in the grade for penetration ofthe corrugation crests into the backfill upon application ofload. Success of this method of bedding is highlydependent on rigid control of grade during reexcavationusing the template. This procedure is probably the mostapplicable where it is necessary to use a cohesive backfill.

(b) Another method of bedding placement is tosluice a clean granular backfill material into the bed afterthe structure is in place. This method is particularly

adapted to areas containing a maze of pipes or conduits.Adequate downward drainage, generally essential to thesuccess of this method, can be provided by sump pumpsor, if necessary, by pumping from well points. Sluicingshould be accompanied by vibrating to ensure adequatesoil density. Concrete vibrators have been usedsuccessfully for this purpose. This method should berestricted to areas where conduits or pipes have beenplaced by trenching or in an excavation that providesconfining sides. Also, this method should not be usedbelow the groundwater table in seismic zones, sinceachieving densities high enough to assure stability in aseismic zone is difficult.

(c) Another method is to place clean, granularbedding material with pneumatic concrete equipmentunder the haunches of pipes, tunnels, and tanks. Thematerial is placed wet and should have an in-place watercontent of approximately 15 to 18 percent. A nozzlepressure of 40 pounds per square inch is required toobtain proper density. Considerable rebound of material(as much as 25 percent by volume when placed with thehose nozzle pointed vertically downward and 50 percentwith the nozzle pointed horizontally) occurs at thispressure. Rebound is the material that bounces off thesurface and falls back in a loose state. However, themethod is very satisfactory if all rebound material isremoved. The material can be effectively removed fromthe backfill by dragging the surface in the area wherematerial is being placed with a flat-end shovel. Two orthree men will be needed for each gunite hose operated.

(d) For structures and pipes that can tolerate littleor no settlement, lean grouts containing granular materialand various cementing agents, such as portland cementor fly ash, can be used. This grout may be placed byeither method discussed in (b) and (c) above.However, grouts may develop hard spots (particularlywhere the sluice method is used that could causesegregation of the granular material and the cementingagent), which could generate stress concentrations inrigid structures such as concrete pipes. Stressconcentrations may be severe enough to cause structuraldistress. If lean grouts are used as backfill around a rigidstructure, the structure must be designed to withstandany additional stress generated by possible hard spots.5-2. Installation of Instruments. Installation ofinstrumentation devices should be supervised, if notactually done, by experienced personnel from within theCorps of Engineers or by firms that specialize ininstrumentation installation. The resident engineer staffmust be familiar with the planned locations of allinstruments and necessary apparatus or structures (suchas trenches and terminal houses) so that necessaryarrangements and a schedule for installation

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can be made with the contractor and with the office orfirm that will install the devices. Inspectors shouldinspect any instrumentation furnished and installed bythe contractor. Records must be made of the exactlocations and procedures used for installation and initialobservations. Inspectors should ensure that necessaryextensions are added for the apparatus (such as leadlines and piezometer tubes) installed within the backfillas the backfill is constructed to higher elevations. Caremust be used in placing and compacting backfill aroundinstruments that are installed within or through backfill.Where necessary to prevent damage to instruments,backfill must be placed manually and compacted withsmall compaction equpment such as rammers orvibratory plates.

5-3. Postconstruction distress. Good backfillconstruction practices and control will minimize thepotential for postconstruction distress. Nevertheless, thepossibility of distress occurring is real, and measuresmust be taken to correct any problems before theybecome so critical as to cause functional problems withthe facility. Therefore, early detection of distress isessential. Some early signs of possible distress include:settlement or swelling of the backfill around the structure;sudden or gradual change of instrumentation data;development of cracks in structural walls; and adverseseepage problems. Detailed construction records areimportant for defining potential distress areas andassessing the mechanisms causing the distress.

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CHAPTER 6

SPECIFICATION PROVISIONS

6-1. General.

a. The plans and specifications define the project indetail and show how it is to be constructed. They are thebasis of the contractor’s estimate and of the constructioncontract itself. The drawings show the physicalcharacteristics of the structure, and the specificationscover the quality of materials, workmanship, andtechnical requirements. Together they form the guideand standard of performance that will be required in theconstruction of the project. Once the contract is let, theplans and specifications are binding on both theContracting Officer and the contractor and are changedonly by written agreement. For this reason, it is essentialthat the contractor and the Contracting Officer’srepresentative anticipate and resolve differences thatmay arise in interpreting the intent and requirements ofthe specifications. The ease with which this can beaccomplished will depend on the clarity of thespecifications and the background and experience of theindividuals concerned. Understanding of requirementsand working coordination can be improved if unusualrequirements are brought to the attention of prospectivebidders and meetings for discussion are held prior toconstruction. Situations will undoubtedly arise that arenot covered by the specifications, or conditions mayoccur that are different from those anticipated. Closecooperation is required between the contractor and theinspection personnel in resolving situations of this nature;if necessary, to be fair to both parties a change ordershould be issued.

b. Preparation of contract specifications is easier ifan outline of general requirements is available to thespecification writer. However, it would be virtuallyimpossible to prepare a guide specification thatanticipates all problems that may occur on all projects.Therefore, contract specifications must be written tosatisfy the specific requirement of each project. Somealternate specification requirements that might beconsidered for some projects are discussed below.6-2. Excavation. The section of the specificationsdealing with excavation contains information ondrainage, shoring and bracing, removal and stockpiling,and other items, and refers to the plans for graderequirements and slope lines to be followed in excavatingoverburden soils and rock.

a. Drainage. For some projects the specifications

will require the contractor to submit a plan of hisexcavation operations to the Contracting Officer forreview. The plans and specifications will require that theexcavation and subsequent construction and backfill becarried out in the dry. To meet this requirement, adewatering system based on the results of groundwaterstudies may be included in the plans. Also, for someprojects the specifications may require the contractor tosubmit his plan for controlling groundwater conditions.The specifications should likewise indicate the possibilityof groundwater conditions being different from thoseshown in the subsurface investigation report due toseasonal or unusual variations or insufficient information,since the contractor will be held responsible forcontrolling the groundwater flow into the excavationregardless of the amount. To this end, the specificationsshould provide for requiring the contractor to submit arevised dewatering plan for review where the originaldewatering plan is found to be inadequate.

b. Shoring and bracing. The specifications eitherwill require the contractor to submit for review his plansfor the shoring and bracing required for excavation or willspecify shoring and bracing required by subsurface andgroundwater conditions and details of the lines andgrades of the excavation. In the latter case, thecontractor may be given the option to submit alternateplans for shoring and bracing for review by theContracting Officer. The plans will present thenecessary information for the design of such a system ifthe contractor is allowed this option.

c. Stockpiling. Provisions for stockpiling materialsfrom required excavation according to type of backfillmay or may not be included in the specifications.Generally, procedures for stockpiling are left to thediscretion of the contractor, and a thorough study shouldbe made to substantiate the need for stockpiling beforesuch procedures -are specified. There are severalconditions under which inclusion of stockpilingprocedures in the specifications would be desirable andjustified. Two such conditions are discussed in thefollowing paragraphs.

(1) Under certain conditions, such as those thatexisted in the early stages of missile base constructionwhere time was an important factor, it may be necessaryor desirable to award contracts for the work in phases.As a result, one contractor may do the excavating

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and another place the backfill. It is probable that theexcavation contractor will have little or no interest instockpiling the excavated materials in a manner conduciveto good backfilling procedures. When such a situation canbe foreseen, the specifications should set forth stockpilingprocedures. The justification for such requirements wouldbe economy and optimum use of materials available fromrequired excavation as backfill.

(2) The specifications will contain provisions forremoving, segregating, and stockpiling or disposing ofmaterial from the excavation and will refer to the plans forlocations of the stockpiles. The subsoil conditions andengineering characteristics requirements may state that thespecifications must be quite definite concerning segregationand stockpiling procedures so that the excavated materialscan be used most advantageously in the backfill. Thespecification may require that water be added to thematerial or the material be aerated as it is stockpiled toapproximate optimum water content, that the stockpile beshaped to drain and be sealed from accumulation of excesswater, and that the end dumping of material on the stockpilebe prohibited to prevent segregation of material size or typealong the length of the stockpile.

(3) An alternative to this latter action would be to specifythe various classes of backfill required and leave theprocedure for stockpiling the materials by type to thediscretion of the contractor. In this case, the contractorshould be required to submit a detailed plan for excavatingand stockpiling the material. The plan should indicate thelocation of stockpiles for various classes of backfill so thatthe material can be tested for compliance with thespecifications. The contractor may elect to obtain backfillmaterial from borrow or commercial sources rather than toseparate and process excavated materials. Then thespecifications should require that stockpiles of the variousclasses of needed backfill be established at the constructionsite in sufficient quantity and far enough in advance of theiruse to allow for the necessary testing for approval unlessconditions are such that approval of the supplier’s stockpileor borrow source can be given.6-3. Foundation preparation. The provisions forpreparation for structures will generally not be groupedtogether in the specifications but will appear throughout theearthwork section of the specifications under paragraphs onexcavation, protection of foundation materials, backfillconstruction, and concrete placement. When a structure isto be founded on rock, the specifications will require that therock be firm, unshattered by blasting operations, and notdeteriorated from exposure to the weather. The contractorwill be required to remove shattered or weathered rock andto fill the space with concrete.

a. Specifications for structures founded on soilrequire the removal of all loose material and allunsuitable material, such as organic clay or silt, belowthe foundation grade. When doubt exists as to thesuitability of the foundation materials, a soils engineershould inspect the area and his recommendations shouldbe followed. When removal of rock material below theplanned foundation level is required, the overexcavationwill usually require filling with concrete. Thespecifications also require dewatering to the extent thatno backfill or structural foundation is placed in the wet.

b. Specifications for preparation of the soilfoundation to receive backfill require removing all debrisand foreign matter, making the area generally level, andscarifying, moistening, and compacting the foundation toa specified depth, generally 12 inches. Specificprovisions may or may not be given with respect toleveling procedures.

6-4. Backfill operations. The specifications definethe type or types of material to be used for backfillconstruction and provide specific instructions as to wherethese materials will be used in the backfill. Thepercentage of CE 55 maximum dry density to beobtained, determined by a designated standardlaboratory compaction procedure, will be specified for thevarious zones of backfill. The maximum loose-liftthickness for placement will also be specified. Becauseof the shape of the compaction curve (see discussion ofcompaction characteristics in Section B-1, app. B), thedegree of compaction specified can be achieved onlywithin a certain range of water contents for a particularcompaction effort. Though not generally specified inmilitary construction, the range of water contents is animportant factor affecting compaction.

a. The specifications sometimes stipulate thecharacteristics and general type of compactionequipment to be used for each of the various types ofbackfill. Sheepsfoot or rubber-tired rollers, rammer orimpact compactors, or other suitable equipment arespecified for fine-grained, plastic materials.Noncohesive, freedraining materials are specified to becompacted by saturating the material and operatingcrawler-type tractor, surface or internal vibrators,vibratory compactors, or other similar suitable equipment.The specifications generally will prohibit the use of rockor rock-soil mixtures as backfill in this type ofconstruction. However, when the use of backfillcontaining rock is permitted, the maximum size of therock is given in the specifications along with maximum liftthickness, loading, hauling, dumping, and spreadingprocedures, type of compaction equipment, and methodof equipment operation. The specifications shouldprohibit the use of rock or rock-soil mixtures as backfill in

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areas where heavy equipment cannot operate. Rock-soilmixtures having greater than 8 to 10 percent bindershould be prohibited in all areas. In the case of backfillcontaining rock, the density is not generally specified.Obtaining adequate density is usually achieved byspecifying the compaction procedures. Thespecifications may require that these procedures bedeveloped in field test sections.

b. Specifications may also require specificequipment and procedures to ensure adequate beddingfor round-bottom structures such as tunnels, culverts,conduits, and tanks. Procedures normally specified forplacement of bedding for these types of structures arediscussed in paragraph 5-1c (2).

c. The specifications will state when backfill maybe placed against permanent concrete construction withrespect to the time after completion; this time period isusually from 7 to 14 days. To provide adequateprotection of the structures during backfill construction,the specifications require that the backfill be built upsymmetrically on all sides and that the area of operationof heavy equipment adjacent to a structure be limited.Also, the minimum thickness of compacted materials tobe placed over the structures by small compaction

equipment, such as vibratory plate or rammer type, willbe specified before heavy equipment is allowed tooperate over the structure. The specifications requirethat the surface of the backfill be sloped to drain at alltimes when necessary to prevent ponding of water on thefill. The specifications also provide for groundwatercontrol, so that all compacted backfill will be constructedin the dry. Where select, free-draining, cohesionlesssoils of high permeability are required in areas wherecompaction is critical, the specifications list gradationrequirements. Gradation requirements are also specifiedfor materials used for drains and filters.

d. Unusually severe specification requirementsmay be necessary for backfill operations in confinedareas. The requirements may include strict backfillmaterial type limitation, placement procedures, andcompaction equipment.

e. It is not the policy of the Government to informthe contractor of ways to accomplish the necessaryprotection from freezing temperatures. However, toensure that adequate protection is provided, it may benecessary to specify that the contractor submit detailedplans for approval for such protection.

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CHAPTER 7

CONSTRUCTION CONTROL

7-1. General. The heterogeneous nature of soil makesit the most variable construction material with whichengineers are required to work. Research in soilmechanics and experience gained recently inconstructing large earth embankments have providedadditional knowledge toward understanding andpredicting the behavior of a soil as a constructionmaterial. However, only with careful control canengineers ensure that backfill construction willsatisfactorily fulfill the intended functions. Both thecontractor and the Government share dual responsibilityin achieving a satisfactory product. The contractor isresponsible for inspection and tests through his qualitycontrol system. The Government’s responsibility isassuring that the contractor’s quality control system isachieving the desired results through its qualityacceptance system.

a. Contractor quality control. The contractor isresponsible for all of the activities that are necessary toensure that the finished work complies with the plansand specifications to include quality control requirements,supervision, inspection, and testing. The constructioncontract special provisions explain the quality controlsystem that the contractor must establish; the technicalprovisions specify the construction requirements with thetests, inspections, and submittals that the contractormust follow to produce acceptable work.

(1) Prior to construction, the contractor mustsubmit for approval by the Contracting Officer his plan forcontrolling construction quality. The plan must contain allof the elements outlined in the special provisions anddemonstrate a capability for controlling all of theconstruction operations specified in the technicalprovisions. The plan must include the personnel(whether contractor’s personnel or outside private firm)and procedures the contractor intends to use forcontrolling quality, instructions and authority he is givinghis personnel, and the report form he will use.The plan should be coordinated with his projectconstruction schedule.

(2) During construction, the contractor isresponsible for exercising day by day construction qualitycontrol in consonance with his accepted control plan. Hemust maintain current records of his quality controloperations. Reports of his operations must be submittedat specified intervals and be in sufficient detail to identifyeach specific test.

(3) The prime contractor is responsible for thequality control of all work including any work bysubcontractors.

b. Corps acceptance control. In contrast to thecontractor’s quality control, the Government isresponsible for quality assurance, which includes: thechecks, inspections, and tests of the products thatcomprise the construction; the processes used in thework; and the finished work for the purpose ofdetermining whether the contractor’s quality control iseffective and he is meeting the requirements of thecontract. These activities are to assure that defectivework or materials are not incorporated in theconstruction.

c. Coordination between Government andcontractor. The contractor’s quality control does notrelieve the Contracting Officer from his responsibility forsafeguarding the Government’s interest. The qualityassurance inspections and tests made by theGovernment may be carried out at the same time andadjacent to the contractor’s quality control operations.Quality control and quality assurance supplement oneanother and assist in avoidance of constructiondeficiencies or in early detection of such deficiencieswhen they can be easily corrected without requiring latercostly tear out and rebuild. The remainder of thischapter discusses the Corps quality assurance activities.

7-2. Corps acceptance control organization.a. General. Difficulties in construction of a

compacted backfill can be attributed at least in part toinexperience of the control personnel in this phase ofconstruction work or lack of emphasis as to theimportance of proper procedure and control. Since it isessential that policies with regard to control beestablished prior to the initiation of construction, thoroughknowledge of the capabilities of the control organizationand of the intent of the plans and specifications isrequired. Control is achieved by a review of constructionplans and specifications, visual inspection of constructionoperations and procedures, and physical testing. A well-organized, experienced inspection force can mean thedifference between a good job and a poor one. A goodfield inspection organization must be staffed andorganized so that inspection personnel and laboratorytechnicians are on the job when and where they areneeded. Thus the organization must have knowledge ofthe construction at all times.

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b. Inspection personnel training program. Prior toconstruction, the training, guidance, and support requiredto ensure that the inspection force is fully competentshould be determined. If experience is lacking, trainingand supervision become more important and necessary.

(1) The training program for earthworkinspection personnel should consist of both classroomand field instruction. During the classroom sessions, thespecifications should be studied, discussed, andinterpreted as to the intent of the designer. The criticalareas of compaction should be pointed out as well as thelocation of zoned and transitional areas. The inspectionpersonnel should be instructed on the various zones ofbackfill, types of backfill, density requirements, andclassification and compaction characteristics for eachclass of backfill. Inspection personnel should also beinstructed as to approved sources of borrow for eachtype of backfill and borrow pit operations, such asloading procedures to provide uniform materials and pre-wetting to provide uniform moisture.The various types of backfill should be studied, soinspection personnel can recognize and readily identifythese materials. Jar samples may be furnished for laterreference and comparison; preferably these should besamples of the particular soils on which laboratorycompaction tests were performed in design studies.Instructions should be given as to water content control,lift thickness, and most suitable compaction equipmentfor each type of backfill. Inspection personnel should becapable of recommending alternate procedures toachieve the desired results when the contractor’sprocedure is unsuccessful.

(2) Inspection personnel should be madeaware of the importance of their work by explaining theengineering features of the design on which theconstruction requirements are based. Every opportunityshould be taken to assemble the inspection force fordiscussion of construction problems and procedures sothat all can gain knowledge from the experience ofothers. Inspection personnel should be kept informed ofall decisions and agreements pertinent to their work thatare made at higher levels of administration. They shouldbe advised of the limits of their authority and contact withcontractor personnel.

(3) Field training of inspection personnelshould include observation of their control techniquesand additional instruction on elements of fieldworkrequiring correction. Inspection personnel should beinstructed in the telltale signs that give visual indicationswhether sufficient compaction is being applied andproper water content is being maintained (see para 7-5b(4) and EM 1110-2-1911 for discussion of telltale signs).They should develop the ability to determine from visualobservations (based on correlations with tests on theproject) that satisfactory compaction is being obtained so

that considerable emphasis can be placed on suchmethods as a control procedure rather than relying onfield tests alone. Inspection personnel should becapable of selecting locations at which field density andmoisture determinations should be made. To meet thisrequirement they must be present almost continuouslyduring compaction operations to observe and note areaswhere tests appear to be needed. Laboratorytechnicians should be made available to perform tests sothat the inspection personnel will be free to observe theplacement and compaction process on another portion ofthe backfill. Inspection personnel should be able to useexpedient quick-check field apparatus such as theProctor and hand-cone penetrometers (sec. B-3, app. B)to make a rapid check of the field water content tosupplement acceptance testing and to serve as a guidein determining areas that should be tested. Inspectionpersonnel should also be well versed in normal testingprocedures so they can properly supervise testing orexplain the procedure in case they are questioned bycontractor personnel.

(4) It is necessary and important thatinspection personnel ascertain their authority andresponsibility at an early stage in the construction. Theirpolicy should be one of firmness coupled withpracticality. The quality of the work should not becompromised; however, unreasonable requirements andrestrictions should not be placed on the contractor inenforcing the specifications. If the inspection personnelknow their job and are fair and cooperative in dealingwith the contractor, they will gain his or her respect andcooperation and be able to efficiently carry out theirresponsibilities.

b. Field laboratory facilities. The field laboratoryis used for routine testing of construction materials (suchas gradation, water content, compaction, and Atterberglimits tests) and for determining the adequacy of fieldcompaction. The data obtained from tests performed byinspection personnel serve as a basis for determiningand ensuring compliance with the specifications, forobtaining the maximum benefit from the materials beingused, and for providing a complete record of thematerials placed in every part of the project. The sizeand type of laboratory required are dependent on themagnitude of the job and the type of structures beingbuilt. Where excavation and backfill construction areextensive and widespread, the establishment of acentrally located field laboratory is generally beneficial.This laboratory in addition to having equipment for on-the-job control will provide a nucleus of experienced soilsengineers or engineering technicians for generalsupervision and training of inspection personnel. Fieldcontrol laboratories on the sites may be established asnecessary during the excavation and backfill phases ofthe construction. They may be set up

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in an enclosed space allocated by the project officer or inmobile testing laboratories, such as pickup trucks with acamper and equipped with the necessary testingequipment for performance of field density tests, watercontent tests, and gradation tests. Another possibility isthe use of large portable boxes in which equipment isstored. When special problems arise and the requiredtesting equipment is not available at the site laboratory,the testing should be performed at the central laboratory.

7-3. Excavation control techniques. Control to obtaina satisfactory excavation is exercised by enforcement ofapproved plans, visual observations, a thoroughknowledge of the contractor’s plan of operation andconstruction schedule, the dimensions and engineeringfeatures of the structure(s) to be placed in theexcavation, and vertical and horizontal controlmeasurements to ensure that the proper line and graderequirements are met.

7-4. Foundation preparation control techniques. Themain control technique for ensuring proper foundationpreparation is visual inspection.Prior to backfill placement, all uncompacted fill should beremoved from those portions of the excavation to bebackfilled. The items included are road fills, loosematerial that has fallen into overexcavated areasadjacent to foundations, and construction ramps otherthan those required for access to the excavation.Identification of such items will be easier if the inspectionpersonnel have charted the items on the plans as theywere created, since they are not always easilydiscernible by visual inspection. It is desirable to controlearth backfill placed in foundation leveling operations bywater content and density tests. Care should beexercised to ensure that all subdrains required in thefoundation are protected by filters and transitional zonesthat are adequate to prevent infiltration of fines from thesurrounding backfill that might otherwise clog the drainsand undermine structures.

7-5. Backfill quality acceptance control.The necessary authority to assure that compactedbackfill is in compliance with the specifications is given inthe specifications. The control consists of inspecting andtesting materials to be used, checking the amount anduniformity of soil water content, maintaining the properthickness of the lifts being placed, and determining thedry unit weight being obtained by the compactionprocess. While control consists of all of these things,good inspection involves much more.

a. Inspection activities. One of the bestinducements to proper placement and compaction ofbackfill is the presence of the inspection personnel whenbackfill is being placed. However, to be of value theinspector must know his job. He should be familiar with

all aspects of backfill operation, such as selection andavailability of materials, processing, hauling, compaction,and inspection procedures. Some of the most commondeficiencies in inspection personnel activities are asfollows:

(1) Failing to enforce specificationrequirements for preparation of the area for backfill.Often temporary fills, the working platform, debris, andother undesirable materials are left in the excavationcausing weak areas and resulting in greaterconsolidation in the backfill.

(2) Failing to be cognizant of detailed site-adapted plans for stockpiling and placing backfill atspecific locations. Without knowledge of these plans,inspection personnel are sometimes forced to makeengineering decisions beyond their capability, such ason-the-site approval of a new material or mixture ofmaterials, and stockpile locations.

(3) Allowing processing of backfill materialand adjustment of water content on the fill that shouldhave been accomplished prior to placement. The resultsare the segregation of grain sizes and the non-uniformdistribution of water content. Al major processing,including crushing, raking, mixing, and adjusting of watercontent, must be done in the stockpile or borrow areas.

(4) Allowing lift thickness that is inconsistentwith equipment capabilities and thicker than that allowedby specifications. Field density determinations will notnecessarily detect this inconsistency.

(5) Allowing construction of backfill slopesthat are too steep to obtain the full effect of compactionequipment.

(6) Failing to require that the fill be built upuniformly in a well-defined pattern. Since thecontractor’s next move cannot be predicted, theinspection personnel cannot adequately plan theiroperations, and it is difficult to determine which areas ofbackfill have been tested and approved when the backfillis built up in an unorganized manner.

(7) Allowing segregation of coarse-grained,non-cohesive materials. This condition is caused byimproper hauling, dumping, and spreading techniques.

(8) Allowing the use of compaction equipmentnot suited to material being compacted.

(9) Failing to perform sufficient field densitytesting in critical areas.

(10) Allowing material that is too wet or too dryto be compacted.

(11) Failing to require that intermediate backfillsurfaces be shaped to drain during backfilling at otherlocations.

b. Inspection requirements. To properly controland inspect backfill operations, the inspection personnelmust keep informed of the construction schedule at alltimes and be at the site where backfill is being placed.

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The inspection personnel must be thoroughly familiarwith every aspect of the earthwork section of thespecifications and know boundary locations for thevarious zones of material. They should be able to readilyidentify the various classes of backfill and know theircompaction characteristics and requirements.Good inspection personnel will also know thecompaction capabilities of various types of equipmentand the materials that each type is best suited tocompact.

(1) To maintain adequate control ofcompaction operations, a staff of earthwork inspectorsand laboratory personnel commensurate with theimportance of the work and size of the operation isessential. There should be at least one inspector at thefill when backfill is being placed. His sole duty should beinspection of earthwork. Although he should be familiarwith the testing procedures and capable of directingtesting operations and selecting locations for testing, heshould not be required to perform the tests. Laboratorytechnicians should be available for this purpose. Adiscussion of the methods and procedures for fielddensity testing of the compacted fill is contained insection B-3, appendix B.

(2) The specifications should require thatnecessary processing of backfill materials be performedin the stockpile or borrow pit. Processing includes rakingor crushing to remove oversize material, mixing toprovide uniformity, and watering or aerating to attain awater content approximating optimum for compaction.An earthwork inspector is required at the stockpile orborrow pit to enforce these provisions. In addition, thisinspector has the duties of classifying the materials,determining their suitability, and directing the zone ofbackfill in which they are to be placed. He is chargedwith the responsibility of seeing that the contractor usesthe materials available for backfill in the mostadvantageous manner. Generally, the stockpile orborrow pit inspector relies upon visual inspection andexperience to exercise control over these operations.Occasionally, he may require that appropriate tests beperformed to confirm his judgment.

(3) The duties of the backfill inspector consistof checking the material for suitability as it is placed onthe fill and spread, ensuring that any oversize material,roots, or trash found in the material is removed, checkingthe thickness of the lift prior to compaction, checking foruniformity and amount of water content, observingcompaction operations, and directing or monitoringtesting of the compacted material for compliance withdensity and water content requirements.

(4) There are many techniques and rule-of-thumb procedures that the earthwork inspector can andmust resort to for assistance in his work. A few of themare discussed below; others can be ascertained by

inspectors meeting together to discuss problems andcorrective action.

(a) The thickness of loose lifts can bechecked easily by probing with a calibrated rod just priorto compaction. Compaction of lifts too thick for theequipment will not normally be detected by performingdensity tests on the lift, since adequate compaction maybe indicated by a test made in the upper portion of the liftand the lower portion may still have too low a density. Itis therefore a requisite that lift thickness be controlled ona loose-thickness basis prior to compaction.

(b) Checks for proper bond betweenlayers can be made by digging through a lift aftercompaction and using a shovel to check this bond. If thesoil can be separated easily along the plane betweenlifts, sufficient bond is not being provided. Backfillmaterials should not be placed on dried or smoothsurfaces, as bond will be difficult to obtain.

(c) Inspection personnel should bethoroughly aware of areas where compaction is critical.These areas are the confined spaces around andadjacent to structures that are not accessible to therolling and spreading equipment. Although the volume ofbackfill is usually rather small in these areas, a muchhigher frequency of check testing for density is requiredas well as a careful check of the quality and watercontent of the materials to be placed.

c. Compaction control tests. Compaction testswill have been performed on representative specimensobtained from exploratory sampling prior to construction.The selection of suitable backfill material are in factgenerally made based on these and other tests. At leastduring the early phases of the backfill operation, densityrequirements are based on these and in some casesadditional preconstruction compaction tests. Conditionsmay develop that require compaction tests during backfilloperations to establish new density requirements.Generally, these changes are the results of backfillmaterial deviations. The need for additional control testsmay be ascertained from visual observation and changesin compaction characteristics during field compaction.For most backfill materials, quality acceptancecompaction control tests must be performed according tothe CE 55 test procedure specified in MIL-STD-621, theequivalent procedure in ASTM D 1557, or the two-pointtest procedure (app B). For some cohesionless soilswhere higher maximum dry densities can be obtainedusing the vibratory (relative density) compactionprocedure, the specifications may require the vibratorytest procedures as specified in EM 1110-2-1906 orASTM D 2049. Field compaction control and rapidcompaction check tests that are used to supplement theCorps acceptance control tests are discussed inappendix B.

d. Field moisture-density control techniques.Moisture-density control is the most important phase of

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backfill operations. The success of ensuring requiredbackfill density often determines the functional service ofthe imbedded structure. Good control involves manytechniques. An experienced inspector will not rely onany one technique but from experience will base hiscontrol on a combination of techniques. Moisture-densitycontrol techniques may be grouped into three categories:rule-of-thumb techniques, and indirect and directmoisture density measurements.

(1) Rule-of-thumb methods. Rule-of-thumbtechniques are derived from experience and are basedon visual observations and feel of the material. A rule-othumb for judging if the water content of a fine grained,plastic material is near the optimum water contentconsists of rolling the material between the hands until itforms a thread approximately Y inch in diameter. If thematerial at this stage tends to crack or crumble, it is inthe proper water content range for compaction. It will berecognized that this method is similar to the method ofdetermining the plastic limit of a soil. The methods aresimilar because the optimum water content forcompaction of a cohesive soil roughly approximates theplastic limit of the soil.

(a) Another good indication of whetherthe proper water content has been obtained can bedetermined by observing the compacting equipment.When a sheepsfoot roller is being used and the soilsticks to the roller to any great extent, the material isbeing rolled too wet for the equipment being used; atoptimum water content it may be expected that a fewclods will be picked up by the roller but a general stickingwill not occur. If the compacted fill does not definitelyspring (noticeable to visual observation) under haulingand compaction equipment, it is probable that severallifts of fill have been placed too dry. The roller should rollevenly over the surface of the backfill if water content isuniform throughout the lift and should not ride higher onsome portions of the backfill than on others. If on thefirst pass of a rubber-tired roller the tires sink to a depthequal to or greater than one half the tire width, if afterseveral passes the soil is rutting excessively, or if at anytime during rolling the weaving or undulating (as opposedto normal "springing" of the surface) of the material istaking place ahead of the roller, either the tire pressure istoo high or the water content of the material is too high.On the other hand, if the roller tracks only very slightly ornot at all and leaves the surface hard and stiff afterseveral passes, the soil is probably too dry. For mostsoils having proper water contents, the roller will trackevenly on the first pass and the wheels will embed 3 to 4inches. Some penetration should be made into soil at itsproper water content, though the penetration willdecrease as the number of passes increases. Afterseveral passes of a sheepsfoot roller, the roller shouldstart walking out if adequate and efficient compaction isbeing obtained. Walking out means the roller beginsbearing on the soil through its feet only-the drum is riding

a few inches above the soil surface. If the roller walksout after only a few passes, the soil is probably too dry; ifit does not walk out but continues churning up thematerial after the desired number of passes, the soil istoo wet or the foot contact pressure is too high.

(b) A trained inspector will spend sometime in the field laboratory, performing severalcompaction tests on each type of backfill material tobecome familiar with the differences in looks, feel, andbehavior and learning to recognize when they are too dryor too wet, as well as when they are at optimum watercontent.

(2) Indirect methods. Indirect methods ofdetermining the density and water content involvemeasurement of the characteristic of the material thathas been previously correlated to the maximum densityand optimum water content. These methods ofmeasuring in-place density and water content canusually effect a more detailed control of a job than canbe accomplished by direct methods alone because theycan provide quicker determinations. However, noindirect method should ever be used without firstchecking and calibrating it with results obtained fromdirect methods, and periodic checks by direct methodsshould be made during construction. Indirect methodsinclude the use of the nuclear moisture-density meter,the Proctor penetrometer (often referred to as the"Proctor needle"), the hand cone penetrometer, and inthe hands of an experienced inspector even a shovel.

(a) The nuclear moisture-density methodconducted in accordance with ASTM D 2922 (for densitydetermination) and ASTM D 3017 (for water contentdetermination) is the only indirect control method usedfor the Corps quality acceptance control. The methodprovides a relatively rapid means for determining bothmoisture content and density. Of the three methodspresented in ASTM D 2922, Method B - DirectTransmission is the best suited for a compacted liftthickness exceeding approximately 4 inches. Thenuclear moisture-density method is discussed in moredetail in section B-3, appendix B.

(b) Penetrometers, such as the Proctorand hand cone penetrometers, are useful under certainconditions for approximating density. However, bothmethods require careful calibration using soils of knowndensity and water content and considerable operatingexperience. Even then, the results may be questionablebecause non-uniform water content (in fine-grainedmaterial) or a small piece of gravel can affect thepenetration resistance. Penetrometers, therefore, arenot recommended for general use in compaction control;however, they can be a very useful tool in supplementingthe inspector’s visual observations and providing ageneral guide for detecting areas of doubtful compac

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tion. The procedure using the Proctor penetrometer fordetermining the relation between wet density, penetrationresistance, and water content is described in ASTM D1558 and in section B-3, appendix B. The hand conepenetrometer procedure also is discussed in section B-3.

(c) Many inspectors in the past have hadgood success in estimating density by simply observingthe resistance of the compacted soil to penetration by aspade. This method requires considerable experienceand is useful only in detecting areas that might requirefurther density tests.

(3) Direct methods. Direct field densitydetermination consists of volume and weightmeasurements to determine the wet density of in-placebackfill and water content measurements to determinein-place water contents and dry densities. The threemethods used for the Corps quality acceptance densitydetermination are: (a) the sand-cone method accordingto MIL-STD-621 (Method 106) and ASTM D 1556; (b) therubber-balloon method according to ASTM D 2167; andfor soft, fine-grained cohesive soils, the drive-cylindermethod according to MIL-STD-621 (Method 102) andASTM D 2937. In addition to the approved methods, amethod sometimes employed to measure densities ofcoarse-grained cohesionless material consists of thelarge-scale, water-displacement method. The large-scale, water-displacement method is discussed in EM1110-2-1911. The sand cone method is considered tobe the most reliable method and is recommended as theproof or calibration test for calibrating other methodssuch as the nuclear density method. The direct fielddensity methods are discussed in section B-3, appendixB.

e. Water content by microwave oven. Thebiggest problem associated with both field compactiontests and in-place density and water content control testsis the length of time required to determine water content.Conventional overdrying methods require from 15 to 16hours for most fine-grained cohesive soils. In somecases, such as confined zones, the contractor may haveplaced and compacted several layers of backfill over thelayer for which density tests were made before qualityacceptance test data are available. Even though thecontractor places successive layers at his own risk, arapid turn around between testing and test results couldprevent costly-tear out and recompact procedures.Drying specimens in microwave ovens offers a practicalmeans for rapid determination of water content for mostbackfill materials if properly conducted. Times requiredfor drying in a microwave oven are primarily governed bythe mass of water present in the specimen and thepower-load output of the oven. Therefore, drying timemust be calibrated with respect to water content andoven output. Also, it may not be possible to successfullydry certain soils containing gypsum or highly metallicsoils such as iron ore, aluminum rich soils, and bauxite.

Details of the microwave-oven method used for rapiddetermination of soil water content is given in section B-3, appendix B.

f. Frequency and location of quality acceptancedensity tests. Acceptance control testing should be morefrequent at the start of backfill placement. Aftercompaction effort requirements have been firmlyestablished and inspection personnel have becomefamiliar with materials behavior and acceptablecompaction procedures, the amount of testing can bereduced. Many factors influence the frequency andlocation of tests. The frequency will be dependent on thetype of material, adequacy of the compactionprocedures, and how critical the backfill beingcompacted is in relation to the performance of thestructure.

(1) A systematic testing program should beestablished at the beginning of the job. Acceptancecontrol tests laid out in a predetermined manner areusually designated as routine control tests and areperformed either at designated locations or at randomrepresentative locations, no matter how smoothly thecompaction operations are being carried out. A routineacceptance control test should be conducted for at leastevery 200 cubic yards of compacted backfill material incritical areas where settlement of backfill may lead tostructural distress and for at least every 500 cubic yardsin open areas not adjacent to structures.

(2) In addition to routine acceptance controltests, tests should be made in the following areas:where the inspector has reason to doubt the adequacy ofthe compaction; where the contractor is concentrating filloperations over relatively small areas; where smallcompaction equipment is being used such as in confinedareas; and where field instrumentation is installed, mainlyaround riser pipes.

g. Errors in field density measurements. Densityand water content measurements determined by any ofthe methods discussed above are subject to threepossible sources of errors. The three categories ofpossible error sources are human errors, errorsassociated with equipment and method, and errorsattributed to material property behavior.

(1) Human error includes such factors asimproper equipment readings and following improper testprocedures. Human errors are not quantitative.However, errors of this type may be minimized byutilizing competent testing personnel familiar with testingprocedures.

(2) There are two types of possible errorsrelated to test equipment. One type of error relates tothe sensitivity of the equipment with respect to itscapability to accurately measure the true density or watercontent. Sensitivity errors are quantitative only in thesense that limiting ranges of possible error can be es

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tablished. An example of sensitivity error would be thenuclear density device that is capable of determiningdensities only to within 3 to 5 pounds per cubic foot ofthe true density. The second type of error relates toconstant deviations between measured and true density.Constant deviation errors can be corrected by calibratingtest equipment against known densities.

(3) Material property errors are primarilylimited to density determinations using either the sand-cone or the rubber-balloon method in sands. When asoil is physically sampled during the process ofconducting an in-place density measurement using thesetwo methods, a shearing action of the soil is unavoidable.Cohesionless soils are sensitive to volume changeduring shear, dense sands tend to expand and increasein volume, and loose sands tend to contract anddecrease in volume. Errors of this nature cannot bequantified or detected in the field. However, such errorscan be as high as 6 percent for sand using the rubber-balloon method for volume measurements.

h. Acceptance or rejection. The inspectionpersonnel have the responsibility to accept or reject thebackfill or any part thereof based on the qualityacceptance control tests. On the surface, this taskseems straightforward. If a segment of the backfilltested at several locations for acceptance passes or failsto pass minimum requirements by a wide margin, then itis generally safe to assume that the backfill within thatsegment either has or has not been adequatelycompacted and the acceptance or rejection of thatsegment can be made based on the test results. On theother hand, if the tests indicated insufficient compaction,the size of the affected area may be questionable; it ispossible that the test(s) represents only a small area andthe lift being tested may be sufficiently compactedelsewhere. In view of the possible errors associated withcontrol tests, tests that indicate marginal passage orfailure should be treated with caution. The borderlinecase requires a close look at several factors: how theresult compares with all previous results on the job, howmuch compaction effort was used and did it differ fromprevious efforts, how does this particular materialcompare with previously compacted materials, theimportance of the lift location in relation to the entirestructure, and the importance of obtaining the correctdensity or water content from the designer’s standpoint.When all factors have been considered, a decision ismade as to which corrective measures are required.What makes such decisions so difficult is that they mustbe made immediately; time will not permit the problem tobe pondered. Discussion with design engineers prior tobeginning compaction operations may help in theevaluation of many of these factors.

(1) On jobs requiring large volumes ofbackfill, it may be advantageous to base the decision toaccept or reject on statistical methods. Statistical

methods require separate analysis for each backfillmaterial type and compaction effort, complete randomselection of test locations, and a large number of controltests as compared with the conventional decisionmethod. In addition, statistical methods include watercontent control, which is not normally included in militaryspecifications.

(2) The theory and details concerning theapplication of statistical methods for compaction controlare well developed. Figure 7-1 shows a sequentialinspection plan example of how the end results of astatistical analysis might be used for the purpose ofacceptance or rejection. In this example, it wasestablished by statistical analysis that adequate densitiescould probably be obtained with reasonable confidenceby a given compaction effort for desired water contentsranging from 3 percentage points below to 1 percentagepoint above optimum. It was also established that adensity corresponding to 95 percent of CE 55 maximumdry density was the minimum acceptable density basedon required engineering performance of the backfill. Thesequential inspection plan consists of examining, insequence, single tests that are obtained at random froma segment of the backfill being considered foracceptance or rejection and, for each test, making one ofthree possible decisions: the segment is acceptable; thesegment is unacceptable; and the evidence is notsufficient for either decision without too great a risk oferror as indicated by the retest block in figure 7-1(a).The reject areas in figure 7-1 indicate conditions thatcannot be corrected by additional rolling. The materialmust be replaced in thinner lifts and be within the desiredwater content range before adequate compaction can beachieved with the compaction equipment being used. Ifthe retest decision is reached, an additional test is madeat a second random location, and the same threedecisions are reconsidered in light of this additionalinformation. If the second test falls below the acceptblocks, the segment of backfill representative of that testshould be rejected; or if compaction procedures thathave produced acceptable tests in the past have notbeen altered, then the compaction characteristics of thatpart of the backfill should be reevaluated.

(3) The primary advantage of statisticalmethods is that they offer a means of systematicallyevaluating acceptance or rejection decisions rather thanleaving such decisions entirely to the judgment of theinspection personnel. However, if experienced and welltrained inspection personnel are available, this approachmay not be necessary.

i. Construction reports. A record should bemaintained of construction operations. It is valuable inthe event repairs or modifications of the structure arerequired at a later time. A record is necessary in theevent claims are made either by the contractor or the

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Figure 7-1. Acceptance Rejection Scheme for a Backfill Area.

Contracting Officer that work required or performed wasnot in accordance with the contract. Recorded data arealso beneficial in improving knowledge and practices forfuture work. The basic documents of the constructionrecord are the plans and specifications, modificationsadopted that were considered to come within the termsof the contract, amendments to the contract such asextra work orders or orders for change, results of tests,and measurements of work performed. The amount ofreporting required varies according to the importanceand magnitude of the earthwork construction phase ofthe project and the degree of available engineeringsupervision. The forms to be used should be carefullyplanned in advance, and the inspection personnel should

be apprised of the importance of their reports and theneed for thorough reporting. Records should be madeon every test performed in the laboratory and in the field.All information necessary to clearly define the locationsat which field tests are made should be presented. Inthe daily reports, inspection personnel should includeinformation concerning progress, adequacy of the workperformed, and retesting of areas requiring additionalwork to meet specifications. These daily reports couldbe of vital importance in subsequent actions. It is goodpractice for the inspection personnel to keep a daily diaryin which are recorded the work area, work accomplished,test results, weather conditions, pertinent conversationswith the contractor, and instructions received and given.

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

APPENDIX A

REFERENCES

Government Publications

Department of Defense

Military StandardsMIL-STD-619 Unified Soil Classification System for Roads, Airfields, Embankments,

and Foundations.MIL-STD-621 Test Methods for Pavement Subgrade, Subbase, and Base Course

Materials

Department of the Army, the Navy, and the Air Force

Technical Manuals

AFM 89-3, Chapter 2 Materials TestingTM 5-818-1/AFM 88-3, Chap. 7 Soils and Geology: Procedures for Foundation Design of Buildings and

Other Structures (Except Hydraulic Structures)TM 5-818-2/AFM 88-6, Chap. 4 Pavement Design for Frost ConditionTM 5-818-5/NAVFAC P-418/ Dewatering and Groundwater Control for Deep Excavations

AFM 88-5, Chap. 6TM 5-818-6 Grouting Methods and EquipmentTM 5-820-2/AFM 88-5, Chap. 2 Subsurface Drainage Facilities for AirfieldsTM 5-820-4/AFM 88-5, Chap. 4 Drainage for Areas Other Than AirfieldsNAVFAC DM-7 Soil Mechanics, Foundations, and Earth Structures

Department of the Army, Corps of Engineers

USACE Publications Depot, 809South Pickett, Alexandria, VA22304

EM 385-1-1 Safety and Health Requirements ManualEM 1110-2-1906 Laboratory Soils TestingEm 1110-2-1908 Instrumentation for Earth and Rockfill DamsEM 1110-2-1911 Construction Control for Earth and Rockfill DamsEM 1110-2-2502 Retaining WallsEM 1110-2-2902 Conduits, Culverts, and PipesER 1110-2-1806 Earthquake Design and Analysis for Corps of Engineers Dams

Department of Interior

Bureau of Reclamation, Manual, Earth ManualSecond Edition

Department of Transportation (DOT) 1920 L Street, N.W., Washington, DC 20036

Report No. FHWA-RD-78-141 Design and Construction of Compacted Shale Embankments, Vol 5.Technical Guidelines (December 1978)

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

Report No. FHWA-RD-79-51 Technical Guidelines for Expansive Soils in Highway Subgrades (June1979)

Non-Government Publications

American Society for Testing and Materials; 1916 Race Street, Philadelphia, PA 19103

D 1556 Standard Test Method for Density of Soil in Place by the Sand-ConeMethod

D 1557 Standard Test Methods for Moisture-Density Relations of Soils Using 10-lb. (4.5 kg) Rammer and 18-in. (457-mm) Drop

D 1558 Standard Test Method for Moisture-Penetration Resistance Relations ofFine-Grained Soils

D 2049 Standard Test Method for Relative Density of Cohesionless SoilsD 2167 Standard Test Method for Density of Soil In Place by the Rubber-Balloon

MethodD 2216 Standard Method of Laboratory Determination of Moisture ContentD 2922 Standard Test Method for Density of Soil and Soil-Aggregate In Place by

the Nuclear MethodsD 2937 Standard Test Method for Density of Soil In Place by the Drive-Cylinder

MethodD 3017 Standard Test Method for Moisture Content of Soil and Soil-Aggregate In

Place by the Nuclear MethodsSpecial Technical Publication Symposium on Nuclear Methods for Measuring Soil Density and Moisture

STP No. 293 (June 1960)Special Technical Publication Special Procedures for Testing Soil and Rock for Engineering Purposes

STP No. 479 (June 1970)Special Technical Publication Soil Specimen Preparation for Laboratory Testing (June 1976) STP No.

599

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APPENDIX B

FUNDAMENTALS OF COMPACTION, FIELD COMPACTION TEST METHODS,

AND FIELD MOISTURE-DENSITY TEST METHODS

SECTION B-1. FUNDAMENTALS OF COMPACTION

B-1. Factors Influencing compaction. Soilcompaction is the act of increasing the density (unitweight) of the soil by manipulation by pressing, ramming,or vibrating the soil particles into a closer state ofcontact. The most important factors in soil compactionare type of soil, water content, compaction effort, and liftthickness. It is the purpose of field inspection to ensurethat the proper water content, lift thickness, andcompaction effort are used for each soil type so that thedesired degree of compaction is obtained. When thewater content, lift thickness, or compaction effort beingused does not produce the desired degree ofcompaction, changes may be necessary. Thedetermination of the necessary changes of these factorsto produce the desired degree of compaction requiresknowledge of the principles governing the compaction ofsoils. Therefore, it is important that inspection personnelhave a general understanding of the fundamentals ofcompaction.

a. General. It has been established throughresearch and construction experience that there is amaximum density to which a given soil can becompacted using a particular compaction effect. Foreach soil and a given compaction effort, there is a uniquewater content, which is called the optimum watercontent, that produces the maximum density. Thepurpose of the laboratory compaction test is to determinethe variation in density of a given soil at different watercontents when compacted at a particular effort or efforts.Normally, the soil to be used is compacted in thelaboratory over a range of water contents using theimpact compaction procedures given in MIL-STD-621Aand ASTM D 1557. The compaction effort used isselected on the basis of the requirements of thestructure. In foundation or backfill design for most majorstructures, the CE 55 (also termed modified) compactioneffort that produces approximately 56,000 foot-poundsper cubic foot of compacted soil should be used.

(1) For some cohesionless soils, a greatermaximum density can be obtained using vibratory-typecompaction procedures given in EM 1110-2-1906 andASTM D 2049 than can be obtained using MIL-STD-621A or ASTM D 1557 impact-compaction procedure.Thus, there may be cases where the vibratorycompaction method may be more appropriate in

determining the maximum density. The compactioneffort used for design purposes should be the basis forconstruction control.

(2) A compaction curve is developed in theimpact-compaction test by plotting densities (dry unitweights) as ordinates and the corresponding watercontents (as percent of dry soil weight) as abscissas.For most soils the curve produced is generally parabolicin form. Figure B-1 shows a compaction curve.The water content corresponding to the peak of thecurve is the optimum water content. The dry unit weightof the soil at the optimum water content is the maximumdry density. The zero air voids curve represents therelation between water content and dry density for 100percent saturation of the particular material tested.Thus, it shows the dry density for a given water contentbased on the condition that all the air is forced out of thevoids by the compaction process.

b. Influence of soil type. Compactioncharacteristics vary considerably with the type of soil.Figure B-2 shows four compaction curves representingthe water content-density relation for four general soiltypes for standard compaction. The maximum drydensity for a uniform sand occurs at about zero watercontent, although density approaching maximum can beobtained when the sand is saturated. A very sharppeaked curve of dry density versus water content isusually obtained for a silt, and water content is critical toachieving maximum density. A small change in watercontent (as small as 0.5 percentage point) above orbelow optimum causes a significant decrease in thedensity (as much as 2 to 4 pounds per cubic foot) for agiven compaction effort. The compaction curve for alean clay is not as sharp as that for the silt, and watercontent control is not as critical. Optimum watercontents for silts and lean clays generally range between15 and 20 percent. The compaction curve for fat clays israther flat and water content is not particularly critical toobtaining maximum density; a 2 to 3 percentage pointchange in water content from optimum for fat clayscauses only a small decrease (1 pound per cubic foot orless) in density. The maximum dry density, as obtainedin laboratory compaction tests using MIL-STD-621A and

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Figure B-1. Compaction curve.

Figure B-2. Typical compaction test curves.

ASTM D 1557 or modified compaction effort, depends onthe soil type and varies generally from about 125 to 140pounds per cubic foot for well-graded, sand-gravelmixtures to about 90 to 115 pounds per cubic foot for fatclays. The optimum water content generally ranges fromzero for the sand-gravel mixtures to about 30 percent forthe fat clays.

c. Influence of water content. For a given fine-grained soil and a given compaction effort, the watercontent determines the state at which maximum drydensity occurs. At low water contents when the soil is

stiff and hard to compress, low, dry densities and highvalues of air content result. As the water content isincreased, higher dry densities and lower air contentvalues are obtained. Increased densities result with anincrease in water content up to optimum water content.Beyond this point, the water in the voids becomesexcessive, and pore pressures develop under theapplication of the compaction effort to resist a closerpacking; lower dry densities are the result.

d. Influence of compaction effort. For most soils,increasing the energy applied (compaction effort) per unitvolume of soil results in an increase in the maximumdensity (unit weight). This greater density occursgenerally at a lower water content. This phenomenon isevident in both field and laboratory compactions. Thus,for each compaction effort, there is a unique optimumwater content and maximum dry density for a given soil.Figure B-3 shows the effect of variation in compactioneffort on the maximum dry density and optimum watercontent for a lean clay (CL). Where values of maximumdry density and optimum water content are specified,they should be referenced to the compaction effort used.

e. Influence of lift thickness. Compaction effortapplied to a soil surface dissipates with depth.Therefore, it is important that the lift thickness to becompacted be commensurate with the type of soil andthe compaction effort. With proper consideration andcontrol over factors influencing compaction, most soilscan be compacted to provide a stable backfill, with the

Figure B-3. Molding water content versus density-leanclay (laboratory impact compaction).

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

exception of certain bouldery soils and soils containingsignificant amounts of soluble, soft, or organic materials.

B-2. Mechanics of compaction. The influence of thewater content on compaction is markedly different oncoarse-grained, cohesionless soils and fine-grained,cohesive soils. As a result, the mechanics ormanipulation of soil grains in the two types of soil duringthe compaction process are different. The mechanics ofcompaction for the two soil types are discussed insubsequent paragraphs.

a. Compaction of coarse-grained soils.Compaction of coarse-grained soils that contain little orno fines and thus exhibit no plasticity (termedcohesionless soils) is achieved by causing the individualparticles to move into a closer, more compactarrangement, with smaller particles filling in voidsbetween larger particles. The compaction energyovercomes friction at contact points between particles asthey move past one another into closer packing.

(1) A loose volume of coarse-grained soil,such as gravel or sand, contains spaces or "voids"between individual particles that are filled with air and/orwater. The density that can be obtained in such a soilunder a given amount of compaction effort depends onthe gradation and shapes of the particles and on thewater content. For a well-graded gravel or sand, therange of particle sizes is sufficient to allow a fairlycompact arrangement of particles, with smaller particlesfilling in the voids between larger particles. For poorlygraded soil, either of uniform gradation or skip-graded(lacking a specific range of particle sizes), the distributionof particle sizes limits the density that can be obtained.Segregation of similar size particles in a skip-gradedmaterial tends to occur and prevents the voids frombeing greatly reduced. In a uniform soil, point-to-pointcontact occurs at very low compaction effort and lowdensity results; further increase in density can only beaccomplished by crushing the grains. Therefore, a well-graded, coarse-grained material can generally becompacted to a greater density under a givencompaction effort than a poorly graded, coarse-grainedsoil. The increase in maximum density with increase incompaction effort will be greater for a well-graded soilthan that for a poorly graded soil.

(2) Rounded particle shapes facilitatemovement and sliding of particles, while angular particleshapes restrict movement and sliding of grains in relationto one another. For either a well-graded, or a poorlygraded, coarse-grained material, increase in angularily ofgrains requires a corresponding increase in compactioneffort to obtain a given density. However, a higherdensity can usually be attained with angular soilsbecause the particle shapes are more conducive to fillingthe voids.

(3) For coarse-grained soils containing only asmall percentage (5 or less) of fine-grained particles,maximum density is more readily obtained when the soilis either dry or saturated. For water contents betweenthese limits, the water in the soil forms menisci betweenthe particle contacts, which tend to hold the soil particlestogether. This resistance to movement of particles into amore compact structure, termed apparent cohesion or"bulking," results in lower densities than those for either adry or saturated cohesionless soil under the samecompaction effort.

(4) It is to be noted that in the precedingparagraphs, the discussion has centered around thedensity in weight per unit volume of coarse-grained soilswith different gradation characteristics. A more realisticparameter that is often used is the relative density ofcohesionless coarse-grained soils. Relative densityexpresses the degree of compactness of a cohesionlesssoil with respect to the loosest and the densestconditions of the soil that can be attained by specifiedlaboratory procedures. A soil in the loosest state wouldhave a relative density of zero percent and in the denseststate, a relative density of 100 percent. The dry unitweight of a cohesionless soil does not, by itself, revealhow loose or how dense the soil is due to the influence ofparticle shape and gradation on the density.Only when viewed against the possible range ofvariation, in terms of relative density, can the dry unitweight be related to the compaction effort used to placethe soil in a backfill or indicate the volume changetendency of the soil when subjected to foundation loads.

(5) Most coarse-grained soils can becompacted to a density such that detrimental additionalconsolidation will not take place under the prototypeloading. This factor is the first important consideration.Another important consideration may be that thecompacted soil be sufficiently pervious to provide gooddrainage. Proper consideration of these two basicfactors will allow the use of most coarse-grained soils forbackfill purposes.

b. Compaction of fine-grained soils. Themechanics by which fine-grained soils are compacted isquite complex because capillary pressures, hysteresis,pore air pressure, pore water pressure, permeability,surface phenomena, osmotic pressures, and theconcepts of effective stress, shear strength, andcompressibility are involved. Numerous theories havebeen developed in an attempt to explain the compactionmechanics. The current state-of-the-art theoriesinvolving effective stress give satisfactory explanations.The basic concepts of these theories are discussedbelow.

(1) Fine-grained soils are compacted in apartially saturated state; therefore, voids or pores containboth pore air and pore water between the soil particles.

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Initial compaction water contents below optimum result ininitial high pore air pressures and pore water pressures,which reduce shear strength and allow soil particles toslide over one another displacing the pore air to form amore dense mass. This process continues as long asthe trapped pore air pressure can escape but requiresincreasing amounts of compaction effort to achievehigher densities since the soil particles carry increasingamounts of the compaction energy. For a givencompaction effort, enough water may eventually beadded to the soil so that air channels becomediscontinuous, and the air is trapped. When the air voidsbecome completely discontinuous, the air permeability ofthe soil drops to zero; no further densification is possiblebecause at this condition transient pore air pressurescan develop that resist the compaction effort. At zeropermeability the soil has reached its so-called "optimumwater content." Since zero permeability may also be

established by closer packing of soil particles, it isevident that lower optimum water contents are possibleat higher compaction efforts.

(2) The addition of water above optimumwater content causes the voids to become completelyfilled with trapped pore air and pore water and therebyprevents the soil particles from moving into a morecompact arrangement no matter what the compactioneffort. Pore water pressure increases significantly withincreasing water contents and causes increasedreduction in shear strength. This fact is evident in thelaboratory compaction mold when the compaction footsinks deeper and deeper into the soil as water contentincreases past optimum. The same process occurs inthe field when sheepsfoot rollers sink into the soil untilthe weight is carried by the drum or excessive ruttingwith rubber-tired rollers

Section B-2. FIELD COMPACTION TEST METHODS

B-3. General. Laboratory test data obtained fromlaboratory-compacted specimens provide a basis fordesign, and it is assumed that the engineeringcharacteristics that will be built into the field-compactedbackfill will be approximately the same as those of thespecimens. Experience has indicated that for most soils,laboratory densities, water contents, and strengthcharacteristics can be satisfactorily reproduced in a field-compacted backfill.B-4. Field compaction tests.

a. Compaction control tests. Compaction controlof soils requires comparison of fill water content and drydensity values obtained in field density tests withoptimum water content and maximum dry density, ordetermination of relative density if more appropriate forthe fill materials that are cohesionless. For fine-grainedor coarse-grained soils with appreciable fines, fieldresults are compared with results of CE 55 laboratory(modified effort) compaction tests performed accordingto procedures presented in MIL-STD-621A and ASTM D1557. For free-draining cohesionless soils, relativedensity of the fill material is determined, if appropriate,using vibratory test procedures prescribed in EM 1110-2-1906 and ASTM D 2049.

b. Frequency compaction control tests. Theperformance of a standard laboratory compaction test onmaterial from each field density test would give the mostaccurate relation of the in-place material to optimumwater content and maximum density, but this test is notgenerally feasible to do because testing could not keeppace with the rate of fill placement. However, standardcompaction tests should be performed duringconstruction (1) when an insufficient number of thecompaction curves were developed during the designphase, (2) when borrow material is obtained from a new

source, and (3) when material similar to that beingplaced has not been tested previously. In any event,laboratory compaction tests should be performedperiodically on each type of fill material (preferably 1 testfor every 10 field density tests) to check the optimumwater content and maximum dry density values beingused for correlation with field density test results.

c. Quick field compaction tests. In addition to thestandard compaction or relative density tests (para B-2a), at least four relatively quick compaction testmethods can provide good approximations of maximumdry density comparable to the standard methods.The quick compaction methods include: one-point andtwo-point compaction methods; the Water and PowerResource Service (WPRS), formerly U.S. Bureau ofReclamation (USBR) rapid compaction control method;and for granular cohesionless material, compactioncontrol by gradation. Since only the one-point and two-point methods are currently accepted by the Corps ofEngineers for compaction control tests, only these twomethods will be discussed in detail. The USBR andgradation methods are briefly summarized.

(1) One-point compaction method. In theone-point compaction method, material from the fielddensity test is allowed to dry with thorough mixing toobtain a uniform water content on the dry side ofestimated optimum, and then compacted using the sameequipment and procedure used in the five-point standardcompaction test. The water content and dry density ofthe compacted sample are then used to estimate itsoptimum water content and maximum dry density asillustrated in figure B-4. The line of optimums is

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

well defined in the figure, and the compaction curves areapproximately parallel to each other, consequently, theone-point compaction method could be used with arelatively high degree of confidence. In figure B-5,however, the optimums do not define a line, but a broadband. Also, the compaction curves are not parallel toeach other and in several instances cross on the dryside. To illustrate the error that could result from usingthe one-point method, consider the field density andwater content shown by point B in figure B-5. Point B isclose to three compaction curves. Consequently, thecorrect curve cannot be determined from the one point.The estimated maximum dry density and optimum watercontent could vary from about 92.8 pounds per cubic footand 26 percent, respectively, to 95.0 pounds per cubicfoot and 24 percent, respectively, depending on whichcurve was used. Therefore, the one-point methodshould be used only when the basic compaction curvesdefine a relatively good line of optimums.

Figure B-4. Illustration of one-point compaction method.

(2) Two-point compaction test results. In thetwo-point test, one sample of material from the locationof the field density test is compacted at the fill watercontent if thought to be at or on the dry side of optimumwater content (otherwise, reduced by drying to thiscondition) using the same equipment and proceduresused in the five-point compaction test. A second sampleof material is allowed to dry back about 2 to 3 percentagepoints dry of the water content of the first sample, andthen compacted in the same manner. After compaction,water contents of the two samples are determined byovendrying or other more rapid means

Figure B-5. Illustration of possible error using one- andtwo-point compaction methods.

(para B-8a), and dry densities are computed. Theresults are used to identify the appropriate compactioncurve for the material test (fig. B-6). The data shown infigure B-6 warrant the use of the two-point compactiontest since the five-point compaction curves are notparallel. Using point A only as in the one-point testmethod would result in appreciable error as the shape ofthe curve would not be defined. The estimatedcompaction curve can be more accurately defined by twocompaction points as shown. Although the two-pointmethod is more accurate than the one-point method,neither method would have acceptable accuracy whenapplied to the set of compaction curves shown in figureB-5.

(3) Rapid one-point test for sands. A rapidcheck test for compaction of uniform sands (SP to SM)with less than 10 percent fines (minus No. 200 sieve) isa modified one-point test. The overdry sand iscompacted in a 4-inch-diameter mold using CE 55(modified) effort. Correlation with standard compactiontests is required to confirm the validity of test results fordifferent sands used on each project.

(4) USBR rapid compaction control method.Details of this method are described in the USBR EarthManual (app A). The test is applicable to fine-grained(100 percent minus No. 4 sieve) cohesive soils withliquid limits less than 50. The method, however, isapplicable to soils containing oversize particles providingthe proper corrections, as stated in EM 1110-2-1911,Appendix B, are applied. It is a faster method than thestandard compaction test and is often more accuratethan other methods. The method usually requiresadding water to or drying back sampled fill material, andthorough mixing is needed to obtain uniform drying or

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

Figure B-6. Illustration of two-point compaction method.

distribution of added water. Otherwise, the results maybe erroneous, especially for highly plastic clays. In toughclays, it is likely to be inaccurate because of insufficientcuring time for the specimens.

(5) Grain-size gradation compaction controlmethod. This test method developed in 1938 isapplicable to coarse, medium, and fine-grained sands.The method involves sieve analysis to establish grain-size gradation curves, whose shapes are then correlatedwith maximum dry density obtained from the standardfive-point compaction tests or relative density tests. Fora given compaction effort, the maximum dry density ofcohesionless material (sand) is also a function of particleshape. Thus, the correlation between grain-sizedistribution and density would, by necessity, have toinclude consideration of particle shape. It is doubtful thatthis method would provide test results more rapidly thanthe one-point and two-point methods or the relativedensity method currently accepted by the Corps sincesamples must be dried for sieve analysis. Therefore,this method is not recommended for routine compactioncontrol.

d. Possible errors. All tests involving mechanicaldevices and human judgment are subject to errors thatcould affect the results. In order to properly evaluate testresults, the inspector must be familiar with the possiblesources of such errors.

(1) Five-point compaction tests. Thefollowing errors can cause inaccurate results: (a)Aggregations of air-dried soil not completely reduced tofiner particles during processing.

(b) Water not thoroughly absorbed intodried material because of insufficient mixing and curingtime.

(c) Material reused after compaction.

(d) Insufficient number of tests to definecompaction curve accurately.

(e) Improper foundation for mold duringcompaction.

(f) Incorrect volume or weight ofcompaction mold.

(g) Incorrect rammer weight and height offall.

(h) Excessive material extending into theextension collar at the end of compaction.

(i) Improper or insufficient distribution ofblows over the soil surface.

(j) Tendency to press the head of therammer against the specimen before letting the weight fall.

(k) Insufficient drying of sample for watercontent determination.

(2) One-point and two-point compaction test.The possible sources of errors for the one-point and two-point compaction test are essentially the same as those forthe five-point method discussed in (1) above. In addition,appreciable inaccuracy in results may occur for bothmethods if attempts are made to extrapolate maximumdensity and optimum water contents from non-uniformfamilies of compaction curves (fig. B-5).B-5. Field compaction and test sections.For most soils, laboratory densities, water contents, andstrength characteristics can be satisfactorily reproduced in afield-compacted backfill. However, during the initial stage ofconstruction frequent checks of density and water contentshould be made for comparison with design requirementsand adjustments should be made in the field compactionprocedure as necessary to ensure adequate compaction.

a. When a compacted backfill is constructed asfoundation support for critical structures, or when otherrequirements, materials, and conditions are unusual, thespecifications may provide for the construction of testsections. The test section is used to determine the bestprocedures for processing, placing, and compacting thematerials that will produce compacted backfill havingengineering properties compatible with design requirements.Therefore, construction of a test section may involve usingdifferent types and different weights of compactionequipment, using different lift thicknesses, using differentamounts of compaction applications (different numbers ofpasses or coverages), processing materials differently withrespect to water content control, and mixing to obtainimproved gradation. A discussion on test sections for shalematerials is presented in Appendix A of FHWA-RD-78-141and illustrates a wide variation in test results, even for verycarefully conducted field tests.

b. By exercising rigid control over the water content,processing, placement, and compaction procedures, byfrequent density sampling, by keeping complete

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

records of the procedures and tests, and then bystudying and evaluating these records, a procedure touse on the job can be established. In addition to watercontent and density check tests, undisturbed samplesshould be obtained to determine that the shear andconsolidation characteristics are consistent with designrequirements. Once control for field conditions has beenestablished, the backfill can proceed at a normal rate.The contractor should be required to adhere to theestablished processing, placement, and compactionprocedures.

c. If provisions for construction of a test section

are not contained in the specifications, the fieldengineers and inspection personnel should providemaximum guidance to the contractor to aid him inestablishing adequate processing, placement, andcompaction procedures. To meet this problem thecontractor must be provided with suggestedimprovements of equipment type, if they have not beenspecified, and procedures during the initial stages ofbackfill operations. The establishment of the proceduresand equipment type that will produce adequatecompaction of the backfill material must be supported bya comprehensive program of control testing.

Section B-3. FIELD MOISTURE-DENSITY TEST METHODS

B-6. General. Field density measurements of thecompacted backfill are essential to ensure that backfillmeets the required design densities necessary for theproper functioning of the structure within that backfill.Although water content requirements are not generallyspecified in military specifications, the measurement andcontrol of water content is important in obtaining requireddensities. The four density measurement test methodsused for the Corps record and contract acceptanceenforcement are listed below.

a. The sand-cone method as described in MIL-STD-621A (Method 106) and ASTM D 1556.

b. The rubber-balloon method as described inASTM D 2167.

c. The nuclear moisture-density method asdescribed in ASTM D 2922 (for density) and ASTM D3017 (for water content).

d. The drive-cylinder method as described in MIL-STD-621A (Method 102) and ASTM D 2937 for soft, fine-grained cohesive soils. The water-displacement methoddescribed in EM 1110-2-1911, although not currentlyused for Corps contract enforcement, may be used forsupplementary density testing for rocky materials. Rapidfield methods of determining or approximating watercontent-density are also discussed in the followingparagraphs.B-7. Water content and density test methods. Fielddensity can be determined by direct or indirect methods.In the direct methods, the weight of soil removed from ahole and the volume of the hole are determined andused to compute the density. In the indirect methods, acharacteristic of the soil, such as radiation scattering orpenetration resistance, is measured with an instrumentsuch as a nuclear density meter or penetrometer, andthen a previously determined relation between densityand the characteristic measured is used to determine thedensity.

a. Direct methods. The sand-displacementmethod is considered to be the most reliable direct

method and should be used as the standard test bywhich indirect test results are correlated with density.Other direct methods are the drive-cylinder method,rubber-balloon method, and water-displacement method.

(1) Sand-cone method. Procedures andequipment for the sand-cone method are described inMIL-STD-621A (Method 106) and ASTM D 1556. Theprocedure as described in the references involvespreparation of the ground surface, measurement of aninitial volume for the purpose of correcting for surfaceirregularities, and measurement of a second volumeafter a small hole is dug. The difference in the volumesis the volume of the hole. The sand used is a standardsand (Ottawa or other sands having rounded grains anda uniform gradation) that has been calibrated for weightversus volume occupied when falling from a standard,constant height. The weight of sand used is measuredby weighing the sand density cylinder before and aftereach volume measurement, and the volume isdetermined from the weight versus volume calibration.The soil removed from the hole is weighed, the watercontent determined (MIL-STD-621A), and the dry weightcomputed. The wet density and dry density of the soilare computed by dividing the appropriate weights by thecomputed volume. The sand-cone method can be usedto determine the in-place density of practically all soilsexcept those containing large quantities of large gravelsizes.

(2) Drive-cylinder method. Procedures andequipment for the drive cylinder method are described indetail in MIL-STD-621A (Method 102) and ASTM D2937. The procedure consists of driving a 3-inch-diameter by 3-inch-high sampling tube of known volumeinto the soil, excavating the sampling tube and soil, andtrimming off the soil protruding from the ends of the tube.The weight and water content of the soil are measuredand the dry weight is computed. The wet density and drydensity of the soil are computed by dividing theappropriate weights by the computed volume. The drive-cylinder method is limited to moist, fine-grained cohesivesoils.

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

(3) Rubber-balloon method. Procedures andequipment for the rubber-balloon method are describedin ASTM D 2167. This method utilizes a rubber balloonattached to a glass or metal cylinder containing waterand having a scale graduated in cubic feet. An annulardevice is seated on the prepared ground surface, andthe balloon apparatus is placed and held down firmly onthe ring. Then water is forced into the balloon underpressures of 2 to 3 pounds per square inch to obtain aninitial volume measurement to correct for ground surfaceirregularities. The apparatus is removed, a small hole isdug, and the apparatus is replaced on the ring. Water isagain pumped into the balloon and causes the balloon toconform to the boundary of the hole; then the volume ismeasured. This volume less the initial volume is thevolume of the hole. The volumeter apparatus is simpleand easy to operate, and the volume measurement canbe made directly and in somewhat less time than thatwith the sand-cone volume apparatus. The resultsobtained are considered to be as accurate as thoseobtained from the sand-cone apparatus. Like the sand-cone method, the rubber-balloon method can be used todetermine the in-place density of practically all soilsexcept those containing large quantities of large gravelsizes.

(4) Water-displacement method. Where it isnecessary to determine the in-place density for a largevolume of soil, as in coarse-grained soils containingsignificant quantities of large gravel sizes, anapproximate density can be obtained by excavating alarge hole (several cubic feet) and determining thevolume by lining the hole with thin plastic sheeting andmeasuring the quantity of water required to fill the hole.A relatively small sample representative of the materialfrom the excavation is used for determining the watercontent. Using the wet and dry weights of the materialexcavated and the measured volume of the hole, the wetand dry densities of the soil can be determined.Although the procedure is not contained in a MilitaryStandard, it is about the only means of determining anapproximate density for soils with large sizes of gravel orrock.

b. Size and preparation of test hole. The size ofthe hole and the care used in preparing the test hole forthe sand volume and balloon methods influence theaccuracy of the volume measurement. The proper sizeof the hole is not well established; however, the largerthe hole, the less significant small errors in measurementof volume become. The instructions in TM 5-824-2indicate that a volume of at least 0.05 cubic foot shouldbe used when testing materials with a maximum particlesize of 1 inch and that larger volumes should be used forlarger maximum particle sizes. ASTM D 1556 suggestscertain relations between particle size and the test holevolume and weight of water content specimen. It also

recommends increasing the size of the sample used forwater content determination with increasing maximumparticle size. The relations suggested by the AmericanSociety for Testing and Materials are shown in the followingtabulation:

Minimum test Water contentMaximum particle hole volume sample

size, in. cu ft g0.187 (No. 4) 0.025 1001/2 0.050 2501 0.075 5002 0.100 1,000

For significant quantities of larger particles the volumesabove should be doubled. The accuracy of the test resultsis influenced by not only the care taken in preparing a testhole but also the degree of recovery of the excavatedmaterial. A hole with irregular surfaces will cause thevolume measurement to be less accurate than a hole withsmooth surfaces. Thus, the inside of the hole should bekept as free of pockets and sharp projections as possible.Digging a smooth test hole in cohesionless coarse-grainedmaterial is particularly difficult. In fine-grained soils withoutgravel particles, the hole may be bored with an auger, buthand tools will be required to smooth the walls and base ofthe hole and to recover loose material. For coarser-grainedsoils and soils containing a significant amount of gravel-sizeparticles, hand tools will generally be required to excavatethe hole to prevent disturbing the material in the walls andbase of the test hole. Should it become necessary indigging a test hole in highly compacted material to loosenthe material by using a chisel and hammer, care must betaken not to disturb the soil around the limits of the hole. Allloose particles must be removed after the final depth hasbeen reached, and all particles must be recovered. All soilshould be placed in a waterproof container as the soil istaken from the hole. This measure will prevent loss of waterbefore the soil can be weighed.

c. Indirect methods. The indirect methods includeuse of the nuclear moisture-density apparatus, Proctorpenetrometer, and cone penetrometer. Both the Proctorpenetrometer and cone penetrometer methods fordetermining the density require very careful calibration usingsoils of known density and water content, and considerableexperience in operating the device; even so, the accuracy ofthese methods may be subject to question because of thegreat influence that non-uniformity of water content or asmall piece of gravel can have on the penetrationresistance. The Proctor penetrometer may also be used toapproximate water content of fine-grained soils.

(1) Nuclear moisture-density method.Procedures and equipment for the nuclear moisture-densitymethod are described in ASTM D 2922 (for density) andASTM D 3017 (for water content). The three methods fordetermining in-place densities described in ASTM

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

D 2922 are Method A-Backscatter, Method B-DirectTransmission, and Method C-Air Gap. Of the threemethods, Method B-Direct Transmission isrecommended over Method A and Method C because iteliminates the effect of vertical density variations.

(a) Modern nuclear-moisture densityequipment incorporates a radioactive source emittingneutrons and gamma rays and measuring elements(geiger tubes) or "scalers" into a single, self-containedunit. The determination of moisture by the nuclearmethod is dependent on the modifying of high energy or"fast" neutrons into low energy or "slow" neutrons(ASTM, STP No. 293). Any material containinghydrogen will moderate fast neutrons. Since hydrogen ispresent primarily in the molecules of free water, thedegree of interaction between the fast neutrons andhydrogen atoms represents a measure of the watercontent of the soil. Density measurements are based onthe scattering of gamma rays by the orbital electrons onthe atoms comprising the soil. Since the scattering is afunction of the electron density, which in turn isapproximately proportional to the density of the soil, it ispossible to correlate the backscatter of the gamma rayswith the soil density.

(b) To obtain a water content or densitymeasurement, the appropriate meter is set in place andthe voltage setting is adjusted to the correct operatingvoltage. After the scaler is turned on, a short warmupperiod (not exceeding 1 minute) is allowed before thetest count is started. Intimate contact at the interfacebetween meter and soil is necessary for Method ABackscatter because the scattering of the gamma raysfor the density measurement is quite sensitive to evenminute air gaps. The normal counting period is 1 minute,with one or two repeat counts taken as a check.Calibration curves for both moisture and densitydeterminations, once the count rates have beenestablished, are furnished by the manufacturers for eachindividual unit. In general, the calibration curve formoisture determination is more reliable than the curvefor density determination. However, it is advisable tocorrelate both calibration curves on each type of soil withwhich the instrument is to be used. Such a correlationshould be accomplished by using current standardmethods for moisture and density determinations or bycalibrating on blocks of material of known moisture anddensity. Examples of calibration for shale materials aregiven in Appendix A of FHWARD-78-141.

(c) For all nuclear-moisture densitydevices, separate standards are provided so that thecount rate can be determined on each instrument at anytime in the field. A standard count should be taken threeor four times during a day’s operation. Althoughadjustments can generally be made on the instrumentsso that the count will coincide with the standard count,

even a slight adjustment is not usually justified. A moresatisfactory procedure is to record the field measurementin terms of percent of this standard count rate, whichshould be within a reasonable percentage (+ 5) of thegiven reference count. Use of the percent of standardcount, rather than simply the counts per minute, isrecommended for increased accuracy. Use of thisprocedure largely cancels out the effects of suchvariables as reduction in source strength, backgroundcount, and changes in sensitivity of the detector tubes.

(d) The calibration curve for the soilbeing tested is entered with the value of the densitymeter count rate (taking into consideration the variationfrom the standard count) to obtain the wet unit weight ofthe soil. Similarly, the moisture meter yields the weightof water per cubic foot of soil. The unit dry weight of thesoil is simply the wet unit weight obtained by the densitymeter minus the weight of water obtained by themoisture meter. By dividing the water measurement bythe dry density, the water content can be expressed inthe more familiar terms of percentage of dry weight.

(e) Anyone working with nuclear metersmust recognize that a possibility of exposure to radiationexists if the safety rules listed by the manufacturer arenot followed. When proper procedures and safety rulesare followed, the radiation hazard is negligible. Forcertain instruments, operating personnel must wear abody radiation film badge and carry a pocket dosimeter.These instruments must be ready weekly to ensure thatthe maximum permissible weekly dosage is less than100 milliroentgen. Other safety rules deal with handlingthe devices and being aware of the built-in safetydevices. The safety precautions mentioned above mayvary or not be applicable for some of the newer devicesbeing manufactured. Therefore, the manufacturer’sliterature should be carefully studied to determineappropriate safety requirements.

(f) It is possible, using nuclear-moisturedensity apparatus, for one inspector to conduct perhaps30 water content and 30 density tests per 8-hour workingday. The time required per test is only 20 or 25 percentof that required in direct sampling methods. A largenumber of tests with the nuclear meter correlated with amuch smaller number of direct sampling determinationscan be of great benefit in ensuring that adequatecompaction of the backfill is being obtained. A simplestatistical analysis of the data can be made, such as aplot of dry density versus number of tests (ASTM STPNo. 293). The resulting bell-shaped curve is a veryuseful tool since each day's results can easily be addedto the plot of previous test results. This procedure canprovide an up-to-date picture of the fill densities beingobtained and can show the effect of changes made infield compaction procedures.

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

(2) Hand cone penetrometer. The hand conepenetrometer offers a rapid means of checking densityrequirement of some compacted backfills. The processinvolves the correlation of penetration resistance withknown in-place densities as determined by either thesand-cone or the rubber-balloon method.

(a) Cone penetration resistance is ameasurement of soil bearing capacity. Since bearingcapacity is dependent on shear strength and thusdensity, the hand cone penetrometer is an indirectmeasurement of density. Because shear strength is afunction of any pore air and pore water pressures thatmay be generated by a shearing action of soilscontaining pore water, the method is applicable only tofree-draining materials where pore pressures aredissipated as fast as they are generated. Penetrationresistance can also be drastically influenced by theobstruction of gravel size particles. Therefore, themethod is applicable only to sands with 100 percentpassing the U.S. Standard No. 4 sieve (4.76 mm) andno more than 15 percent passing the U.S. Standard No.200 sieve (0.074 mm).

(b) A plot of hand-cone soundingresistance versus depth of sounding will result in anapproximate linear relationship for homogenousmaterials of relatively constant density for depths ofsounding ranging from approximately 2 inches to 20inches depending on the geometry and size of the conepoint and material type. Correlations may be madebetween known in-place densities and either the angle ofinclination between sounding resistance and depth ofpenetration or the sounding resistance at a given depth.The range of known in-place density must be sufficient toestablish a trend between sounding resistance anddensity. Correlations between density and soundingresistance at a given depth is the simplest correlationsince the angle of inclination does not have to becomputed. Figure B-7 shows a case example of acorrelation between dry density and sounding resistancemeasured at 6 inches below the surface. Contractspecification required a minimum acceptable dry densityof 104.7 pounds per cubic foot (98 percent of themaximum dry density according to the compactionmethod described in ASTM D 1557). Figure B-7 alsoindicates that all soundings with resistances of 110pounds or more corresponded to densities greater than104.6 pounds per cubic foot. Therefore, no additionalstandard density checks are needed beyond the routinetests. When all soundings with resistance of 86 poundsand below correspond to densities below 104.6 poundsper cubic foot, it is evident that sufficient compaction hasnot been achieved and additional standard densitychecks are definitely needed for an acceptance orrejection decision. Sounding with resistances between86 and 110 pounds may or may not need additionaldensity checks depending on whether the inspector hasreason to suspect adequate compaction has or has notbeen achieved.

Figure B-7. Correlation between dry density and handcone resistance at a depth of 6 inches below the surface.

(c) The correlation between soundingresistance and known in-place dry densities (fig. B-7) ismade directly without knowing water content at eachsounding location. Although sounding resistances areaffected by water content for the dry, moist, and 1 to 2percentage points above optimum state, the range ofpossible water content in the moist state does notsignificantly affect sounding resistance.

(d) The hand cone penetrometer isideally suited for use in confined zones where sand isused as backfill and where rapid control aids are neededto determine if adequate compaction has been achieved.With a little practice, a hand-cone sounding can be madein less than 1 minute.

d. Possible sources of errors. Since the decisionto accept or reject a particular part of a backfill isprimarily dependent upon the results of in-place densitycontrol tests, it is important for the inspector to befamiliar with the possible sources of errors that mightcause an inaccurate test result. Some of the more likelysources of errors for the sand-cone, rubber-balloon, andnuclear moisture-density methods are discussed below.All tests that are suspected to be in error must berepeated.

(1) Sand-cone method. The major sources ofpossible error are as follows: (a) The sand-cone methodrelates the bulk density of a standard sand to the knownweight of the

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

same sand occupying an in-place volume of sampledmaterial. Changes in effective gradation between orwithin batches of sand may significantly affect the testresults. This error can be minimized by frequentcalibration of the sand’s bulk density.

(b) Loose sand increases in densitywhen subjected to vibrations. Care must be taken not tojar the sand container while calibrating bulk density in thelaboratory or during in-place volume measurements inthe field. A common error is to use the sand conemethod for in-place volume measurements adjacent tothe operation of heavy equipment. Heavy equipment cangenerate vibrations that densify the sand and result inerroneously high-volume measurements and low in-place densities.

(c) Appreciable time intervals betweenbulk density determination of the sand and its use in thefield may result in change in the bulk density caused by achange in the moisture content of the sand.

(2) Rubber-balloon method. The majorsources of possible error are as follows: (a) New rubber-balloon volumeters should be calibrated against severalknown volumes of different sizes covering the volumerange of in-place measurements.

(b) For stiff soils such as clay, it ispossible to trap air between the sides of the sample holeand balloon. This error can be minimized by placinglengths of small-diameter string over the edge of the holeand down the inside wall slightly beyond the bottomcenter.

(c) The application of the 2- to 3-pounds-per-square-inch pressure to extend the balloon intoexisting irregularities in the hole will cause a noticeableupward force on the volumeter. Care must be taken toensure that the volumeter remains in intimate contactwith the base plate.

(d) The rubber balloon must befrequently checked for leaks.

(3) Nuclear moisture-density method. Themajor sources of possible error are as follows:

(a) The single consistent source of erroris related to the accuracy of the system. The overallsystem accuracy in determining densities is statistical innature and appears to vary with the equipment used, testconditions, materials tested, and operators. If properprocedures are followed, the standard deviations interms of accuracy will vary on the order of 3 to 5 poundsper cubic foot for density tests and 0.5 to 1.0 pound ofwater per cubic foot of material for water content tests.

(b) Manufacturers furnish calibrationcurves for each piece of equipment. Due to the effectsof differing chemical compositions, calibration curvesmay not be applicable to materials not represented inestablishing the calibration curve. Apparent variations incalibration curves may also be induced by differences in

the seating, background count, sample heterogeneity andsurface texture of the material being tested.

B-8. Rapid field water content control procedures. Inmany cases, particularly in confined zones, it is important torapidly determine the dry density of a given part of thebackfill in order to prevent the possibility of costly tear outand rebuild operations. The test procedures for determiningdry densities using the sand-cone and rubber-balloonmethods sometimes require extensive drying times (dependon material type up to 16 hours) to determine water content.Alternate techniques for rapidly determining water contentare discussed below.

a. Microwave ovens. Microwave energy may beused to dry soil rapidly and thus enable quick determinationof water content (ASTM STP No. 599). However, in dryingsoils with microwaves, the only control on the amount ofenergy absorbed by the soil is exposure time; consequently,if soils are left in the oven too long, severe overheating canoccur. This overheating of the soil can cause bound water,a part of the soil structure, to be driven off and thus result insignificant errors in water content measurements. Inaddition, continuous heating can result in excessive heatbeing generated; certain soils have been observed to fuseor explode and thereby create hazards to both equipmentand personnel.

(1) Times required for drying in a microwaveoven are primarily governed by the mass of water presentand the power-load output of the oven, as expressed by

GT = Mw[(0.2/w + 1) (100 - to) + 539](4.18896) (B-1)P

whereT = time in the microwave oven, secondsMW = mass of water present in the soil-water mixture,gramsw = water content of the specimento = initial temperature of the soil-water mass,degree CentigradeP = power output of the oven, watts

This governing equation indicates that in order to predictaccurately the drying times required, an estimate of thespecimen water content must be made and the oven powerversus load relationship must be established by calibration.

(2) The limitation of having to estimate the initialwater content of the specimen is not insurmountable. Testresults indicate that slight overestimations of the actualwater content, i.e., longer drying times, generally result insmall differences between conventional oven andmicrowave oven water contents. Conversely,underestimations of water content result in more seriouserrors. If an accurate estimate of water content

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

cannot be made, experience has shown that close visualobservation often can be used to determine if soiloverheating is occurring. An alternative approach is toincrementally dry a duplicate specimen until a constantweight is obtained, calculate the water content, and inputthis value into equation (B- 1).

(3) The useful power output "P" is determinedin the laboratory by subjecting a mass of distilled water tomicrowaves for a given time and then measuring the risein temperature induced in the water. Power in watts iscalculated from

MwtP = Tc 4.18896 (B-2)

whereMW = mass of distilled water in the oven, gramst = increase in temperature of the distilled water,

degree CentigradeTc, = time in the oven for calibration, seconds

A plot is then made of power output and oven load (massof water in oven) in grams of water as shown in figure B-8.

Figure B-8. Power Applied by the Oven to Dry MoistSoils.

(4) The water content estimate is used tocalculate the mass of water in the specimen from

(Wwet) (W)MW = (1 + w) (B-3)

whereMW = mass of the water in the specimen, and

equivalent to oven load in figure B-8, gramsWw,t = wet weight of the specimen, gramsBy calculating MW (oven load in fig. B-8) from equation(B-3) and finding a comparable value of power from aplot similar to figure B-8 for the particular oven used, thedrying time may be calculated from equation (B-1).

(5) It may not be possible to successfully drycertain soils in the microwave oven. Gypsum maydecompose and dehydrate under microwave excitation.Highly metallic soils (iron ore, aluminum rich soils, andbauxite) have a high affinity for microwave energy andoverheat rapidly after all the free water has beenvaporized. Hence, extreme care is required when dryingthese soils. For the same reason, metallic tare cans oraluminum plates are not permissible as specimencontainers.

(6) Because microwaves are a type ofradiation, normal safety precautions to avoid undueexposure should be observed.

b. Proctor penetrometer. The Proctor penetrationresistance method in the hands of inspection personnelexperienced in its use provides a rapid expedient checkon whether the field water content is adequate for propercompaction. However, the method is suitable only forfine-grained soils because coarse sand or gravel maycause erroneously high resistance readings. Themethod consists of compacting by the procedure usedfor control of a representative sample of soil taken fromthe loose lift being placed. The compacted specimen isweighed, and the wet unit weight is determined. Thepenetration resistance of the compacted specimen in themold is then measured with the soil penetrometer. Themoisture content can then be estimated by comparingthe penetration resistance of field compacted specimenswith a relation previously established in the laboratorybetween wet unit weight, penetration resistance, andmoisture content. The procedure requires about 10minutes and is sufficiently accurate for most fieldpurposes. The procedure to determine the relationbetween wet unit weight, penetration resistance, andmoisture content is described in ASTM D 1558. Therelation is generally developed in conjunction with thecompaction test.

c. Other methods. Other methods fordetermining water content include drying by hot plate oropen flame, drying by forced hot air and a rapid moisturetest that uses calcium carbonate. In the hot platemethod, a small tin pan and a hot plate, oil burner, or gasburner (something to furnish fast heat) are used. Asample of wet soil is weighed, dried by one of the abovementioned methods, and weighed again to determinehow much water was in the sample. This method is fast,but care must be taken to ensure that the material isthoroughly dry. Also, if both organic matter and boundwater are removed, higher water content determinationsthan those obtained by ovendrying sometimes result. Inthe forced hot air, a sample is placed in

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

a commercially available apparatus containing an electricheater and blower. Hot air at 150 to 300 degreesFahrenheit is blown over and around the sample for apreset time. A 110- or 230-volt source is required.Available sizes of apparatus can accommodate sample

weights from 25 to 500 grams. Drying times areestimated to vary from 5 minutes for sand to as long as30 minutes for fat clay. The rapid moisture test andlimitations are described in STP 479.

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

APPENDIX C

BIBLIOGRAPHY

Broms, B., "Lateral Earth Pressures Due to Compaction of Cohesionless Soils". Proceedings, Fourth BudapestConference on Soil Mechanics and Foundation Engineering, Budapest, Hungary (1971).

Burmister, D. M., "The Grading-Density Relations of Granular Materials". Proceedings, American Society for Testing andMaterials, Volume 38, Part II, Philadelphia, Pennsylvania (1938).

Davis, F. J., "Quality Control of Earth Embankments". International Conference on Soil Mechanics and FoundationEngineering, Volume 1, Switzerland (1953).

de Mello, U. F. B., Sonto Silveira, E. B., and Silveira, A., "True Representation of the Quality of a CompactedEmbankment". First Pan-American Conference on Soil Mechanics and Foundations Engineering, Volume II, Mexico(1960).

Hilf, J. W., "Compacting Earth Dams with Heavy Tamping Rollers". Journal of the Soil Mechanics and FoundationsDivision, American Society of Civil Engineers, Volume 83, No. SM2, Paper 1205, Ann Arbor, Michigan(1957).

Ingold, T. S., "The Effects of Compaction on Retaining Walls". Geotechnique, Volume 29, No. 3. London (1979).

Ingold, T. S., "Retaining Wall Performance During Backfilling". Journal of the Geotechnical Engineering Division,American Society of Civil Engineers, Volume 105, GT5, Paper 14580, New York, New York (1979).

Li, C. Y., "Basic Concepts on the Compaction of Soil". Journal of the Soil Mechanics and Foundations Division, AmericanSociety of Civil Engineers, Volume 82, SM1, Paper 862, Ann Arbor, Michigan (1956).

Joshi, R. C., Duncan, D. M. and Durbin, W. L., "Performance Record of Fly Ash as a Construction Material".Proceedings, Fourth International Ash Utilization Symposium, St. Louis, Missouri, 24-25 March 1976. John H. Faber,editor, Energy Research and Development Administration Morgantown, West Virginia (1976) (NTIS: MERC/SP-76/4).

Olson, R. E. "Effective Stress Theory of Soil Compaction". Journal of the Soil Mechanics and Foundations Division,American Society of Civil Engineers, Volume 89, No. SM2, Paper 3457, Ann Arbor, Michigan (1963).

Plantema, G., "Influence of Density on Sounding Results in Dry, Moist, and Saturated Sands". Proceedings of the FourthInternational Conference on Soil Mechanics and Foundation Engineering, Long Volume I (1957).

Proctor, R. S., "The Design and Construction of Rolled Earth Dams". Engineering News-Record, Volume m (1933).

Quigley, D. W. and Duncan, J. M., "Earth Pressures on Conduits and Retaining Walls". Report No. UCB/GT/78-06,University of California, Department of Civil Engineering (1978).

Turnbull, W. J. and Foster, C. R., "Stabilization of Materials by Compaction". Journal of Soil Mechanics and FoundationsDivision, American Society of Civil Engineers, Volume 82, No. SM2, Paper 934, Ann Arbor, Michigan (1956).

Winterkorn, H. F. and Fang, Hsai-Yang, Foundation Engineering Handbook. Van Nostrand Reinhold Company, NewYork, New York (1975).

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

The proponent agency of this publication is the Office of the Chief ofEngineers, United States Army.Users are invited to send comments and suggested improvements on DAForm 2028 (Recommended Changes to Publications and Blank Forms) directto HQDA (DAEN-ECE-G), WASH DC 20314.

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