EM 1110-2-191130 September 1995

US Army Corpsof Engineers

ENGINEERING AND DESIGN

Construction Control for Earthand Rock-Fill Dams

ENGINEER MANUAL

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DEPARTMENT OF THE ARMY EM 1110-2-1911U.S. Army Corps of Engineers

CECW-ED Washington, DC 20314-1000

ManualNo. 1110-2-1911 30 September 1995

Engineering and DesignCONSTRUCTION CONTROL FOR EARTH AND ROCK-FILL DAMS

1. Purpose. The purpose of this manual is to present principles and methods for constructioncontrol of earth and rock-fill dams.

2. Applicability. This manual applies to all Corps of Engineers divisions and districts havingresponsibility for construction of earth and rock-fill dams.

3. General. This manual is a guide to construction and inspection of earth and rock-fill dams inthose aspects that pertain to safe and satisfactory performance.

FOR THE COMMANDER:

ROBERT H. GRIFFINColonel, Corps of EngineersChief of Staff

This manual supersedes EM 1110-2-1911, dated 17 January 1977.

DEPARTMENT OF THE ARMY EM 1110-2-1911U.S. Army Corps of Engineers

CECW-ED Washington, DC 20314-1000

ManualNo. 1110-2-1911 30 Septembe r 1995

Engineering and DesignCONSTRUCTION CONTROL FOR EARTH AND ROCK-FILL DAMS

Table of Contents

Subject Paragraph Page Subject Paragraph Page

Chapter 1IntroductionReferences . . . . . . . . . . . . . . . . . . . . . . 1-1 1-1General Considerations . . . . . . . . . . . . . . 1-2 1-1

Chapter 2Field Organization andResponsibilityResident Inspection Force . . . . . . . . . . . . 2-1 2-1Field Laboratory . . . . . . . . . . . . . . . . . . 2-2 2-3Assistance by Higher Echelon . . . . . . . . . 2-3 2-4Records and Reports . . . . . . . . . . . . . . . 2-4 2-4

Chapter 3Foundation and AbutmentTreatmentGeneral . . . . . . . . . . . . . . . . . . . . . . . . . 3-1 3-1Clearing, Grubbing, Stripping, and

Cleaning . . . . . . . . . . . . . . . . . . . . . 3-2 3-1Seepage Control . . . . . . . . . . . . . . . . . . 3-3 3-3Treatment of Unfavorable Conditions . . . . 3-4 3-6Dewatering and Drainage of

Excavated Areas . . . . . . . . . . . . . . . . 3-5 3-10

Chapter 4Borrow Areas and Quarries

Section IEarth FillExcavation, Handling, and

Hauling Equipment . . . . . . . . . . . . . . 4-1 4-1

Borrow Area Operation . . . . . . . . . . . . . 4-2 4-5

Section IIQuarries and Rock ExcavationGeneral . . . . . . . . . . . . . . . . . . . . . . . . . 4-3 4-8Equipment . . . . . . . . . . . . . . . . . . . . . . 4-4 4-8Test Quarries . . . . . . . . . . . . . . . . . . . . . 4-5 4-9Obtaining Specified Rock Fill . . . . . . . . . 4-6 4-10

Section IIIFinal Condition of Borrow Areas,Quarries, and Spoil AreasBorrow Areas . . . . . . . . . . . . . . . . . . . . 4-7 4-11Quarries . . . . . . . . . . . . . . . . . . . . . . . . 4-8 4-11Spoil or Waste Areas . . . . . . . . . . . . . . . 4-9 4-11

Chapter 5Earth-Fill and Rock-FillConstruction

Section IFill Processing and CompactionEquipmentHeavy Compaction Equipment . . . . . . . . 5-1 5-1Hand-Operated Compaction Equipment . . 5-2 5-1Spreading and Processing Equipment . . . . 5-3 5-1

Section IITest FillsRock Test Fills . . . . . . . . . . . . . . . . . . . 5-4 5-1Earth Test Fills . . . . . . . . . . . . . . . . . . . 5-5 5-5

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Subject Paragraph Page Subject Paragraph Page

Section IIIImpervious and Semipervious FillDefinitions . . . . . . . . . . . . . . . . . . . . . . 5-6 5-5Compaction Fundamentals . . . . . . . . . . . 5-7 5-6Compaction Specifications . . . . . . . . . . . 5-8 5-6Simple Control Procedures . . . . . . . . . . . 5-9 5-7Field Control Testing and Sampling . . . . . 5-10 5-8Operations in Adverse Weather . . . . . . . . 5-11 5-15Compaction in Confined Areas . . . . . . . . 5-12 5-16

Section IVPervious FillDefinition . . . . . . . . . . . . . . . . . . . . . . . 5-13 5-17Compaction Equipment . . . . . . . . . . . . . . 5-14 5-17Material Gradations . . . . . . . . . . . . . . . . 5-15 5-18Water Content Control . . . . . . . . . . . . . . 5-16 5-18Lift Thicknesses and Number of

Passes or Coverages . . . . . . . . . . . . . 5-17 5-18Density Requirements . . . . . . . . . . . . . . . 5-18 5-19Construction Control . . . . . . . . . . . . . . . 5-19 5-19Test Results and Actions to be Taken . . . . 5-20 5-20

Section VRock FillGeneral . . . . . . . . . . . . . . . . . . . . . . . . . 5-21 5-21Hard Rock . . . . . . . . . . . . . . . . . . . . . . 5-22 5-21Soft Rock . . . . . . . . . . . . . . . . . . . . . . . 5-23 5-22

Section VISemicompacted Earth FillsUses . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-24 5-22Specifications . . . . . . . . . . . . . . . . . . . . 5-25 5-22Construction Control . . . . . . . . . . . . . . . 5-26 5-22

Section VIISequence of Placement andMeasurement of QuantitiesSchedule of Construction . . . . . . . . . . . . 5-27 5-22Placement Sequence . . . . . . . . . . . . . . . . 5-28 5-22Measurement of Quantities . . . . . . . . . . . 5-29 5-23

Section VIIISlope ProtectionAreas to be Protected . . . . . . . . . . . . . . . 5-30 5-23Upstream Slope Protection . . . . . . . . . . . 5-31 5-23Downstream Slope Protection . . . . . . . . . 5-32 5-25

Chapter 6Miscellaneous ConstructionFeaturesRiver Diversion . . . . . . . . . . . . . . . . . . . 6-1 6-1Stage Construction . . . . . . . . . . . . . . . . . 6-2 6-3Surface Drainage Facilities . . . . . . . . . . . 6-3 6-3Service Bridge Pier Foundations . . . . . . . 6-4 6-3Instrumentation . . . . . . . . . . . . . . . . . . . 6-5 6-3Haul Roads, Maintenance Roads,

and Public Roads . . . . . . . . . . . . . . . 6-6 6-4

Chapter 7Records and ReportsDaily Reports . . . . . . . . . . . . . . . . . . . . 7-1 7-1Compaction Control Reports . . . . . . . . . . 7-2 7-1Instrumentation Observations . . . . . . . . . . 7-3 7-1Construction Foundation Report . . . . . . . . 7-4 7-6Final Construction Report . . . . . . . . . . . . 7-5 7-6

Appendix AReferences

Appendix BMethods of Relating Field DensityData to Desired or SpecifiedValues

Appendix CField Compaction Control Formsand Supplemental Instructions

Appendix DInstructions for Preparing PeriodicSummaries of Field CompactionControl Data on Earth andRock-Fill Dams

Appendix EDescription and Use of InstrumentsDuring Earth and Rock-Fill DamConstruction

Index

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Chapter 1Introduction

1-1. References

References pertaining to this manual are listed inAppendix A. Additional reference materials pertaining tothe subject matter addressed in this manual are also includedin Appendix A.

1-2. General Considerations

The safety and satisfactory performance of earth and rock-fill dams require competent and adequate supervision,careful inspection, and control testing. It is the responsi-bility of the Resident Engineer to bring to the attention ofthe Engineering Division any design or construction detailnot adequately covered by the plans and specifications. TheResident Engineer must provide the field supervision andcontrol required to accomplish the intent of the plans andspecifications. The resident engineer must also coordinatethe work with design engineers and provide guidance to thecontractor if unexpected conditions are discovered duringconstruction.

a. Importance of construction control. Many earth androck-fill dams have shown signs of distress or experiencedpartial failure (necessitating expensive remedial measures)from causes traceable to poor construction practices or tounexpected adverse conditions. Close observations by soilsengineers and geologists of foundation and abutmentpreparation, excavation, fill operations, movements anddeformations of fill and foundation, and seepage can oftenenable early detection and correction of undesirableconditions. Construction control should ensure that:

(1) Necessary actions are taken to remedy or allow forunexpected conditions. Frequent and careful observationsby inspectors, geologists, and field engineers, who arefamiliar with conditions assumed for design, are essentialduring stripping of the foundation, opening of borrow areas,and excavating operations. Immediate reporting of unex-pected conditions will enable the Resident Engineer to

coordinate and plan, with design engineers, any additionalinvestigations needed to establish design modifications.

(2) Equipment and procedures are adequate to satisfac-torily accomplish the work. Review of the contractorsplans for quality control, dewatering and draining workareas, and haul roads, together with inspections of the actualoperations, is an important aspect of construction control.

(3) The completed structure meets the requirements andintent of the plans and specifications. This involves con-tinuous inspection of foundation and abutment preparation,material processing, and embankment construction, and acomprehensive control testing program to ensure propermaterial placement and compaction.

(4) Adequate construction records are maintained.Preparation of completion reports of construction operationsand maintenance of records of test results are essentialaspects of field control. Such reports and records are oftenrequired to evaluate claims by the contractor and todetermine possible causes of distress that might later occurin portions of the completed work. These documents shouldinclude as many detailed photographs as necessary. Videophotography may also be included in the methods todocument and record construction procedure and methods.

b. Relation of construction and design. The design ofan earth or rock-fill dam is not finished until construction iscomplete, the reservoir has been filled, and the dam isfunctioning satisfactorily. During construction, designengineers should frequently reassess design concepts andassumed conditions in light of actual conditions observed inthe field. This involves frequent visits to the project toobserve actual conditions and construction procedures.Consultation with specialists may be required to evaluateunusual problems or foundation conditions. Design eval-uation must include analyses of compaction control results.It may also require reanalysis of stability conditions basedon results of laboratory tests on record samples andadditional foundation samples and field observations of porewater pressures, settlements, and lateral displacements. Ahigh degree of coordination between design and constructionengineers is mandatory.

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Chapter 2Field Organization andResponsibility

2-1. Resident Inspection Force

Each levee, earth or rock-fill dam, or other embankmentmust be adequately inspected to ensure that plans andspecifications are observed and followed by the contractor.This requirement applies to all divisions and districts havingresponsibility for design and construction of civil worksprojects. The size and composition of the residentinspection force for foundation and earthwork controloperations should be adequate to provide for continuousinspection of construction activities, field testing andsampling, observation of field instrumentation, laboratorytesting, data compilation, maintenance of records, andpreparation of reports. On large projects, the contractor mayoperate as many as three shifts, and materials-handling forfill operations may be highly mechanized to obtain highproduction. The inspection force must be so staffed andorganized that inspectors and technicians are available forcontinuous inspection of the contractors operations. Indiscussing contractor quality control, ER 1180-1-6 states,The Government is responsible for all phases of the con-struction project, including the activities necessary to assurethat the contractor has complied with the requirements ofthe contract plans and specifications... and In contrast tothe Contractors quality control the government is re-sponsible for quality assurance. This includes checks,inspections, and tests of the products which comprise theconstruction, the processes used in the work and the finishedwork for the purpose of determining whether the Con-tractors quality control is effective and he is meeting therequirements of the contract. These activities are to assurethat defective work or materials are not incorporated in theconstruction.

a. Technical responsibilities. The Resident Engineer isresponsible for constructing the embankment and relatedappurtenances in compliance with plans and specifications.On large projects, resident geologists and soils engineersprovide technical assistance. Assisting the Resident Engi-neer are office and field engineering staffs. The officeengineering staff is responsible for preparing fieldmodifications to the plans and specifications in accordancewith applicable district regulations, reviewing planssubmitted by the contractor such as those for quality controland dewatering, evaluating results of construction controltests, and compiling instrumentation data to send to theEngineering Division for evaluation. The field engineer incharge of field supervision and inspection is responsible for

planning, executing, and coordinating all field inspection andtesting to ensure compliance with established standards anddetail drawings and specifications. The field engineer isassisted by one or more chief construction inspectors oneach shift who coordinate the activities of subordinateinspectors, and by a materials testing or soils engineer whosupervises a number of technicians in obtaining samples andperforming required field and laboratory tests. Detailedtechnical and organizational responsibilities may differ inthe various districts and divisions; however, constructionprojects are to be staffed with the number of experiencedGovernment laboratory technicians needed to performGovernment acceptance testing on compacted fill, includingfilter and drainage fills. Acceptance testing should beperformed immediately after placement and compaction ofthe lift material to be sampled and tested. Attention shouldbe given to selection of samples for acceptance testing sothat all materials of generally different descriptions beingplaced in the same compacted zone of the embankment willbe tested.

b. Preconstruction training. Every earth or rock-filldam is designed for specific foundation conditions and toutilize locally available materials. Unique features areinherent in each project, and a wide variety of constructionmethods may be utilized. Therefore, good communicationbetween design and construction personnel is essential. Theconstruction staff should be familiar with the designmemoranda pertinent to the work. Preconstruction in-structions and training should be given to field inspectionpersonnel to acquaint them with the design concepts and toprovide them with a clear understanding of expectedconditions, methods of construction, and the scope of plansand specifications. This may be done by training sessions,preferably with design personnel present, using a manual ofwritten instructions prepared especially for field personnel,to discuss engineering considerations involved and toexplain control procedures and required results. Inspectionpersonnel should be familiar with the plans and specifi-cations; excavation boundaries; types of materials to beexcavated; temporary and permanent drainage and seepagecontrol measures; approved sources of borrow materials;procedures and equipment most suitable for excavating,processing, and hauling borrow materials; characteristics offill materials and compaction requirements; capabilities ofvarious types of compaction equipment; and proceduresrequired to obtain desired or specified compaction. Closelysupervised on-the-job training should be given to inspectorsand materials testing personnel during initial stages ofconstruction to increase their proficiency in recognizingsigns of inadequate compaction, in using expedient methodsof checking water content and density of fill materials, inusing selected methods for field density measurements and

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laboratory compaction, and in detecting inadequate con-struction procedures and unsafe conditions.

c. Number of personnel and skills required. Ex-perienced construction engineers, inspectors, and techniciansare required for construction control operations on earth androck-fill dams. The Resident Engineer, the field engineer incharge of supervision and inspection, chief inspector,materials engineer or chief soil technician, and geologistsshould have been associated with the project from the timeof any preliminary construction operations such as test fills,quarry blasting and rock production tests, and excavationsof tunneling made to inspect subsurface conditions.Augmentation of this cadre with less experienced inspectorsand technicians will provide a sufficiently capable inspectionforce. An example of a resident inspection force organi-zation for a large earth and rock-fill dam is shown inFigure 2-1. Additional inspectors and technicians may be

Figure 2-1. Example of resident engineers staff organization for large earth dam project

required during certain phases when construction operationsare at their peak or when several major portions of the earthor rock-fill dam are being constructed concurrently. Anexample of a typical similar organization for a smaller earthdam project is shown in Figure 2-2. It should be noted thatthe organization for a small earth dam is very flexible, beingdependent on the magnitude and extent of the construction.Small projects often require temporary assignment ofspecialized personnel, such as soils engineers and geologists,during certain construction phases.

d. Quality control.

(1) The contractor is responsible for quality control, andthe contract specifications give requirements for thecontractor quality-control organization, personnel qualifi-cations, facilities and types of tests, and reporting of testdata and inspections. The Government field inspection forcehas the responsibility of accepting completed work and musthave a staff large enough to accomplish the following:

(a) Check the effectiveness and adequacy of the con-tractors quality-control system and take action to havedeficiencies corrected.

(b) Inspect construction operations to prevent defectivework and placement of unsatisfactory materials.

(c) Monitor progress.

(d) Perform check tests and acceptance tests.

(e) Resolve or report field problems and conflicts withcontract documents to higher authority.

(f) Make acceptance inspections.

(2) Contractor quality-control operations will, if properlyimplemented, assist in achieving adequate construction,

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particularly on those operations where specifications contain

Figure 2-2. Example of resident engineers staff for small earth dam project

product requirements such as water content limits andgradation of filter material. However, Government fieldinspectors must be present to witness operations for whichconstruction procedures are specified and to conduct tests toensure that results obtained are those required by design.Several important considerations are listed below:

(a) The contractor is not required to conduct tests on thequality or durability of materials used for pervious fill, filterzones, bedding, spalls, or rip-rap. The Government isresponsible for determining such properties in approvingsources of natural or processed (blasted, ripped, or crushed)pervious materials for use in embankments.

(b) The contractor is not required to provide tests oncompaction characteristics when fill placement procedure isspecified. Government inspection forces are responsible forinspecting the contractors specified construction activitiesand for testing embankment or slope materials to establishthat excavation and fill placement procedures result in anexcavated slope or fill that conforms to properties assumedin the design.

e. Relationship with contractor. Mutual understandingbetween the Resident Engineer and his staff and thecontractor of the requirements of the contract specificationsis necessary to obtain desired construction quality. Thequality of the work must never be compromised. Un-necessary requirements and restrictions should not beimposed over and above the specification requirements.Firmness by Resident Engineers and inspectors who areefficient and know their job will gain the respect andcooperation of the contractor.

2-2. Field Laboratory

Foundation and earth-fill materials are tested in the fieldlaboratory to determine gradation, water content, in situdensity, compaction, relative density, and Atterberg limits.The data obtained serve as a basis for ensuring compliancewith specifications and design requirements, for guidancetoward maximum utilization of available materials, and forproviding a record of the properties of materials placed inall parts of the project.

a. Size. The size of the field laboratory and satellitetesting units depends on the magnitude and areal extent ofconstruction operations. Remote borrow areas, dikes, orrelocation fills may require a central laboratory, supple-mented by one or more skid- or wheel-mounted portableunits. An existing structure, a prefabricated structure, ortrailer may be specified. For relatively large projects, about500 to 1,500 sq ft may be required for a laboratory space.The laboratory should have a sound floor, necessaryworktables, benches, storage cabinets, equipment pedestals(for compaction mold base and sieve shakers), service sinks,and utilities. Additionally, it may be very advantageous tohave an awning-covered work slab with an area of from 500to 1,000 sq ft to serve as a work space on which to dry andwork large samples, to perform coarse aggregate gradations,etc.

b. Types of tests and facilities required. The majorportion of testing in the field laboratory is for acceptancetesting of compacted fill. These tests must be conducted ina manner that will yield results of a quality comparable tothe initial laboratory tests upon which the design was based.

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Discrepancies resulting from variations in testing equipmentand techniques should be carefully avoided. Water content,compaction, relative density gradation, and Atterberg limitstests are the most common tests conducted. Water contentand compaction tests are used for control of cohesive soilsin impervious and random fills. Atterberg limits tests maybe used for control of fills of fine-grained cohesive soilswhere good correlations of optimum water content andmaximum dry density with the Atterberg limits have beenestablished. Gradation and relative density tests are used forcontrol of pervious fills. Gradation tests are also used forcontrol of rock fill. Field density tests are performed on thefill, but compaction tests on the material and water contentdeterminations may be made either in the central laboratoryor in the field. The central laboratory must have equipmentfor these tests, but supplemental portable units may beadvisable for gradation, compaction, and water contenttesting at remote locations. Panel pickup trucks are oftenused to transport equipment for field density testing,undisturbed (record) sampling in the fill, and sampling inborrow areas. Specially equipped pickup trucks with asmall hoist may be required where large-scale field densitytests are to be performed on material such as rock fill orsoils with a high percentage of large gravel sizes.

2-3. Assistance by Higher Echelon

Unusual conditions encountered during constructiongenerally require special attention. The advice of specialistsin soil and rock mechanics, geology, and instrumentation ofearth and rock-fill dams, and additional evaluation by designengineers are often required to obtain effective solution tounusual problems and conditions.

a. Geologists and soils engineers. Specialists withexperience in soil and rock mechanics, geology, and in-strumentation are found in division and district offices, atHeadquarters, U.S. Army Corps of Engineers (HQUSACE),Waterways Experiment Station (WES), and on Corps ofEngineers boards of consultants. The services of soil androck mechanics engineers and geologists are particularlyvaluable during early stages of construction when the foun-dation, abutment, and any diversion tunnels or excavations(such as for spillway foundations) expose existing condi-tions. At this time, it is vital that actual conditions beevaluated to determine if they are consistent with conditionsassumed for design. In addition, it is necessary to recognizeany unusual conditions that may affect construction. Theadvice of specialists in soil mechanics and rock mechanicsis also valuable in establishing, from observed field con-ditions, modifications that may significantly improve thedesign without increasing the cost of the project.

b. Design engineers. The engineer who designs the

earth or rock-fill dam should visit the project duringconstruction and assist field personnel in interpreting plansand specifications and observe problems that may not havebeen fully evaluated in the design. Visits should also bemade whenever unexpected conditions are encountered thatmay require changes in the plans or specifications. Acooperative attitude must be maintained between designengineers and construction personnel so that mutualunderstanding is reached on existing problems and feasiblesolutions are developed. In some cases, conferences at theconstruction site may be necessary between constructionpersonnel, designers, and specialists to review conditionsand determine if design modifications are required. Aregular schedule of visits should be set up so that designpersonnel and representatives from the division office andHQUSACE can inspect field conditions at critical con-struction stages.

c. Instrumentation. Instrumentation of earth and rock-fill dams is becoming increasingly important. The mainreasons are that many higher earth and rock-fill dams arebeing constructed, sites having unfavorable foundation con-ditions must be used more frequently, interest is increasingin obtaining meaningful data for evaluating dam behavior,and continually increasing downstream land development isincreasing the consequences of failure on property damageand loss of life. Monitoring of pore water pressure, settle-ments, and deformations of the foundation and embankmentis necessary to check the safety of the dam during con-struction and to control the rate of construction. Theinstrumentation must be of the proper type, placed in criticallocations, and installed properly. Valid readings depend ontechniques and procedures used in installing and observingthe instrumentation. For this reason, specialists experiencedin field instrumentation should plan and supervise the fieldinstallations. These specialists can be from the districtand/or division office, from WES, or from firms specializingin installations of instrumentation of earth and rock-filldams. This applies whether the instrumentation is furnishedand installed by the Government or furnished and installedby the contractor. Because proper interpretation of in-strumentation data is vital to the safety of the dam, theresponsibility for collecting and reporting data to the Engi-neering Division should be carefully delegated. Installationand observations of instrumentation are discussed in para-graph 6-5; the general use of instrumentation is described inAppendix E and by Dunnicliff (1988).

2-4. Records and Reports

Construction records and reports are needed to documentresults of tests made to verify specification requirements andaction taken to correct deficiencies and to provide a recorddescribing the field conditions, modifications to plans and

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specifications, construction procedures, sequence of opera-tions, and the location and as-built dimensions of importantfeatures. These are necessary to evaluate claims made bythe contractor based on changed conditions, or claims by theContracting Officer that work performed does not meetcontract requirements. Progress reports are also needed forthe district office and to provide a basis for payments to thecontractor for work accomplished. Inspectors must maintaina daily inspection report (or log), and a master diary mustbe kept by the Resident Engineer. The required content ofthese documents is outlined in EM 415-1-302, Inspectionand Work Records. Details of specific construction controlrecords and reports are described in Chapter 7.

a. Construction records. These records provide usefuldata for designing future alterations and additions to thestructure, determining causes of later undesirable movementor seepage, or interpreting piezometric data. As-builtdrawings, construction photographs, descriptions of foun-dation conditions encountered and various treatments,compaction data, and test data on record samples should beincluded in the records.

b. Construction reports. The construction foundationreport should include details such as dip and strike of rock,

faults, artesian and other groundwater conditions, and othercharacteristics or conditions of foundation materials. Acomplete history of the project in narrative form should bewritten, giving the schedule of starting and completingvarious phases of the work, describing construction methodsand equipment used, summarizing quantities of materialsinvolved, and including other pertinent data. Foundationreports should be supplemented by photographs that clearlydepict foundation conditions. Routine photographs shouldbe taken at regular intervals, and additional pictures shouldbe taken of items of specific interest, such as the preparationof foundations and dam abutments. For these items, coloredphotographs should be taken to provide a better depiction ofconstruction conditions. The captions of all photographsshould contain the name of the project, the date on whichthe photograph was taken, the identity of the feature beingreported, and the location of the camera. In reportscontaining a number of photographs, an alternative would bean index map with a circle indicating the location of thecamera with an arrow pointing in the direction the camerawas pointing, with each location keyed to the numbers onthe accompanying photographs. Details concerning the useand preparation of construction foundation reports arepresented in ER 1110-1-1801.

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Chapter 3Foundation and AbutmentTreatment

3-1. General

The preparation of the foundation and abutments for anearth or rock-fill dam is a most difficult and important phaseof construction; the thoroughness with which it is done isreflected in the performance of the completed structure. Itis often difficult or sometimes impossible to correctfoundation and abutment deficiencies that show up afterconstruction is well underway or completed. The primarypurpose of foundation and abutment treatment is to obtainpositive control of under seepage, prepare surfaces toachieve satisfactory contact with overlying compacted fill,and minimize differential settlements and thereby preventcracking in the fill. Inspection of the work must ensure thatthe foundation and abutments are stripped to depthssufficient to remove soft, organic, fractured, weathered, orotherwise undesirable materials; depressions and joints inrock surfaces are cleaned and adequately filled; rocksurfaces are made relatively smooth and uniform by shapingand filling; subsurface cavities are detected and grouted; andcutoffs extend to suitable impervious materials. During thisphase of construction, close liaison must be maintainedbetween construction and design personnel since mostdiscrepancies between design and field construction occur inthis portion of the work. Few dams are constructed withoutencountering some undesirable foundation conditions thatwere not discovered in exploration for design, such as zonesof weathered or fractured rock, cavities, soft soil areas,abandoned pipes or drains, or abandoned stream channelsfilled with sand and gravel. This is the reason thatinspection trenches are generally required beneath theimpervious zone of a dam when cutoff trenches are notspecified. These inspection trenches provide the means forcareful examination of the foundation along the entire lengthof the dam to ensure that undesirable foundation conditionsare detected.

3-2. Clearing, Grubbing, Stripping, and Cleaning

Clearing, grubbing, stripping, and cleaning of areas uponwhich a compacted earth or rock-fill dam will lie arerequired to remove those materials having undesirableengineering qualities such as low shear strength, high com-pressibility, undesirable permeability, or other characteristicswhich would interfere with compaction operations; andprovide a surface favorable for a good bond with theoverlying fill. Specifications should provide adequate timefor inspection by the Contracting Officers representative ofexposed foundation and abutments. In some cases where

abutments will require special treatment, a separate contractfor such work is awarded.

a. Soil foundation and abutments.

(1) Clearing consists of removal of all abovegroundobstructions, including trees, vegetation, felled timber, brush,abandoned structures, local dams, bridges, and debris.Grubbing includes removal of all objectionable below-ground obstructions or material including stumps, roots,logs, drain tiles, and buried structures or debris. Foundationor abutment soils disturbed during clearing and grubbingoperations must be removed. Blasting should be avoided ifpossible; if unavoidable, explosive charges should be keptas small as possible.

(2) Foundation and abutment stripping generally followclearing and grubbing operations. Stripping consists of theremoval of sod, topsoil, boulders, and organic or foreignmaterials. Stripping beneath closure sections should beperformed in the dry after diversion of the river. Necessaryor inadvertent deviation from stripping limits identified inthe plans and specifications should be reported to the designoffice so that effects of the changes can be evaluated.Personnel inspecting stripping operations should be able toidentify the materials to be removed. Inspectors should lookfor soft pockets as well as old sloughs or river meandersthat may not have been found during design investigations;the resident geologist can assist in locating such features.Several passes of a heavy roller should be made over thestripped surface to proof test the area to reveal anyunsuitable materials overlooked during stripping.

(3) The sides of holes and depressions left by grubbingand stripping should be flattened and scarified and thedepressions filled with material of the same type andcompacted to a density at least equal to that of the surround-ing foundation material by the specified method andequipment. Where areal dimensions of depressions aresmall, power hand tampers are required to compact fill.Final preparation of the foundation surface, immediatelyprior to placing embankment fill, should include adjustingsoil water contents as near optimum as possible, compactingas prescribed for the overlying fill, and scarifying thecompacted surface to receive the initial embankment lift.

(4) Compaction of some types of saturated soils in wetfoundation areas may do more harm than good. When it isnot feasible to dry such areas out, it may be necessary toplace a thick initial lift to permit compaction equipment tooperate without remolding and disturbing the foundationsoil. Also, the weight of compacting equipment operatingon the initial lift might be reduced and progressivelyincreased as more lifts are placed. However, this should not

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be done for foundation areas under the embankment unlessspecifically permitted by the plans and specifications orapproved by the design office, as the effects of a lightlycompacted layer at the base of the dam could adverselyaffect stability.

(5) Preparation of soil abutments prior to fill placementshould be the same as that for soil foundations. To ensurebonding of the embankment to the natural soil of theabutments, it is necessary to remove some of the abutmentsurface soil. Inspection should confirm that all loose, wet,or soft soils are removed. In addition, abutment slopesshould be smooth and as flat as economically feasible atcontact with the embankment to improve compaction of fillagainst the abutment and to minimize the probability ofdifferential settlement causing cracking (paragraph 3-4a(5)).Depressions should be filled with concrete or soil compactedat proper water contents to densities equal to or greater thanthose of the materials to be placed above them in theembankment fill. See paragraph 3-4a(2) for discussion oftreatment of abutment slopes of clay shales.

b. Rock foundations and abutments.

(1) After all rough excavations of overburden and/orweathered rock have been completed, all grouting iscompleted, and the surface of the rock foundation isexposed, shaping and cleaning operations should begin.Shaping and cleaning a rough rock foundation are necessaryto provide a smooth, uniform, and clean surface againstwhich fill can be compacted. The procedure generallyconsists of removing large loose rocks, overhangs, andprojecting knobs by scaling, handpicking and wedging, andlight blasting pressure washing followed by some form ofdental treatment to fill all holes, cracks, joints, crevices,and depressions. Dental treatment involves cleaning thecavities and backfilling them with concrete, and is discussedin more detail in paragraph 3-4b(3). The resident geologistor embankment engineer should inspect and approve thisphase of the work.

(2) The final preparation of almost all rock foundationsrequires hand labor. The use of heavy or tracked vehicleson the final foundation should be avoided, especially if therock is thinly bedded or badly jointed. Blasting to removeknobs or overhangs may prove more harmful than helpful,and extreme caution must be exercised to prevent theopening of cracks or actual displacement of blocks or rocksthat would otherwise provide adequate bearing. It isgenerally desirable to place concrete fill beneath or aroundprojections if, by so doing, blasting can be avoided. Whereconcrete fill is used, materials and procedures should bedirected towards ensuring good concrete/rock bond;

subsequent fill operations should avoid dislocating theconcrete. Hand methods involve removal of all loose ordrummy rock (rock that sounds hollow when struck witha steel hammer or bar), and the scaling down of slopedsurfaces to provide an even, uniform slope.

(3) Washing the hard rock foundation surface with waterunder high pressure and dry brooming to remove looseresidue are generally the last step in foundation preparation.This is done to clean the surface to the maximum extentpossible and to remove fines that may have worked intoseams. All seams or cracks should be cleaned to a depth ofat least twice their width. Removal of these fines willfacilitate complete filling of seams in subsequent operations(such as dental treatment) taken to prevent seepage.Pressure washing also serves to detect rock projectionsoverlooked during hand excavation which might otherwisework loose during compaction of the first lift or lifts of fill.Washing should be performed to clean from higherelevations to lower elevations.

(4) Particular attention should be given to cleaningopenings that cross the axis of the dam. Accumulated waterfrom the washing process must be removed. Small airpumps, hand bailers, or aspirators may be used to emptynarrow, water-filled fissures. If the foundation consists ofblocky rock with frequent joints, caution must be used toavoid removal of satisfactory foundation material (such asstiff clay in joints) by overzealous pressure washing. Whenthe nature of the rock is such that it could be softened bywashing with water, compressed air should be used insteadof water. Air pressure is also often used as a final step incleaning sound rock surfaces. Figure 3-1 shows the rockfoundation at DeGray Dam being cleaned with compressedair.

Figure 3-1. Final foundation cleaning using com-pressed air, DeGray Dam, Arkansas

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(5) Where rough and irregular surfaces remain after handexcavation, troughs, pits, and other depressions are filledwith concrete to provide a more even surface on which thefirst layer of the embankment may be compacted. Aspreviously noted, this procedure is termed dental treatmentand is discussed further in paragraph 3-4b(3). If foundationgrouting has been performed, cleanup operations shouldinclude removal of any spilled or washed grout that mightotherwise conceal surface imperfections and pockets ofundesirable material.

(6) Before placing the first layer of embankmentmaterial, the cleaned and prepared rock surface should bemoistened, but no standing water should be permitted.Moistening the rock surface is recommended instead ofusing overly wet soil in the first lift to obtain good contact.Use of heavy pneumatic equipment (preferably a rubber-tired roller) is recommended for compacting the first lift onrock surfaces. This will enable the rock surface to be keptintact, especially where the rock surface is irregular orcomposed of thin beds of alternating hard and soft rock.

(7) Foundations consisting of compaction-type shales andslaking tuffs should be protected from disintegration causedby drying due to exposure to air. The handling of clayshales is discussed in paragraph 3-4a(2).

(8) The same degree of care should be exercised inabutment treatment as in foundation treatment. A goodbond between the embankment and the abutment is criticallyimportant. Areas to be cleaned at rock abutments shouldinclude not only those beneath the embankment core butalso those beneath transition or filter zones. Within theseareas, all irregularities should be removed or trimmed backto form a reasonably uniform slope on the entire abutmentwith vertical surfaces no higher than 5 ft. Benches betweennear-vertical surfaces should be of such width as to providea stepped slope comparable to the slope on adjacent areasbut not steeper than 1V on 1H. Overhangs should not bepermitted at any locations. Methods of overhang removalare discussed in paragraph 3-4b(4).

(9) The treatment of cracks, fissures, and otherundesirable conditions in rock foundations and abutments isdiscussed in paragraph 3-4b(2).

3-3. Seepage Control

a. Cutoffs. Foundation cutoffs or core trenches serve asbarriers to underseepage. The design of foundation cutoffsis based largely on borings made during field investigationsfor design. Therefore, the open excavation of a cutofftrench provides the first real look at actual foundationconditions; frequent inspections, particularly by the field

geologist and embankment engineer, should be made. Somecommon types of cutoffs are discussed in the followingparagraphs.

(1) Compacted backfill trenches. Backfill compactedinto a seepage cutoff trench is one of the most effectiveconstruction devices for blocking foundation seepage.Material and compaction requirements are the same as forthe impervious section of the embankment. When requiredby contract specifications, the trench must fully penetrate thepervious foundation and extend a specified distance intounweathered and relatively impervious foundation soil orrock. Treatment (as described in paragraph 3-2a) of theexposed surface in the bottom and sides of the trench isessential to ensure firm contact between foundation andbackfill. The trench excavation must be kept dry to preventsloughing of the side slopes and to permit proper backfillplacement and compaction. When the water table is nearthe ground surface, dewatering the excavation is requiredand is frequently a major expense in cutoff construction.Dewatering and drainage methods are discussed in para-graph 3-5. In any trenching operation, a qualified geo-technical engineer should inspect the construction at regularintervals to monitor stability of the side slopes.

(2) Slurry trenches.

(a) The slurry trench method of constructing a seepagecutoff involves excavating a relatively narrow trench withnear-vertical walls, keeping the trench filled with a bentoniteslurry to support the walls and prevent inflow of water, andthen backfilling with a plastic impervious mixture of well-graded clayey gravel to protect against piping, to reduceseepage, and to minimize consolidation of the backfillmaterial.

(b) The backfill should be a mixture of imperviousborrow, sand, gravel, and bentonite slurry (U.S. ArmyEngineer District, Savannah 1968). The backfill may be amixture of material excavated from the cutoff trench andother material to provide an acceptable blend.

(c) Depending on the required depth, the excavation maybe accomplished with a dragline, backhoe, clamshell, ortrenching machine. A trenching machine is limited todepths less than about 40 ft, provided no cobbles exist.Unmodified backhoes are limited to depths less than about45 ft but with special modification can reach depths of 55to 60 ft; their main advantage is that they can be used inareas where cobbles exist. Maximum depths of about 100 fthave been achieved with a dragline. Required equipmentmodifications for excavation to a great depth (with a drag-line) include weighting the bucket to overcome the buoyanteffect of the slurry and providing heavy-duty bearings and

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hydraulic systems. A dragline excavating a slurry trench isshown in Figure 3-2.

Figure 3-2. Dragline excavation of slurry trench, WestPoint Dam, Georgia

(d) The specific gravity of the slurry must be highenough to ensure that hydrostatic pressure exerted by theslurry will prevent caving of the sides of the trench and yetnot be so high as to limit the depth to which the excavatingbucket will operate. Typical values of specific gravity ofslurries used in past jobs range from 1.05 to 1.2, with somevalues as high as 1.5. The slurry level is generallymaintained 2 to 3 ft above the groundwater level.

(e) Procedures for cleaning the bottom of the trench,removing sand which settles out of the slurry, continuouscontrol of viscosity and specific gravity of the slurry, andmixing and placing the backfill are critical in achievingsuccessful results. An example of successful slurry trenchconstruction is that at West Point Dam, ChattahoocheeRiver, Alabama and Georgia, in which the bottom of thetrench was cleaned with a modified dragline bucket (Jones

1967; U.S. Army Engineer District, Savannah 1968). Ascraper blade was attached to the bucket which, whendragged along the bottom of the trench, removed coarsersoil particles and some of the finer loose material at the topof the rock. An air jet was used to remove sand, gravel,and other undesirable material from potholes, cracks andcrevices; these materials subsequently became entrained inthe slurry. Suction and discharge pipes were used to removecontaminated slurry (from the trench), which was cleaned bysending it to shallow sediment ponds along the sides of thetrench where the contaminants settled out of suspension.The clean slurry was then placed back into the trench.Mechanical desanders are available and may be desirable oreven required for removing sand (from the bottom of thetrench) in some situations. The bottom of a trench shouldbe sampled after it has been cleaned to ensure that it isproperly free of undesirable material.

(f) After the bottom of the trench has been cleaned,backfill is placed in the trench with a clamshell to form agentle slope parallel to the axis of the trench; backfill isthen successively pushed into the trench with a bulldozerand allowed to slide down the slope, intermixing with anddisplacing the slurry. The slope of the backfill should beflat enough to prevent sliding and sloughing. The trenchsurface should be observed/inspected as long as possible todetect unusual settlements which might indicate slurrypockets entrapped during the backfilling process. A sketch/schematic of the progressive excavation and backfillingscheme used at West Point Dam is shown in Figure 3-3.The ultimate objective is to achieve a positive cutoff by thecombined effects of the backfill and a filter cake formedon the sides of the trench by the slurry. Since the integrityof the filter cake after backfill placement cannot be assured,it is recommended that the added benefit of the cake beconsidered as an additional safety factor with the backfill asthe primary element of seepage cutoff.

(g) The slurry trench as a construction technique forforming a cutoff barrier was first used in the United Statesin the 1940s, and its widespread use began in the late 1960sand early 1970s. Presently, the technique is used routinelyalthough design studies for cutoff walls must be com-prehensive and include parameters such as wall depth,thickness, layout, grade, and preparation of the workingsurface. Design studies must also consider properties of theslurry. Water of adequate quality for slurry mixing must beprovided. Typical water quality requirements for bentoniteslurry are hardness less than 50 parts per million (ppm),total dissolved solids content less than 500 ppm, organiccontent less than 50 ppm, and pH of about 7. A satisfactoryprocedure for the design of slurry trenches and slurry trenchconstruction is given by Winterkorn and Fang (1975).

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(3) Grout curtains.

Figure 3-3. Progressive excavation and backfilling scheme for slurry trench construction

(a) Grouting is the injection by pressure of grout (amixture of water, cement, and other chemical compounds)into openings (voids, cracks, or joints) in a rock mass. Thegrout is designed to be injected as a fluid and to stiffen orsolidify after injection.

(b) The rock foundation and abutments of most largedams require grouting to reduce seepage and to reducehydrostatic uplift pressures in dam foundations. Groutcurtains are frequently tied into the bottom of cutofftrenches which extend through soil overburden to the rockfoundation. Grouting procedures must be tailored to theformation characteristics of the foundation being grouted,and close supervision and inspection are required to obtainsatisfactory and economical results. The resident geologistshould direct and supervise the inspection of grouting work;he should be experienced in this type of work since manydecisions must be made as the work progresses based onjudgment and evaluation of results. Successful and eco-nomical grouting requires a complete and reliable subsurfaceinvestigation to allow determination of the volume whichmust be grouted. Items which must be determined by thegrouting inspector are grout hole location, geometry, lengthand inclination; injection pressure and rate; grout properties(liquid, transition, set); and necessary degree of improve-ment in soil properties.

b. Blankets, relief wells, galleries, and toe drains.

(1) Upstream impervious blankets. A horizontalupstream impervious blanket controls underseepage bylengthening the path of underseepage. The effectiveness ofthe blanket depends on its length, thickness, continuity, andthe permeability of the material/soil from which it is

constructed. At sites where a natural blanket of impervioussoil already exists, the blanket should be closely examinedfor breaches such as outcrops of pervious strata, root holes,boreholes, and similar (seepage) paths in the foundationwhich, if present, should be filled or covered withimpervious material to provide a continuous imperviousblanket. This is especially important in areas where oldstream beds may exist. It may be necessary to makeadditional shallow auger borings during construction todefine the extent of breaches, if any, in the natural blanket.

(2) Pressure relief wells. Relief wells are installed alongthe downstream toe of an embankment to intercept under-seepage water and relieve excess uplift pressures that wouldotherwise develop at the toe of an embankment. Relief ofhydrostatic pressure and removal of the associated watervolume prevents the transport of soil which might occur inthe formation of sand boils and also prevents heaving at thetoe. The installation of relief wells is discussed briefly inEM 1110-2-1901 and TM 5-818-5 and in greater detail inEM 1110-2-1914.

(3) Drainage galleries and tunnels. To facilitatefoundation and abutment grouting and interception ofseepage water, drainage galleries and tunnels are sometimesused in high dams. Drainage tunnels into rock abutmentsshould be examined by a geologist or rock mechanics expertto obtain information on in situ jointing and rock types.Information on the subterranean makeup of the site shouldbe acquired, collated, and interpreted by experiencedpersonnel with geological training for the express purposeof evaluating the site for grouting. The plan for groutingshould include design of the location, spacing, depth, andsize of grout holes as well as a method to install drain pipesrequired to block or intercept seepage. The inspector mustalso be experienced in rock tunneling to ensure satisfactory

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installation of rock bolts and other structural supportfeatures by plans and specifications. Drainage galleries atthe base of a dam or in an abutment of soil or weatheredrock are usually concrete-lined tunnels. Inspection ofconcrete-lined tunnels requires knowledge of concreteplacement and backfill practices around concrete structuresin addition to knowledge of grouting and seepage control inpervious soils. Inspection of concrete, including properplacement techniques, is thoroughly discussed in the ACIManual of Concrete Inspection (1967). EM 1110-2-2000also contains information related to the inspection ofconcrete placement.

(4) Toe drains.

(a) Toe drains collect and facilitate removal of seepagewater at the downstream toe of the dam to prevent form-ation of soft boggy areas and/or boils. Toe drains aregenerally connected to the horizontal drainage blanket andsometimes to the relief well system to collect and removeseepage water in thin pervious strata in the upper foundationthat the deeper relief wells cannot drain.

(b) Toe drains generally consist of a trench containing aperforated collector pipe surrounded by filter gravel with theremainder of the trench backfilled with sand. Particular caremust be exercised in placement of the backfill. Unless thesides of the trench are approximately sloped at the angle ofrepose of the filter material, a wood or steel form must beused to keep the filter layers separated as the backfill isbrought up. Additionally, filter materials must be protectedfrom contamination which could result from inwash duringa rainstorm. Construction (backfilling) of toe drains in shortsections could minimize contamination.

(c) The same control procedures are used for toe drainsas those that are used in construction of impervious fill inthe main embankment; these are described in Section IV ofChapter 5. Gradation tests on filter materials should be runat least twice each day during placement operations. Stock-piled as well as in-place filter material should be tested.Handling and compaction of the filter material must beclosely controlled to avoid segregation and particlebreakage.

3-4. Treatment of Unfavorable Conditions

Unexpected unfavorable conditions are frequently discoveredduring early construction, and may range from undesirabledeposits of material not detected in exploratory drilling toadverse seepage conditions that were impossible to predict.Very often, when undesirable materials are found, additionalexploration by test pits, borings, or calyx holes is necessaryto define the extent of the unexpected deposits and their

characteristics. In this way, the impact of a problem depositcan be properly evaluated in relation to the original design.Some common undesirable conditions are discussed in thefollowing paragraphs.

a. Unfavorable soil conditions.

(1) Highly compressible and low strength soils. Organicsoils exhibit high compressibility and low shear strength andare generally recognized by their dark color, the presence oforganic particles, and often a distinctive organic odor.Inorganic clays with high water content also exhibit highcompressibility and low shear strength. If an embankmentis constructed on a deposit of either highly organic soil orhighly compressible inorganic soil, excessive differentialsettlement could cause cracking of the embankment, orshear failure might occur; if significant deposits of either ofthese materials are discovered during early construction,their extent should be established and, if it is feasible, theyshould be removed and replaced with acceptable compactedbackfill. If extensive and/or deep deposits of such materialsare found, engineering personnel should be consulted todetermine if design modifications (such as flatteningembankment slopes or adding berms) are required.

(2) Clay shales.

(a) Clay shales are among the most troublesome andunpredictable soils. They are often termed compaction orsoil-like shales if they have been highly overconsolidatedby great thicknesses of overlying sediment and have noappreciable cementation. Clay shales tend to slake rapidlywhen subjected to cycles of wetting and drying; someexhibit very high dry strength, but upon wetting swell andslake profusely, losing strength rapidly. They vary in colorfrom brown to green to black and are often slickensided (aslickenside is a smooth, shiny, striated, discontinuoussurface that shows evidence of movement). Problem clayshales can be identified on the basis of slickensides foundby breaking undisturbed blocks or chunks apart, and fromthe speed of slaking during cycles of wetting and drying.Clay shales that are slickensided may be unstable even inrelatively flat slopes. Rapidly slaking shales will deteriorateinto soft clays with low strength upon exposure to air andwater and require protection of exposed surfaces prior to fillplacement. Stability in deposits of problem clay shales isfurther compounded if they are highly fractured or jointedor show evidence of faulting.

(b) Some clay shales also tend to swell or expandconsiderably when unloaded by excavating overlyingmaterial. Expansion may progress deeper into the clay shaledeposit with time and cause nonuniform rebound acrossexcavation surfaces. This is caused by stored strain energy

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that is released with time after overlying materials areremoved. Therefore, excavating in clay shales should becompleted and backfilled without delay. The last foot or soof excavation into a slaking clay shale should be deferreduntil just prior to backfill operations in order to minimizethe time of exposure of the final clay shale surface. Duringwinter, the depth of cover should be no less than the frostpenetration depth; operating in this manner will provide afresh surface to compact the fill against and eliminate thechance of a soft stratum between the unweathered shale andthe fill. This is generally a costly procedure for steepslopes, but becomes more economical for slopes flat enoughfor equipment to work on.

(c) Only rubber-tired equipment should be used in finalexcavation, cleanup, and initial fill placement on clay shalesto minimize disturbance. Final clay shale surfaces shouldnot be scarified prior to covering with fill. If pressurecleaning is required, only air pressure (i.e, no water) shouldbe used.

(d) Various types of coatings have been applied toprotect exposed clay-shale surfaces; they include gunite,sprayed asphalt, and other bituminous materials and resinemulsions. Gunite is reliable when reinforced and anchoredto the shale, but particular care must be exercised to avoida drummy condition. This type of protection was success-fully used at Waco and Proctor Dams in Texas. Althoughbituminous coatings and resins have been used successfully,they do not always provide adequate protection for the clayshale. At Waco Dam, an asphalt emulsion membrane usedon near-vertical cuts was not always adequate, even withmultiple application. Evidence of its inadequacy was thatthe shale surfaces spalled and slaked. Concrete slabs,whether placed specifically for protective purposes or asslabs for an overlying structure, provide good protection.Exposed surfaces may also be protected by wet mats.Burlap has proven to be an unsatisfactory mat because it istoo porous to retain water for any length of time. Maxi-mum allowable exposure time can vary from a few minutesto several hours depending on the characteristics of the shaleand the prevailing weather conditions.

(3) Collapsible soils.

(a) Collapsible soils are generally soils of low densityand plasticity which are susceptible to large decreases inbulk volume when they are exposed to water. Collapsiblesoils are characterized by bulky grains (in the silt-to-fine-sand grain size) along with some clay. Collapse results fromsoftening of clay binder between larger particles or the lossof particle-to-particle cementation due to wetting. Volumechange from collapse occurs rapidly (relative to consoli-dation) and can be very significant especially if the soil is

under high stress. If an embankment is founded on acollapsible soil which is subjected to wetting for the firsttime, substantial settlement and possibly cracking in theoverlying embankment could result. Therefore, unalteredcollapsible soils should not be allowed in a dam foundation.If it is not practicable to remove such deposits, they shouldbe treated to break down their structure prior toconstruction.

(b) Prewetting has been used as a treatment for collapsi-ble soils; the deposits are flooded with water in flat areaswhere ponding is possible, or by continuous sprinkling onslopes where ponding is not possible. Later, as the embank-ment is constructed, its weight compresses the foundationsoil, causing primary consolidation to take place duringconstruction rather than a sudden and possibly catastrophicfoundation collapse when the reservoir is filled for the firsttime.

(4) Loose granular soils. Loose, water-saturated sandsand silts of low plasticity may have adequate shear strengthunder static loading conditions; however, if such materialsare subjected to vibratory loading, they may lose strength tothe point where they flow like a fluid. The process inwhich susceptible soils become unstable and flow whenshocked by vibratory loading is called liquefaction, and itcan be produced by vibration from blasting operations,earthquakes, or reciprocating machinery. In very loose andunstable deposits, liquefaction can occur as the result ofdisturbances so small that they are unidentifiable. Loose siltand sand deposits have been compacted by blasting(Layman 1942), vibroflotation, and driving compactionpiles; however, the effectiveness of these procedures fordeposit densification is not predictable. Vibroflotation hasbeen successfully used in treating limited areas, but it isvery expensive. Blasting is generally not effective indensifying loose granular deposits because the vibratoryenergy produced is of such high frequency.

(5) Steep abutment slopes. Steep abutment slopes ofearth tend to increase the possibility of transverse cracksdeveloping in the embankment after construction. Duringconstruction, they may become unstable and endangerconstruction personnel. Slides can occur in clays, sands,and gravel, particularly in slopes subjected to seepage.Slides may damage completed works and require costlyrepairs. In many cases, it may be necessary to bench theslopes to provide safety against sloughing material andsliding. Frequent inspection should be made by the residentgeologist or other experienced personnel to determinewhether flattening of specified slopes is required.

(6) Old river channels. Old abandoned river channelsfilled with pervious or impervious materials are often

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encountered unexpectedly during construction. As men-tioned earlier, the extent of these deposits is oftenimpossible to establish accurately during the exploratorystages, and in some cases an entire deposit may be missed.Old river channels beneath a dam foundation, filled withcourse-grained pervious material, would constitute adangerous open path of seepage. Channel fillings of softfine-grained materials can cause differential settlements andcracking of the embankment if not removed and replacedwith properly compacted material. Where the existence ofsuch deposits has been revealed, additional exploration byborings, test pits, etc., to establish their extent may benecessary. The design engineer can then decide whatmeasures will have to be taken to modify the design or toremove the deposit. An old river channel found duringfoundation excavation for the core trench at Fall CreekDam, Fall Creek, Oregon is shown in Figure 3-4.

Figure 3-4. Old River Channel in foundation of FallCreek Dam, Oregon

b. Unfavorable rock conditions.

(1) Weathered rock. Weathered rock may haveundesirable characteristics such as high compressibility, lowstrength, and high permeability. Removal of weathered rockis generally required for embankments founded on rock toobtain impervious contact beneath the core and to eliminatethe possibility of differential settlements and low shearstrengths beneath the core and other zones. The weatheringof rock is a transitional process; a sharp line of demarcationdoes not exist between weathered and unweathered zones,and the degree of weathering usually decreases with depth.It may be necessary to excavate deeper in some areas thanin others to remove weathered rock. The elevation to solidrock sometimes shown on plans can be used only as aguide. Excavation in abutment and foundation areas beingstripped should be inspected by a geologist as workprogresses to determine when solid rock is reached.

(2) Open joints and fractures. All open joints, cracks,fissures, and fractures in the foundation rock surface mustbe filled to prevent erosion or scour of embankment materialat the rock contact. A sand-cement mortar is generally usedto fill these openings. The mortar is worked into thefractures using a stiff broom, taking care to prevent theaccumulation of mortar on unfractured surfaces, where itwould not be needed in any event and might be harmful ifit cracked or broke off during rolling of embankment fill.The water-to-solids ratio of the mortar should be varied asrequired to accommodate the conditions encountered. If therock is closely fractured with fine cracks, the water contentmay be increased and a fine sand used to permit easy entryof the mortar into the minute seams. If wide, deep cracksare present, a stiffer mortar with coarser sand should beemployed to reduce the extent of shrinkage cracking (in thecured mortar). A rock surface after mortar treatment isshown in Figure 3-5.

Figure 3-5. Mortar-treated rock surface

(3) Cavities and solution features.

(a) Cavities, potholes, and other voids caused by solutionof the rock are dangerous, and field personnel should alwaysbe on the lookout for such conditions during foundationpreparation. Personnel should be especially alert where adam is being built on rock susceptible to solution, such aslimestone or gypsum. Potholes and cavities exposed ordaylighted on the foundation surface are usually remediedby dental treatment. Concrete should be thoroughly vibratedor rodded into the voids and its upper surface brought up tothe general level of the surrounding rock. Dental treatmentserves to smooth up the foundation to reduce compactiondifficulties as well as provide a nonerodable impervious sealas a measure of protection against scour of the embankmentfill along the rock contact. Figure 3-6 shows a solutionchannel located directly under the center line of

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Mississinewa Dam, Indiana, discovered during excavation

Figure 3-6. Solution channel, Mississinewa Dam,Indiana

for the cutoff trench. After extensive exploration to trace itslimits, the channel was backfilled with concrete.

(b) The presence of daylighted cavities on the foun-dation surface indicates the possibility of buried cavities oreven caverns below grade. Cavities left untreated are highlydangerous, as emphasized by past experience; there havebeen cases where leakage through underground cavities wasso great that it was impossible to fill the reservoir and thedams were eventually abandoned. Hence, any indication ofunderground cavities (such as sink holes, disappearance ofsurface water, etc.) should be reported so that furtherexploration may be undertaken if required. If an extensivenetwork of solution cavities is found, extensive groutingmay be required to ensure the imperviousness of thefoundation.

(4) Overhangs and surface depressions. Overhangs andother irregularities in the rock surface of an abutment orfoundation must be corrected. An abutment overhang atDeGray Dam, Arkansas, is shown in Figure 3-7. Overhangsshould be removed by drilling and blasting with care so asnot to disturb the adjacent sound rock. Line drilling andblasting (or blasting with presplitting) have been usedsuccessfully to form a relatively smooth surface (EM 1110-2-3800). Presplitting has generally given better results thanline drilling for a variety of rock types. Figure 3-8 showsthe left abutment at J. Percy Priest Dam, Tennessee, formedby the presplitting method. Concrete dental treatment canbe used to fill depressions created by blasting and to remedysome types of overhangs. An example of the use of dentalconcrete to eliminate an abutment overhang is shown inFigure 3-9. Tamping of soil under overhangs instead ofremoval or dental treatment must not be permitted. Surface

Figure 3-7. Abutment overhang, DeGray Dam,Arkansas

depressions are generally filled with select impervious

Figure 3-8. Presplit abutment, J. Percy Priest Dam,Tennessee

borrow using heavy mechanical hand tampers to even up thefoundation surface in preparation for the first lift of materialto be compacted by heavy roller equipment. If the rock isvery irregular, it may be more economical to cover theentire area with a concrete slab. It should be noted that agently undulating rock surface is desirable, and only whensurface depressions interfere with compaction of the first liftshould concrete backfilling be required.

(5) Springs. Springs, often encountered in rock foun-dations and abutments, are simply groundwater sources

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seeping to the ground surface driven by artesian pressure.

Figure 3-9. Abutment overhang after concreting

Attempts to place impervious fill over springs issuing fromjoints or rock fractures will result in extremely wet soil inthe vicinity of the spring which is impossible to properlycompact. Depending on the flow rate and pressure drivingthe spring, seepage will continue through the wet soil,creating an uncompacted weak zone of increasing size if fillplacement is continued without properly removing thissource of water. The zone created around an improperlycontrolled spring is a very dangerous situation which willcause problems both during construction and over the life ofthe embankment. Where the water is under a low head andhas a single point of issue, a standpipe can usually beinstalled. A corrugated metal pipe of a diameter dependingupon the size of the spring is placed over the spring area,and a damp mixture of quick-setting cement, sand, andgravel is packed around the standpipe base. Earth is thencompacted around the outside of the pipe at the base. Thewater is kept pumped down within the standpipe until animpervious seal is obtained and enough pipe sections havebeen added to retain the head of water in the pipe. Thepipe is then filled with vibrated concrete or grout, andconstruction of the fill continued upwards and across the topof the plugged pipe by conventional methods. The area isthen examined for evidence of new springs, which oftenappear after an old spring is plugged. This procedure canalso be used for springs on the abutment when the fillreaches the same elevation as the spring. While fillingoperations are progressing below the spring, a small pipecan be grouted into the source of seepage and dischargedaway from the fill as a temporary measure. This procedurewas used at Green River Dam, Kentucky, to eliminateseepage from the abutment spring shown in Figure 3-10. Ifthe springs are not fully localized in area, more extensivemethods as described in paragraph 3-5 may be required.

3-5. Dewatering and Drainage of Excavated

Figure 3-10. Seepage from spring in abutment atGreen River Dam, Kentucky

Areas

Inadequate control of groundwater seepage and surfacedrainage during construction can cause major problems inmaintaining excavated slopes and foundation surfaces and incompacting fill on the foundation and adjacent to abutmentslopes. Plans and specifications seldom contain detailedprocedures for dewatering and other drainage control mea-sures during construction, and the contractor is responsiblefor dewatering systems adequate to control seepage andhydrostatic uplift in excavations, and for collection anddisposal of surface drainage and seepage into excavations.Inspections and observations must ensure that dewateringand drainage control systems are installed correctly and arefunctioning properly.

a. Dewatering.

(1) Potential troubles can often be detected in earlystages by visual observation of increased seepage flow,piping of material from the foundation of slopes, develop-ment of soft wet areas, uplift of excavated surfaces, lateralmovement of slopes, or failure of piezometer water levels todrop sufficiently as pumping is continued. Water pumpedfrom dewatering systems must be observed daily at the dis-charge outlet; if the discharge water is muddy or containsfine sand, fines are being pumped from the foundation.This can be observed by obtaining a jar of the water andobserving sediment settling near the bottom. The pumpingof fines from the foundation can cause internal erosionchannels or piping to develop in the embankment structure;if this happens it is crucial that corrective measures be

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taken. Wells or wellpoints from which fines are beingdischarged must be sealed and replaced with wells havingadequate filters. Piezometers should be installed withdewatering systems to monitor drawdown levels in theexcavated area. Piezometers should be read daily and thereadings plotted to enable continuous evaluation. Dailypumping records should also be kept and evaluated todetermine the quantity of water removed by dewateringsystems and sump systems. These records are valuable fordetecting inadequate seepage control and for evaluatingclaims by the contractor of changed conditions with respectto the plans and specifications. A detailed description ofvarious types of dewatering systems, installation procedures,and performance evaluation is given in TM 5-818-5. Asketch of a single-stage wellpoint dewatering system isshown in Figure 3-11a, and a sketch of a multistage

Figure 3-11. Dewatering systems

dewatering system with provisions for drainage of surfacewater is shown in Figure 3-11b. The two-stage wellpointsystem used to dewater the core trench at Carlyle Dam,Illinois, in the St. Louis district is shown in Figure 3-12.

(2) Failure of the dewatering system can result inextremely serious problems, often requiring extensive andexpensive remedial work. In excavations bottoming inimpervious material, unchecked artesian pressure in under-lying pervious strata can cause heaving of the excavation

bottom. If the impervious stratum ruptures under these

Figure 3-12. Core trench at Carlyle Dam, Illinois, withtwo-stage wellpoint dewatering systems on each slope

pressures, boils (violent emission of soil and water) willdevelop, causing the loss of the underlying foundationmaterial and thereby endangering the entire structure.Figure 3-13 shows sand boils that developed at Friars Point,Mississippi, landward of a levee during a high river stage.Similar boils could develop on the bottom of an excavationfrom excessive artesian pressures in the underlying strata.Failure of excavation slopes may also occur because ofexcessive artesian pressures. In order to prevent failure ofthe dewatering system, all power sources should have stand-by gas or diesel-powered pumping or generating equipment,and standby pumps should be available.

Figure 3-13. Sand boils at Friars Point, Mississippi

b. Sumps and ditches.

(1) When an excavation such as a cutoff trench isextended to rock or to an impervious stratum, there willusually be some water seeping into the excavation and/or

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wet spots in the bottom of the excavation, even with deepwells or wellpoint systems in operation. Water seeping intothe excavation from the upstream and downstream slopes ofa long cutoff trench can usually be captured by excavatingnarrow longitudinal ditches or drainage trenches at theintersection of the slopes and the bottom of the excavation(see Figure 3-11b), or by forming such trenches with sand-bags, with sumps located as necessary for pumping thewater out. If the bottom of the excavation will still not dryout, smaller ditches can be cut through the problem areasand sloped to drain to the side trenches.

(2) To keep the bottom of the cutoff trench dry whileplacing backfill, the drainage ditch system can be filled withgravel so that the system can continue to function even afterbeing covered with soil. These gravel-filled ditches shouldconstitute only a very small portion of the overall area beingdrained to preclude the necessity of later grouting largeportions of the bottom of the cutoff trench containing gravel.The surface of the gravel can be covered with a layer ofheavy felt paper, burlap, or plastic sheeting, or by a layer ofstiff concrete to prevent migration of fines from the fillmaterial. The ditches are blocked at the ends of the excava-tion by means of concrete plugs. Riser pipes are brought upfrom each sump (low point in the ditch) with the result thatwater can be pumped from one or more of the riser pipes asnecessary, with the remaining riser pipes serving as ventpipes.

(3) After the backfill is brought to a height that willcounteract the hydraulic head, the gravel-filled ditches aregrouted. Cement grout is introduced under gravity throughone riser pipe with the vent pipes serving as an escape forair and water in the gravel. When grout issues from thevent pipes, the vent pipes are shut off and a slight pressureis maintained in the system until the grout has set. Aftergrouting, normal fill placement operations are continued. Ifonly one sump is used in the drainage system, a vent pipewill have to be installed at an appropriate location beforebackfilling starts.

(4) Figures 3-14a and 3-14b show the drainage systemsuccessfully used at Laurel Dam, Kentucky, to removelocalized flow cracks in the shale. The system provided adry foundation for impervious material. Figure 3-14a showsa trench dug along a water-producing crack in the shale. Aperforated collector pipe was placed in the trench with avent hose and grout pipe attached. The collector pipe wassurrounded with stone and the remaining portion of thetrench filled with concrete. Water volume from variouscollector pipes discharged into a collection box from whicha single pipe carried the water to a pump located in a 60-in.corrugated metal pipe (CMP) sump (see Figure 3-14b). Thevent hoses, grout, pipes, and CMP were extended as the fillwas brought up. After 20 to 30 ft of embankment fill had

been placed, the collector pipes were grouted and the CMP

Figure 3-14. Foundation drainage system at LaurelDam, Kentucky

was filled with -3-in. stone and grouted.

(5) In many cases where a dewatering system is beingused, a 4- to 5-ft-high impervious blanket placed at the toeof the slope will prevent the minor seepage flow thatotherwise might occur and will therefore provide a drybottom.

c. Surface erosion. Surface erosion may presentproblems on slopes cut in silts, fine sands, and lean clays.

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Eroded material will wash down and fill in the excavationbelow the slope. The slope itself will be left deeply scouredand rutted, making it necessary for costly smoothingoperations to be performed before the fill can be placedagainst it. Figure 3-15 shows surface erosion on anunprotected excavation at Kaskaskia Dam, Illinois. The bestway to combat surface erosion of temporary excavationslopes is to backfill as soon as possible, thus cutting downon exposure time. This often cannot be done, however, andit becomes necessary to take other measures. Grass coveron the slopes is a good means of preventing surface erosionif it can be readily established and if the slopes are toremain open for a season or two. Other slope protectionmeasures such as rip-rap or asphaltic treatment are rarelyjustified for construction slopes. Thus, it is necessary tokeep as much water off the slope as possible. Most slopescan withstand rain falling directly on them with only minorsloughing. Perimeter ditches and/or dikes (see Fig-ure 3-11b) at the top of the slope are needed to carry othersurface waters away from the excavation if surface watersoutside the excavation would otherwise run into it. Ditchesmay be needed at several elevations along the excavationslopes to catch surface waters, as shown in Figure 3-11b.

d. Other seepage control measures. Other means of

Figure 3-15. Surface erosion of unprotectedconstruction slope of Kaskaskia Dam, Illinois

stabilizing excavation slopes and preventing seepage fromentering an excavation (such as electro-osmosis, freezing,sheet-piling, and grouting) have been used for structureexcavations. These methods are not economically feasiblefor extensive foundation excavations for dams but might beused in structures where conventional dewatering methodsare not suitable for various reasons.

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Chapter 4Borrow Areas and Quarries

Section IEarth Fill

4-1. Excavation, Handling, and HaulingEquipment

Over the past several decades, significant improvements inearth-moving equipment have been made at an increasingrate, and there is no indication that this trend will slow.While the basic types of equipment have remained virtuallythe same, speed, power, and capacity have continuouslyincreased. Some of the basic principles of the morecommon units are discussed in the following paragraphs.

a. Excavation equipment.

(1) Power shovels, draglines, elevating graders, wheelexcavators, and scrapers. Excavation is usually accom-plished with power shovels, draglines, scrapers, wheelexcavators, or side-delivery loaders. Each offers certainadvantages and has certain disadvantages; therefore, severaltypes are often used on the same job. Discussion of thefour major types of excavation units is given in Figures 4-1through 4-3.

(2) Dredges. Dredging is sometimes employed to movematerial from borrow areas to the damsite. Dredges areparticularly suitable for use when large quantities of materialare to be obtained from borrow areas submerged in rivers,lakes, etc. The two basic types of dredges are the bucketdredge and the hydraulic dredge.

(a) Bucket dredge. A limiting disadvantage of bucketdredges is that the discharge is alongside the place ofexcavation. They can best be used for localized dredging orwhere the borrow area is located so that the material can beeconomically transported by trucks or barges to the site.There are three types of bucket dredges: grab dredges,dipper dredges, and ladder dredges. The grab dredge isessentially a grab bucket operated from a derrick mountedon a flat-topped barge. The dipper dredge is simply apower shovel operating from a barge. The ladder dredgeexcavates with a continuous chain of buckets supported onan inclined ladder. Bucket dredges have the advantage thatthey can excavate in most any material. Dredging depthsgreater than 100 ft are not uncommon for grab dredges.Digging depths of the dipper dredge are limited by thelength of the boom (65 ft is about the maximum, althoughgreater lengths are available for special projects) while thedigging depth of the ladder dredge is limited by the length

of the ladder (usually around 40 ft, but lengths greater than75 ft are not uncommon).

(b) Hydraulic dredges. All hydraulic dredges basicallyconsist of a centrifugal pump and a suction line throughwhich the pump is supplied with material, but differentmeans are used to loosen and pick up material. Hydraulicdredges can transport material without rehandling for prac-tically unlimited distance if booster pumps are used; thisfeature is particularly useful in the construction of temporarydikes and permanent structures by the hydraulic fill method.However, it must be mentioned that material placed by thehydraulic fill method will be loose, essentially water-saturated, and probably susceptible to liquefaction ifsufficiently disturbed by the mechanisms specified inparagraph 3-4a(4). The most common dredge in current useis the cutterhead dredge, which excavates material with arotating cutterhead then removes it by suction. The cutteris attached to the forward end of a ladder or to the suctionpipe itself; rotation of the cutter agitates soft material and/orcuts hard material. Various types of cutters are available,and the type to be used depends on the hardness of thematerial to be excavated. Some cutters can cut into soundrock. The maximum suction available at the pump is about28 ft of water; therefore, head losses must be kept to aminimum in order to provide adequate suction.

b. Hauling, handling, and separating equipment.Borrow material excavated by scrapers or by dredging isusually taken directly to its point of deposition on theembankment under construction. However, when most otherexcavation procedures and equipment are used, additionalhauling and handling are required. Regardless of theequipment used, borrow material may require sorting and/orseparation according to size or material type. Some of theequipment used for hauling, handling, and separating borrowmaterials is discussed in the following paragraphs.

(1) Trucks. Trucks of many types are used to transportembankment material from the borrow pit to the dam,capacities of up to 50 tons being common. They can becategorized according to method of unloading as bottomdump, end dump, or side dump.

(2) Belt conveyors. Belt conveyors (Figure 4-4) areused to transport material from the borrow area to thedamsite when very large quantities of material must bemoved under difficult conditions. Belt conveyors are mostsuitable for moving large quantities of material over roughterrain where there are large differences in elevationbetween the borrow pit and the dam, and where the cost ofbuilding and maintaining haul roads would be high. Con-veyor systems are adaptable to different types of automatedprocessing procedures: screening plants, crushing plants,

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blending operations, water additions, etc. Transfer points

Figure 4-4. Belt conveyor system Figure 4-5. Vibrating screen separating plant

(where the material is transferred from one belt to another)are usually required. Automatic facilities for loading trucksat the terminal points can be easily provided, or sometimesthe material can be dumped directly from the belt onto theembankment, spread with bulldozers or grader, andcompacted.

(3) Separation plants. Separation or screening plants(Figure 4-5) are employed where it is desired to separatedifferent particle sizes of a granular material. Genera