DEPARTMENT OF THE ARMY ETL 1110-1-163U.S. Army Corps of Engineers

CEMP-RT Washington, DC 20314-1000

Technical LetterNo. 1110-1-163 30 June 1996

Engineering and DesignCHECKLIST FOR DESIGN OF VERTICAL BARRIER

WALLS FOR HAZARDOUS WASTE SITES

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Title and Subtitle Engineering and Design: Checklist for Design of VerticalBarrier Walls for Hazardous Waste Sites

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DEPARTMENT OF THE ARMY ETL 1110-1-163U.S. Army Corps of Engineers

CEMP-RT Washington, DC 20314-1000

Technical LetterNo. 1110-1-163 30 June 1996

Engineering and DesignCHECKLIST FOR DESIGN OF VERTICAL BARRIER

WALLS FOR HAZARDOUS WASTE SITES

1. Purpose

a. This engineer technical letter transmits adocument for the design of vertical barrier walls forhazardous waste sites. The document was developedto aid in project planning, scheduling, and budgeting;scoping predesign investigations and architect/engineer (A/E) services; as a guidance document fordesign of vertical barrier walls; for use in reviewingA/E products; and as a source for technicalreferences.

2. Applicability

This letter applies to HQUSACE elements andUSACE commands having hazardous, toxic, andradioactive waste (HTRW) investigation, design, andremedial action responsibility.

3. References

References are listed in Appendix A.

4. Discussion

The document will focus on slurry walls; however,other types of vertical barrier walls will also be dis-cussed. The document is divided into two sections, anarrative section and a checklist. The narrative sec-tion briefly discusses design aspects for verticalbarrier walls and provides design references. Thechecklist section contains a list of questions coveringpertinent aspects of design that were discussed in thenarrative section.

5. Actions Required

The topics listed in this checklist are to be consideredin the design of vertical barrier walls for hazardouswaste sites. It is strongly recommended that input besought from the appropriate technical staff for allphases of scoping, design, and construction of verticalbarrier walls. The involvement of in-house technicalexpertise is essential to providing a cost-effective,high-quality service to the customer.

FOR THE DIRECTOR OF MILITARY PROGRAMS:

3 AppendicesAPP A - References Chief, Environmental Restoration DivisionAPP B - Vertical Barrier Wall Directorate of Military Programs

Design ConsiderationsAPP C - Vertical Barrier Wall

Design Checklist

ETL 1110-1-16330 Jun 96

APPENDIX A: REFERENCES

A-1. Required References

a. Code of Federal Regulations.

40 CFR 26040 CFR 260, Hazardous Waste Management System:General.

40 CFR 26140 CFR 261, Identification and Listing of HazardousWaste.

40 CFR 26240 CFR 262, Standards Applicable to Generators ofHazardous Waste.

40 CFR 26340 CFR 263, Standards Applicable to Transporters ofHazardous Waste.

40 CFR 26440 CFR 264, Standards for Owners and Operators ofHazardous Waste Treatment, Storage, and DisposalFacilities.

40 CFR 26540 CFR 265, Interim Status Standards for Ownersand Operators of Hazardous Waste Treatment, Stor-age, and Disposal Facilities.

40 CFR 26640 CFR 266, Standards for the Management of Spe-cific Hazardous Wastes and Specific Types of Haz-ardous Waste.

40 CFR 26740 CFR 267, Interim Standards for Owners and Oper-ators of New Hazardous Waste Land DisposalFacilities.

40 CFR 26840 CFR 268, Land Disposal Restrictions.

49 CFR 100-18049 CFR 100-180, DOT Hazardous Materials, Sub-stances, and Waste Regulations.

b. Engineer regulations.

ER 385-1-92ER 385-1-92, Safety and Occupational Health Docu-ment Requirements for Hazardous, Toxic and Radio-active Waste (HTRW) and Ordnance and ExplosiveWastes.

ER 1110-1-263ER 1110-1-263, Chemical Data Quality Managementfor Hazardous Waste Remedial Activities.

c. Predesign investigation requirements.

EM 1110-1-4000EM 1110-1-4000, Monitor Well Design, Installation,and Documentation at Hazardous and/or Toxic WasteSites.

ETL 1110-2-282ETL 1110-2-282, Rock Mass Classification DataRequirements for Rippability.

EPA/530/R-93/001EPA/530/R-93/001, RCRA Ground-Water Monitor-ing: Technical Enforcement Guidance Document.

EPA 600/4-89/034EPA 600/4-89/034, Handbook of Suggested Practicesfor the Design and Installation of Ground-WaterMonitoring Wells.

d. Slurry walls.

EPA/540/2-84-001EPA/540/2-84-001, Slurry Trench Construction forPollution Migration Control. Municipal Environmen-tal Research Laboratory, U.S. Environmental Protec-tion Agency.

Xanthakos 1979Xanthakos, P. P. 1979.Slurry Walls. McGraw-HillBook Company, New York.

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Xanthakos 1994Xanthakos, P. P. 1994.Slurry Walls as StructuralSystems, 2nd ed. McGraw-Hill Book Company, NewYork.

Paul, Davidson, and Cavalli 1992Paul, D. B., Davidson, R. R., and Cavalli, N. J.1992. “Slurry Walls: Design, Construction, andQuality Control,” STP 1129, American Society forTesting and Materials, Philadelphia, PA.

e. Quality assurance/quality control.

EPA/600/R-93/182EPA/600/R-93/182, Quality Assurance and QualityControl for Waste Containment Facilities.

f. Corps of Engineers guide specifications.

CEGS-01110CEGS-01110, Safety, Health and EmergencyResponse.

CEGS-01300CEGS-01300, Submittals Descriptions.

CEGS-01305CEGS-01305, Submittals Procedure.

CEGS-01440CEGS-01440, Contractors Quality Control.

CEGS-01450CEGS-01450, Contractors Chemical Quality Control.

CEGS-02110CEGS-02110, Clearing and Grubbing.

CEGS-02210CEGS-02210, Grading.

CWGS-02214CWGS-02214, Soil-Bentonite Slurry Trench Cutoffs.

CEGS-02222CEGS-02222, Excavation, Trenching and Backfillingfor Utilities.

CEGS-02233CEGS-02233, Graded-Crushed-Aggregate BaseCourse.

CEGS-02234CEGS-02234, Subbase Course.

CEGS-02272CEGS-02272, Separation/Filtration Geotextile.

CWGS-02411CWGS-02411, Metal Sheet Piling.

CEGS-02444CEGS-02444, Soil-Bentonite Slurry Trench forHTRW Projects.

CEGS-02546CEGS-02546, Aggregate Surface Course.

CEGS-02671CEGS-02671, Ground-Water Monitoring Wells.

CEGS-02720CEGS-02720, Storm Drainage System.

CEGS-02831CEGS-02831, Chain-Link Fence.

CEGS-02935CEGS-02935, Turf.

CWGS-03363CWGS-03363, Concrete Cutoff Wall.

CWGS-03365CWGS-03365, Concrete for Concrete Cutoff Walls.

g. American Society for Testing and Materials.

ASTM C 143ASTM C 143, Test Method for the Slump of PortlandCement Concrete.

ASTM D 422ASTM D 422, Method for Particle-Size Analysis ofSoils.

ASTM D 512ASTM D 512, Test Methods for Chloride Ion inWater.

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ASTM D 698ASTM D 698, Laboratory Compaction Characteristicsof Soil Using Standard Effort (12,400 ft-lbf/ft3

(600 kN-m/m3)).

ASTM D 1557ASTM D 1557, Laboratory Compaction Characteris-tics of Soil Using Modified Effort (56,000 ft-lbf/ft3

(2,700 kN-m/m3)).

ASTM D 2216ASTM D 2216, Method for Laboratory Determinationof Water (Moisture) Content of Soil, Rock, and Soil-Aggregate Mixtures.

ASTM D 2434ASTM D 2434, Test Method for Permeability ofGranular Soils (Constant Head).

ASTM D 2487ASTM D 2487, Classification of Soils for Engineer-ing Purposes (Unified Soil Classification System).

ASTM D 4318ASTM D 4318, Test Method for Liquid Limit, PlasticLimit, and Plasticity Index of Soils.

ASTM D 5084ASTM D 5084, Measurement of Hydraulic Conduc-tivity of Saturated Porous Materials Using a FlexibleWall Permeameter.

ASTM D 5092-90ASTM D 5092-90, Recommended Practice for Designand Installation of Ground-Water Monitoring Wells inAquifers.

ASTM D 5299ASTM D 5299, Guide for the Decommissioning ofGround Water Wells, Vadose Zone MonitoringDevices, Boreholes, and Other Devices for Environ-mental Activities.

h. Miscellaneous.

API 13AAmerican Petroleum Institute (API) Specification13A, Drilling Fluid Materials.

API RP 13B-1API RP 13B-1, Field Testing Water-Based DrillingFluids.

EM 1110-1-3500EM 1110-1-3500, Chemical Grouting.

EM 1110-2-2504EM 1110-2-2504, Design of Sheet Pile Walls.

EM 1110-2-3506EM 1110-2-3506, Grouting Technology.

EP 200-1-2EP 200-1-2, Process and Procedures for RCRAManifesting.

TM 5-822-12TM 5-822-12, Design of Aggregate Surfaced Roadsand Airfields.

USP-NF-XVII United States Pharmacopeia.

A-2. Related References

a. Predesign investigation requirements.

EM 200-1-3EM 200-1-3, Requirements for the Preparation ofSampling and Analysis Plans.

EM 1110-1-1802EM 1110-1-1802, Geophysical Exploration for Engi-neering and Environmental Investigations.

EM 1110-1-1804EM 1110-1-1804, Geotechnical Investigations.

EM 1110-2-1907EM 1110-2-1907, Soil Sampling.

EPA/330/9-81/002EPA/330/9-81/002, Manual forGroundwater/Subsurface Investigations at HazardousWaste Sites.

EPA/530/SW-611EPA/530/SW-611, Procedures Manual for GroundWater Monitoring at Solid Waste Disposal Facilities.

EPA 530/SW-846EPA 530/SW-846, Test Methods for Evaluating SolidWaste.

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EPA 600/4-84/076EPA 600/4-84/076, Characterization of HazardousWaste Sites-A Methods Manual: Volume I, SiteInvestigations.

EPA 600/4-91/029EPA 600/4-91/029, Guide to Site and Soil Descrip-tion for Hazardous Waste Sites.

EPA 600/7-84/007EPA 600/7-84/007, Geophysical Techniques for Sens-ing Buried Wastes and Waste Migration.

EPA 600/8-89/046EPA 600/8-89/046, Soil Sampling Quality AssuranceUsers Guide.

EPA 625/4-91/026EPA 625/4-91/026, Site Characterization for Subsur-face Remediation.

EPA 625/6-90/016bEPA 625/6-90/016b, Handbook-Ground Water Vol. I,EPA 625/6-87/016 and Vol. II: Methodology.

EPA 625/12-91/002EPA 625/12-91/002, Description and Sampling ofContaminated Soils: A Field Pocket Guide.

EPA 625/R-92/007EPA 625/R-92/007, Use of Airborne, Surface, andBorehole Geophysical Techniques at ContaminatedSites.

EPA 625/R-93/003EPA 625/R-93/003, Subsurface Characterization andMonitoring Techniques: A Desk Reference Guide.

b. Vertical barrier walls.

Ayres, Lager, and Barvenik 1983Ayres, J. E., Lager, D. C., and Barvenik, M. J. 1983.“Design of Soil-Bentonite Backfill Mix for the FirstEnvironmental Protection Agency Superfund CutoffWall,” Proceedings of the Fourth National Sympo-sium on Aquifer Restoration and GroundwaterMonitoring.

D’Appolonia 1980D’Appolonia, D. J. 1980. “Soil-Bentonite SlurryTrench Cutoffs,”Journal of the Geotechnical Engi-neering Division, American Society of Civil Engi-neers, Vol. 106, No. GT4, 399-417.

EPA-600/2-87/063EPA-600/2-87/063, Investigation of Slurry Cut-OffWall Design and Construction Methods for Contain-ing Hazardous Waste.

Gallinatti and Warner 1994Gallinatti, John D., and Warner, Scott D. 1994.“Hydraulic Design Considerations for Permeable InSitu Ground Water Treatment Walls,” abs.,GroundWater, Vol. 32, 851.

Gillham and O'Hannesin 1994Gillham, Robert W., and O'Hannesin, Stephanie.1994. “Enhanced Degradation of Halogenated Ali-phatics by Zero-valent Iron,”Ground Water, Vol. 32,958-967.

Johnson et al. 1985Johnson, A. I., Frobel, R. K., Cavalli, N. J., andPettersson, C. B. 1985. “Hydraulic Barriers in Soiland Rock,” STP 874, American Society for Testingand Materials, Philadelphia, PA.

Leach, Saugier, and Carter 1995Leach, R. E., Saugier, R. K., and Carter, E. E. 1995.“Falling Beam SoilSawTM, An Advanced Process forForming Underground Cutoff Walls,” TechnicalReport CPAR-GL-95-1, U.S. Army Engineer Water-ways Experiment Station, Vicksburg, MS.

Millet and Perez 1981Millet, R. A., and Perez, J. 1981. “Current USAPractice: Slurry Wall Specifications,”Journal of theGeotechnical Engineering Division, American Societyof Civil Engineers, Vol. 107, No. GT8, 1041-1056.

Rumer and Ryan 1995Rumer, R. R., and Ryan, M. E. 1995.Barrier Con-tainment Technologies for Environmental RemediationApplications. John Wiley & Sons, New York.

Zappi, Shafer, and Adrian 1990Zappi, M. E., Shafer, R. A., and Adrian, D. D. 1990.“Compatibility of Ninth Avenue Superfund SiteGround Water with Two Soil-Bentonite Slurry WallBackfill Mixtures,” Miscellaneous Paper EL-90-9,U.S. Army Engineer Waterways Experiment Station,Vicksburg, MS.

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c. Geotechnical design.

Daniel 1993Daniel, D. E. 1993.Geotechnical Practice for WasteDisposal, Chapman and Hall.

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APPENDIX B: VERTICAL BARRIER WALL DESIGN CONSIDERATIONSFOR HAZARDOUS WASTE CONTAINMENT APPLICATIONS

B-1. General

a. Intended use of document. This documentfor vertical barrier wall design was developed to aidin project planning, scheduling, and budgeting; scop-ing predesign investigations and architect/engineer(A/E) services; as a guidance document for conduct-ing vertical barrier wall designs; reviewing A/E pro-ducts; and as a source for technical references. Thedocument is divided into three appendices: Appen-dix A, References; Appendix B, Design Consider-ations; and Appendix C, Checklist. Appendix Bbriefly discusses design aspects for vertical barrierwall components. Appendix C contains a list ofquestions covering pertinent aspects of design thatwere discussed in Appendix B.

b. Purpose and function of vertical barriers.The primary purpose of a vertical barrier wall is toisolate hazardous waste contamination by minimizingthe movement of groundwater from a contaminatedsite to uncontaminated areas or by preventing cleangroundwater from entering a contaminated area.Vertical barrier walls function as a relatively imper-meable barrier which either contains or redirectsgroundwater flow.

c. Types of vertical barriers. The type of verti-cal barrier wall chosen for use at a hazardous wastesite is dependent on environmental and site-specificparameters. No Environmental Protection Agency(EPA) regulations exist for the design and construc-tion of vertical barrier walls. The type of verticalbarrier wall most commonly used at hazardous wastesites is the soil-bentonite (S-B) slurry wall. EPA-540/2-84-001 provides guidance on the design andconstruction of S-B slurry walls. This document willfocus on design issues for S-B slurry walls; however,several other types of walls will also be discussed.

d. Project team. An idealized design/reviewteam for a vertical barrier wall project is providedbelow. For most projects, the design or review teamwill not include individuals from each of the noteddisciplines. Consequently, it is important that allaspects of design are assigned to a member of theteam. Team members include:

• Customer.

• Project manager.

• Technical manager.

• Project engineer.

• Geotechnical engineer.

• Geologist.

• Mechanical engineer.

• Electrical engineer.

• Hydrologist.

• Environmental engineer.

• Civil engineer.

• Industrial hygienist.

• Chemist.

• Land surveyor.

• Landscape architect.

• Drafter/computer-aided design and drafting(CADD) technician.

• Specifications writer.

• Cost estimator.

• Regulatory specialist.

• Real estate specialist.

• Construction representative.

• Local, state, and Federal regulatory staff.

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B-2. Predesign Investigations

a. General. Prior to preparing a design analysisor plans and specifications for any type of verticalbarrier wall, it is necessary to conduct predesignsurveys and investigations to fill data gaps. Theexisting database available from the remedial investi-gation (RI), feasibility study (FS), and other docu-ments must be reviewed before scoping a predesigneffort. The following information is often required inthe design of a vertical barrier wall.

b. Field surveys and record searches.

(1) Aerial photography. Historic aerial photo-graphs can be used to preliminarily define the natureand extent of contamination at a site. The principalsources of aerial photographs are the U.S. Departmentof Agriculture (USDA) Agricultural Stabilization andConservation Service (ASCS) and the U.S. Geologi-cal Survey (USGS).

(2) Design and operational data. Design andoperational information such as as-built drawings,specifications, and design analyses may help in iden-tifying the nature and extent of contamination.However, documents such as these are rarely avail-able for hazardous waste sites.

(3) Map database. The USGS, USDA, and othergovernment agencies produce topographic, soils,groundwater, and other mapping that may be usefulin the design of a vertical barrier wall.

(4) Topographic surveys. A current topographicsurvey of the site is required. To allow for manipula-tion of data and to expedite the design process, thetopographic survey should be mapped on a computeraided design and drafting (CADD) system. Ideally,topographic mapping will have 300 mm (1 ft) contourintervals. However, larger contour intervals may beacceptable depending on site-specific conditions (e.g.,time, budget, and topography). Topographic mappingshould be accurate to within ± 30.0 mm (± 0.1 ft) inthe vertical and horizontal directions. Elevations ofpiezometers, monitoring wells, or other instrumenta-tion should be accurate to ± 3.0 mm (± 0.01 ft) toallow for accurate interpretation of data. All surfacefeatures such as buildings, utilities, ponds, fences,trees, streams, ditches, and exploratory borings andtrenches should be delineated on the mapping.

(5) Horizontal and vertical control. At a mini-mum, three permanent control monuments need to beestablished. The monuments should be strategicallylocated so that they are not damaged during construc-tion. All monuments should be assigned state planecoordinates and/or tied into the horizontal grid usedin previous studies. The vertical datum should bemean sea level, North American Datum of 1983.

(6) Monitoring baseline survey. To monitordesign concerns, it is necessary to perform surveys toestablish a baseline for monitoring wells, piezometers,and other instrumentation.

(7) Utilities. All onsite above and below groundutilities should be identified, located, and subse-quently shown on the project drawings. A utilitysearch should consist of an onsite inspection, reviewof as-built drawings, and contacts with utility com-panies. The project drawings should show the loca-tion of onsite utilities including horizontal alignment,depth or height, type, and size.

(8) Boundary survey and property search. Aboundary survey should be performed for all proper-ties or parcels within project construction or accesslimits. The boundary survey should be tied to thesite’s horizontal control. A property search shouldalso be performed to identify property owners of allaffected and adjacent parcels of land. Prior to anyinvestigation or construction activity, it is essential toobtain construction easements and project rights-of-way. This may take 12 to 18 months; therefore,coordination with real estate specialists should beginas soon as possible.

c. Geological investigations. After the existingdatabase has been reviewed, geological investigationscan be scoped. The following items need to beinvestigated in order to design a vertical barrier wallsystem.

(1) Limits of waste. The limits of the contami-nated area the vertical barrier wall will surround needto be defined. Depending on the type and composi-tion of the waste, the limits can be tentativelydefined by geophysical methods such as electromag-netic conductivity surveys and soil gas surveys.Intrusive methods such as test pits, borings, and mon-itoring wells will also need to be used to verify theboundaries of the contaminated area. All test pits and

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borings should be logged by a qualified geologist orgeotechnical engineer. Surveys should also beperformed to determine the exact location of anygeotechnical investigations used to define the bound-ary of the contaminated area.

(2) Site geology. The subsurface geologic con-ditions must be determined and understood prior todesign and construction of a vertical barrier wall.Some of the geologic data which may be required fordesign include detailed site stratigraphy, soil or rocktype, grain size distribution, Atterberg limits, moisturecontent, chemical properties of the aquifer materials,degree of weathering, structural discontinuities, rockhardness, and rippability.

(3) Hydrology. In addition to understanding thegeology, it is imperative to have an understanding ofthe site groundwater conditions to define pollutionmigration paths. The types of hydrologic informationtypically required for design include the following:location of the water table, recharge and dischargezones, hydraulic head distribution, hydraulic conduc-tivity, porosity, extent of geologic units, contaminantspresent in the groundwater, and background waterquality. After data gathering has been accomplished,potentiometric surface maps, geologic cross-sectionswith water table elevations, and the depth and extentof the slurry wall can be determined. A groundwatermodel can be used to evaluate and simulate futuregroundwater conditions, and several vertical barrierwall alignments can then be evaluated by the model.

d. Chemical data requirements. Chemical test-ing is often required for the features listed below. Achemist should be involved in these aspects of theproject.

(1) Leachate and groundwater testing.

(2) Determination of limits of waste.

(3) Contaminated materials handling.

(4) Compatibility testing of vertical barrier wallmaterials and site contaminants.

(5) Borrow soil testing (contamination check).

B-3. S-B Slurry Walls

a. History and background. A S-B slurry wall(Figure 1), constructed by the slurry trenchingtechnique, is a subsurface barrier made to impede orredirect the flow of groundwater. This technique waspioneered in the United States in the mid-1940’susing technology developed by the oil industry.Slurry wall construction is a versatile technique thathas been used extensively for cutoff walls in damsand levees, and is very successful in controlling pol-lutants, contaminated groundwater, and landfillleachate migrating from waste sites. Because theyhave been so successful, the use of slurry walls haslargely replaced the use of traditional cutoff barrierssuch as steel sheet pile walls and grout curtain wallsat hazardous waste sites.

Figure 1. Slurry wall schematic

b. General construction. A S-B slurry wall isconstructed by excavating a narrow vertical trench,typically 600 to 1,500 mm (2 to 5 ft) wide, throughpervious soils to a relatively impervious key stratum.During excavation, the trench is filled with slurryconsisting of a bentonite and water mixture. Thetrench is kept full of slurry to prevent the trenchwalls from caving or sloughing. The slurry alsodevelops a filter cake on the walls of the trench thatcontributes to trench stability and to the low perme-ability of the completed cutoff wall. Slurry trencheshave been excavated to depths of more than 30 m(100 ft) with no caving or sloughing of the trench

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walls. Depending on the depth of the trench, typicalexcavation equipment may include extended reachbackhoes, clamshells, or draglines. For depths lessthan about 20 m (70 ft), backhoes are generally mostefficient. After excavation, the slurry filled trench isbackfilled with a soil/bentonite/water mixture engi-neered to create a low-permeability cutoff wall (1 ×10-7 cm/sec to 1 × 10-8 cm/sec). U.S. Army Corpsof Engineers (USACE) guide specificationCEGS 02444, Soil-Bentonite Slurry Trench forHTRW Projects, should be used in the preparation ofplans and specifications for S-B slurry walls.

c. Predesign issues.

(1) Geologic subsurface investigation. A slurrywall should be keyed into an impervious geologicformation along the entire length in order to avoidpotential seepage zones under the wall. Borings aretaken along the proposed alignment of a slurry wall,preferably before the development of plans and speci-fications. These borings are spaced 30 to 60 m (100to 200 ft) apart, depending on the geologic uniformityand thickness of the key layer. Additional boringsmay be taken along the alignment during slurry wallconstruction to verify assumptions made duringdesign. During the design phase, samples can betaken from the slurry wall alignment borings for usein backfill optimization and compatibility studies. Inaddition, groundwater levels should be measuredalong the alignment of the slurry wall, and if possi-ble, seasonal groundwater fluctuations should bedocumented.

(2) Soil-bentonite (S-B) backfill testing andoptimization.

(a) Borrow sources. During predesign investiga-tions, potential borrow sources for backfill materialshould be located and sampled. Soils excavated fromthe trench may be utilized for backfill soil. Thispractice saves the time and money of locating, pur-chasing, and hauling borrow soil to the site. It alsoeliminates the problem of disposing of the excavatedtrench material. If the in situ soils are not suitable,due to problems with gradation or gross contamina-tion, imported borrow is the only viable option. Soilcharacteristics such as classification, gradation, watercontent, permeability, and chemical properties shouldalso be evaluated. If a single borrow area does nothave a suitable soil, it may be necessary to mix twodifferent soils to come up with suitable backfillmaterial.

(b) Backfill soil materials. To obtain a lowpermeability (typically 1 × 10-7 cm/sec or less) S-Bbackfill mixture, soils with an appreciable amount offines (preferably plastic) are necessary. USACEguide specification CEGS 02444 provides the follow-ing gradation criteria for backfill soils:

Screen Size or Number Percent Passing(U.S. Standard) by Dry Weight

75 mm (3 in.) 1004.76 mm (No. 4) 40-800.42 mm (No. 40) 25-60

74 uM (No. 200) 20-40

(c) Bentonite. Bentonite is a natural clay whoseprincipal mineral constituent is sodium montmorillon-ite and is characterized by a very large volumeincrease with wetting. Bentonites which conform toSection 4 of API Spec 13A typically have beentreated with small amounts of polymers. Bentoniteswhich conform to Section 5 of API Spec 13A havenot been chemically treated. Nontreated (Section 5)bentonites are generally more expensive than Section4 bentonites. Bentonites conforming to Section 4may be used for construction, provided the necessarypermeability is obtained during compatibility testing;however, bentonite which conforms to Section 5 ofAPI Spec 13A is generally preferred. During pre-design, several types of bentonite from varioussuppliers should be obtained for compatibility andbackfill optimization testing. USACE guide specifi-cation CEGS-02444 requires the following propertiesfor bentonite used for slurry wall construction:

YP/PV Ratio API Spec 13A <3Viscometer >30Filtrate Loss <15 cm3

MoistureContent ASTM D 2216 <10 percent

(d) Water. During pre-design activities, samplesof the groundwater and samples of tap water to beused for slurry mixing and other operations should beobtained. Generally, 40 liters (10 gallons) of eachshould be obtained for compatibility and backfilloptimization testing. Since a large volume of wateris normally required for any slurry trench installation,an adequate source of mixing water must be identi-fied. The guide specification requires water used formixing slurry and backfill meet the followingrequirements:

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pH API RP 13B-1 6 to 8Hardness <200 ppmTotal dissolved <500 ppm

solidsOil, organics,

acids, alkali, < 50 ppm eachand other

deleterioussubstances

Chloride ASTM D 512 For Record

(e) Bentonite slurry mixture. Once all bentoniteand water samples are obtained, the design of thebentonite slurry mixture may begin. In general, abentonite content of 6 percent by weight should meetmost requirements. The EPA recommends the fol-lowing properties for bentonite slurries: viscosity(measured with a Marsh funnel) greater than 40 sec,unit weight around 1,025 kg/m3 (65 pcf), pH between7 and 10, and a bentonite content of 4 to 8 percent.USACE guide specification CEGS-02444 states thatthe bentonite slurry should meet the following crite-ria:

Viscosity API RP 13B-1 >40 secDensity >1025 kg/m3

(64 pcf)Filtrate loss <20 cm3

pH 6.5 to 10

(f) Initial compatibility testing. The presence ofchemical contaminants in soil and/or groundwatermay significantly alter the rate of water movementthrough a S-B slurry wall. For example, calcium insoil or groundwater will displace some of the sodiumions in bentonite. This results in reduced swelling ofthe bentonite and increased permeability. While theeffects of individual chemicals on S-B slurry wallshave been studied and documented, the effects ofmultiple contaminants, which are common at mostHTRW sites, are largely unknown. The objective ofany compatibility/optimization testing is to determinethe optimum S-B backfill mix design necessary toachieve an in-place permeability of 1 × 10-7 cm/secor less. In addition, the study should determinewhether contaminants present in the soil or ground-water will cause long-term changes to the S-B back-fill. Typically, a compatibility testing program willlast 3 months or longer. Because of the long timeframe involved, it is often preferable to conduct pre-liminary compatibility tests during the predesignphase and long-term compatibility tests concurrentwith design.

A recommended compatibility testing programgenerally consists of the following:

The free swell test measures the increase in volumeof a bentonite sample when poured into water. Freeswell is expressed as a percentage of the original(dry) volume. Two grams (2.2 cm3) of bentonite areslowly poured into 100 ml of water and the volumeof settled solids is recorded after 2 and 24 hr. SeeUnited States Pharmacopeia test method USP-NF-XVII for a test description. Often, two tests areconducted, one using tap water from the site and theother using contaminated groundwater from the site.Several bentonites should be evaluated and the resultsof the free swell tests used in the selection of a ben-tonite. Bentonite samples which exhibit the greatestswell and are the least affected by contaminants areusually chosen for use in the remainder of the com-patibility testing.

Another test which is often run to help determinebentonite selection is the “Filter Cake CompatibilityTest.” As stated previously, the filter cake is animportant component of an S-B slurry wall. Filtercake permeabilities may be as low as 1 × 10-9 cm/sec.For this reason, both filter cake compatibility testsand free swell tests are used to evaluate bentoniteperformance. Bentonite slurry (6 percent bentoniteby weight and site tap water) from each potentialbentonite source is placed in a fixed-wall permeame-ter. Slurry is then forced through filter paper overly-ing a porous stone at the bottom of the chamber witha chamber pressure of about 7 N/cm2 (10 psi) for24 hr. During this time, a filter cake of approxi-mately 10 mm (1/2 in.) thick will form on the filterpaper. The remaining bentonite slurry is thenremoved and replaced with either site tap water orcontaminated groundwater. The water is forcedthrough the filter cake with a pressure of about 1 to2 N/cm2 (2 to 3 psi). Permeabilities are calculatedand plotted for a 48-to 72-hr period. Bentoniteswhich exhibit the least variation between tap waterand groundwater should be considered for use. Atthe conclusion of the free swell tests and the filtercake compatibility tests, one bentonite is chosen forthe remainder of the compatibility testing.

(g) Mix design optimization. The purpose ofthis phase of testing is to determine the most eco-nomical mix of soil, dry bentonite, and bentoniteslurry which will produce an in-place slurry trenchpermeability less than or equal to 1 × 10-7 cm/sec.Because mixing and placing operations are less

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controlled in the field than in the laboratory, it isprudent to attempt to achieve a maximum laboratorypermeability of 5 × 10-8 cm/sec or less. Laboratorytest equipment must be constructed of materials thatare chemically resistant to the groundwater contami-nants. Short-term permeability tests (48 to 72 hr)using fixed-wall or flexible-wall permeameters arerun on various mix designs, varying the amount ofdry bentonite added (ie., 0, 2, and 4 percent). Sam-ples to be tested should be mixed to achieve a 100- to150-mm (4- to 6-in.) slump. One sample should berun with site tap water as the permeant (control sam-ple), and one sample should be run with contaminatedgroundwater as the permeant. It should be noted thatwhen using fixed wall permeameters, the specimenshould be allowed to set up and then applied headpressures should be increased slowly over severaldays, to avoid a sample failure. The most economi-cal S-B backfill which achieves a permeability lessthan or equal to 5 × 10-8 cm/sec, should be selectedfor subsequent long-term testing.

(h) Long-term compatibility tests. Long-termcompatibility testing consists of running flexible wallpermeameter tests on a minimum of three S-B back-fill material samples utilizing the optimum mix designobtained during short-term permeameter testing. Onesample should be run with site tap water only as thepermeant (control sample); one sample with contami-nated groundwater as the permeant (after one porevolume of site tap water permeant); and one samplewith a bentonite content 2 percent greater than theoptimum mix design, with contaminated groundwateras the permeant (after one pore volume of site tapwater permeant). It is generally recommended thatthree pore volumes of groundwater pass through theS-B backfill material. This testing may take up to3 months, depending on the sample size. It is recom-mended that the height-to-width ratio of the samplenot exceed 1:1. For example, a 75 mm (3 in.) diamsample should not be longer than 75 mm (3 in.). It isalso recommended that inflow and outflow measure-ments be taken during long-term tests to ensure thatno leakage is in the system. A chemical analysis isoften run on the effluent that has run through thesamples to see if any changes have occurred in thepermeant. This data may be used to determine along-term effect of contaminants on a soil-bentonitemixture. For example, an increase in the amount ofsodium and a decrease in the amount of calcium inthe permeameter effluent may indicate a displacementof sodium ions in the bentonite by calcium ions in thegroundwater. This reaction may tend to increase the

permeability of the S-B backfill over time. Afterlong-term compatibility tests are completed, theresults are then analyzed to determine if the contami-nants present in the groundwater have increased thepermeability of the S-B backfill in comparison to thecontrol sample.

d. Geotechnical design.

(1) Slurry wall alignment. Based on predesigninvestigations, a slurry wall is often located so that itsurrounds a contaminated groundwater plume, or alocalized “hot spot”. Sometimes a slurry wall may beinstalled to prevent clean groundwater from entering asite. Because of health and safety considerationsduring construction, it is desirable to locate the slurrywall outside the area of gross contamination. Thehandling and stockpiling of contaminated materialsexcavated from a slurry trench is also expensive.When practical, in order to maintain a continuousslurry wall, it is desirable to round corners of a slurrytrench rather than to intersect two straight line seg-ments. Although depth dependent, a 30-m (100-ft)radius curve is usually the shortest radius that mostexcavation equipment can negotiate.

(2) Key depth.

(a) One of the most important elements in aslurry wall design is determining if there is an ade-quate “key” into which the slurry wall can be tied.Usually, a slurry wall is keyed into a soil or rockhorizon which performs as an aquiclude. When thekey material is a soil, the slurry trench should bekeyed a minimum of 600 mm (2 ft) into the confin-ing layer. If the key material is rock, the nature ofthe rock surface must be determined throughout thealignment of the wall. The wall must extend throughareas of broken and/or weathered material into essen-tially impermeable material in order to minimizeseepage along the contact. Some options to removeweathered rock may include mechanical removal,controlled blasting, and debris removal with airliftequipment. See ETL 1110-2-282 for more informa-tion on rock excavation. Grouting along the contactmay be feasible in some instances; however, groutingof bedrock will greatly increase construction costsand may create new seepage fractures in the rockformation or the S-B backfill.

(b) For floating contaminants such as oil prod-ucts leaked from fuel storage tanks, it may not be

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necessary to key into some low permeability layer.These slurry walls are termed “hanging” slurry walls.

(3) Wall thickness. When designing the thick-ness of the wall, the hydrostatic gradient and thepermeability of the backfill material must be evalu-ated. A wall thickness of 900 mm (3 ft) will meetmost design criteria, although walls up to 2 m (7.5 ft)in width have been constructed. Also, a thicker wallwill provide a greater margin of safety if inconsisten-cies exist in the backfill. It is also desirable that thetrench be excavated with one pass of the excavationequipment.

(4) Work platform. If the terrain is highly irreg-ular, has steep slopes, or dense vegetation, extensivesite work and grading will be necessary. A S-Bslurry wall generally cannot be constructed in areaswith a slope greater than 2 percent along the align-ment of the slurry wall, unless site grading can beperformed or a work platform is constructed to main-tain this slope restriction. The width of the workplatform must be adequate for the equipment envi-sioned for construction. A work platform width of12 m (40 ft) with the slurry trench located along thecenterline is typically specified (Figure 2). Widerplatforms may be needed if S-B backfill will bemixed alongside the trench. Materials to constructthe work platform should exhibit good compactioncharacteristics to provide a firm working surface.

Figure 2. Typical slurry trench cross section

(5) Trench stability. While the slurry trenchmethod generally provides adequate trench stability,some soils that are extremely loose must be analyzedto verify the trench will remain stable during con-struction. Other construction control practices suchas stockpile locations and equipment operation near

the trench should be specified to maximize trenchstability.

(6) Control of groundwater. The control ofgroundwater is an integral part of any vertical barrierwall design. To provide positive contaminant con-tainment, groundwater is often extracted from theinside of a barrier wall to provide an inward hydrau-lic gradient from the outside of the wall to the inside.Thus, contaminants in the groundwater may be effec-tively contained, since any wall leakage will beinward rather than outward.

(7) Site layout. In general, slurry wall construc-tion requires a great deal of relatively flat, open spaceto provide room for equipment, slurry hydrationponds, and S-B backfill mixing areas. Mixing areasshould be located as near the trench as possible, withadequate utilities available.

(8) Construction quality control. As with anyconstruction project, the preparation of the finestplans and specifications does not guarantee the instal-lation of a quality product. Construction qualitycontrol for slurry walls is of utmost importance if aslurry wall is to function as designed. The followingitems are considered to be fundamental to ensuring aquality slurry wall installation:

(a) Contractor qualifications. It is imperativethat the slurry wall contractor have the necessaryexperience and qualifications to do the work. Per-haps the two most important individuals involved inthe construction of a slurry wall are the slurry trenchspecialist, and the excavation equipment operator.Submittals should be provided for both of these indi-viduals, prior to construction, showing that they meetthe experience requirements outlined in the specifica-tions. The Contractor should also submit, prior toconstruction, all required plans which describe howthe work will be done. The plans should include fouritems: sequence and layout of operations; trenchingoperations plan (includes trench excavation, use ofexcavated material, trench bottom cleaning, backfillplacement, etc.); preconstruction backfill testing pro-cedures and results; and quality control sampling andtesting plan.

(b) Bentonite. The Contractor should submit abentonite sample and test results from the bentonitemanufacturer for each lot of bentonite. The bentonite

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must meet all specified requirements and be stored ina proper manner on site.

(c) Water. The water must meet all specifiedrequirements. The Contractor should be required tocondition the water or locate another source if thespecified requirements are not met.

(d) Bentonite slurry. The initial bentonite slurrymust be tested prior to placement in the trench toensure that all required properties are met. Bentonitemay be mixed in high-shear mixers or mixed andhydrated in slurry hydration ponds. In general, aminimum hydration time of 8 hr will allow the ben-tonite slurry to meet all criteria. Other tests, such asviscosity, density, and pH, should be run two timeseach shift per batching plant during construction.Slurry in the trench should be tested for viscosity,density, sand content, and pH two times each shift, attwo locations in the trench (approximately 600 mm(2 ft) below the slurry surface and 600 mm (2 ft)above the bottom of the trench.)

Bentonite slurry should be introduced into the trenchwhen excavation begins. The slurry level in opentrenches should be maintained at a minimum of 1 m(3 ft) above the groundwater level, and no more than600 mm (2 ft) below the top of the working platform.If the density of the slurry in the trench exceeds1,025 kg/m3 (85 pcf), the excess solids must beremoved or the slurry must be replaced with freshslurry. If the excess solids are not removed, theymay settle to the bottom of the trench, creating areasof higher permeability within the wall.

(e) S-B backfill material. Mixing S-B backfillmaterials should be done in such a manner that the S-B backfill is consistent. Generally, S-B backfillmaterials are mixed either on a separate mixing pador alongside the slurry trench. It is felt by many thatit is easier to control S-B backfill mixing when doneon a separate mixing pad. However, mixing on aseparate pad requires extra handling of materials andis generally more expensive. It is important that allsoil particles are coated with slurry and that clods arebroken down. Quality control test results for perme-ability, slump, density, and moisture content shouldbe supplied by the Contractor as required in the spec-ifications for the S-B backfill.

(f) Trench excavation. The upgradient portion ofthe slurry wall is generally constructed first. Con-structing the down gradient portion of the wall first

can make construction more difficult by causing thegroundwater elevation to rise in areas where the wallhas not yet been built. The critical aspect of trenchexcavation is ensuring that a proper “key” has beenexcavated into the confining layer. Most often, it isrelatively easy to determine when the key layer hasbeen reached based upon bucket cuttings. However,if the key layer is difficult to determine from bucketcuttings, it may be necessary to push drive samplersto obtain samples. Once S-B backfilling operationshave begun, it is recommended that trench excavationprecede the toe of the S-B backfill slope by at least9 m (30 ft), but not more than 30 m (100 ft). Thispractice will decrease the length of open trench filledwith slurry, decreasing the risk for caving or slough-ing of the trench walls. It is preferable to round thecorners of a slurry trench rather than have intersect-ing walls. This practice will allow S-B backfillmaterials to flow around the curve. Material excav-ated from the trench should be stockpiled as far aspractical away from the trench. If the excavatedtrench spoils produce odors, it may be necessary tocover the stockpiles. During excavation, soundingsshould be obtained to determine the elevation of thetop of the key layer, the bottom of the excavation,and the bottom of the trench prior to backfilling. Ifexcessive sediments (greater than 50 mm (2 in.)) havebuilt up, it will be necessary to clean the trench bot-tom by airlift pumps, or excavation equipment, toremove the sand and sediment that has settled to thebottom of the trench. The trench bottom should becleaned, as a minimum, at the beginning of eachshift. The soundings should be obtained approxi-mately every 6 m (20 ft) along the alignment of thetrench.

(g) S-B backfill placement. Initial S-B backfillplacement should be performed by one of the follow-ing two methods: placement by lowering S-B back-fill to the bottom of the trench with a crane andclamshell bucket or tremie until the surface of theS-B backfill rises above the surface of the slurry or,begin excavation at a point outside the limits of workto allow a S-B backfill face to form prior to reachingthe full depth of the required slurry trench. S-Bbackfill should be placed so that no pockets of slurryare trapped. Free dropping of S-B backfill is notpermitted. It should be placed on top of the previ-ously placed S-B backfill and allowed to push outalong the bottom of the trench, or slide down theforward face of the S-B backfill slope. Periodicsoundings should be taken along the face of the slope

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to ensure that a smooth face has formed and ismaintained.

(h) Post-construction testing. Long-term perfor-mance of the slurry wall is primarily determined bymonitoring wells. Once S-B backfill material isplaced, it is very difficult to detect voids that may bepresent in the slurry wall. Various techniques havebeen attempted to determine if voids are present.One method is to take shelby tube samples of thecompleted wall. However, it is often difficult toobtain good samples and is generally expensive.Pump tests may also provide some indication of theperformance of the wall.

(9) Slurry wall protection.

(a) Clay plug. In order to protect the completedslurry wall from desiccation, a temporary noncom-pacted soil cover is placed over the S-B backfill.Usually, this temporary soil cover is placed within1 day after S-B backfill is completed, over each 30 m(100 ft) reach. After allowing for settlement (approx-imately 2 weeks), the temporary soil cover should beremoved, and replaced with a compacted clay coverover the completed slurry wall. The dimensions ofthe final clay plug are site specific.

(b) Equipment crossings. During construction, itmay be necessary to construct equipment crossingsover the completed wall. Measures should be takenduring construction to minimize the number of equip-ment crossings at the site. For heavy equipment, it isrecommended that the upper portion of (depth varieswith the width of the trench) the S-B backfill beexcavated and a clay plug be placed under the com-pacted trench cover and equipment crossing. Variousgeosynthetic materials may also be used to helpbridge the soft S-B backfill.

(10) Instrumentation.

(a) Monitoring wells. Monitoring wells shouldbe installed to monitor the performance of the slurrywall. Monitoring wells should be spaced evenly onboth sides of the slurry wall. Groundwater and con-taminant levels should be recorded at regular inter-vals, at a frequency determined by the design of thecutoff wall and the regulatory authorities. Periodicmaintenance of the monitoring wells may be neces-sary to ensure proper operation.

(b) Settlement. Settlement monuments may berequired at slurry wall installations which are verydeep (>15 m (50 ft)) or have a thick wall(>1,200 mm (4 ft)). Most settlement will occur dur-ing the first year after installation.

B-4. Alternative Barrier Systems

a. Cement-bentonite slurry walls.

(1) Description. A cement-bentonite (C-B)slurry wall is similar to a S-B slurry wall, except thatcement is added to the trench slurry in order to pro-duce a self-hardening slurry, thus eliminating theneed to backfill the trench. C-B slurries normallycontain water with about 6 percent by weight benton-ite and 18 to 30 percent cement. After hardening,C-B walls generally have strengths from about 14 to35 N/cm2 (20 to 50 psi). The permeability of a C-Bslurry wall is generally around 1 × 10-6 cm/sec. Itshould be noted that the permeability of C-B wallsmay decrease over time, and that measurements takenat 28 days may underestimate long-termpermeabilities.

(2) Applicability. The C-B slurry wall method isbest suited to contain sites with hydrocarbon contami-nation. Other contaminants may be contained;however, chemical compatibility tests should be per-formed. The C-B method provides some advantagesover S-B slurry walls, in that no borrow materials arerequired, and C-B walls may be installed in soils withquestionable stability, due to the relatively quicksetting times of the slurry.

b. Vibratory beam walls.

(1) Description. The vibratory beam installation(VBI) method produces a thin slurry wall (Figure 3)by driving a wide flange beam, which has groutinjection nozzles attached, to a predetermined depthand then extracting the beam and injecting the result-ing void with a slurry mixture. Cement-bentonite iscommonly used, although some bituminous groutshave been successfully used. Successive beam pene-trations are overlapped to form a cutoff wall. TheVBI method produces a cutoff wall approximately 75to 125 mm (3 to 5 in.) wide, depending on the widthof web on the beam or welded fin near the bottom ofthe beam.

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Figure 3. Vibratory beam wall-plan view

(2) Applicability. The VBI method is best suitedfor areas with flat topography and where loose gran-ular soils are present. Beam penetration is difficult instiff clays and rocky soils. The major advantage ofthe VBI method over conventional slurry wall sys-tems is that no excavation of site soils is required,thus creating a safer work environment for site work-ers during installation. However, because a relativelythin wall is produced by the VBI method, it is notrecommended as a primary barrier for contaminatedgroundwater. There are also concerns regarding wallcontinuity, as the tip location of the beam may vary,especially during deep penetrations. It may be diffi-cult to determine when a key layer has been reachedas well. VBI walls should be considered for use inconjunction with a groundwater removal system orfor diverting clean groundwater around a contamin-ated site.

c. Sheet pile walls.

(1) Description. Interlocking sections of steelsheet pile are driven to a predetermined depth andmay key into a subsurface aquiclude consisting ofclay or another relatively impermeable horizon. Steelsheet pile cutoff walls are considered to be a some-what permeable groundwater barrier. Each sectioninterlocks with an adjacent section. The connectionbetween sections may initially be a pathway forgroundwater migration. This leakage may be reducednaturally with time as fines are trapped in the connec-tion. If the piles are driven in material with little orno fines, the connections may never completely seal.Leakage in joints may also be sealed artificially bybeing filled with impermeable material. Varioussheet configurations are available in varying weights.

It is difficult to key steel sheet pile into hard rock,and keys should preferably be made in clay.

(2) Applicability. The major advantage of usingsheet piling is that no excavation is required. Thedisadvantages are the lack of an effective seal of thepile connections and problems with pile corrosion.Sheet piles are typically driven in soils that areloosely packed and predominately sand and gravel innature. Piling longevity depends on groundwatercharacteristics. For steel piles, pH is of particularimportance. If the pH of the groundwater is in therange of 5.8 to 7.8, sheet piles can last up to40 years; however, a pH as low as 2.3 can shortenthe lifetime to 7 years or less.

d. Grout curtains.

(1) Description. In general, grouting is the injec-tion of one of a variety of special grouts into soil orrock, thus greatly decreasing the soil’s permeability.The injection process involves drilling holes to thedesired depth and injecting grout under pressure byusing special equipment. To produce a curtain wall,a line of holes is drilled in single or double staggeredrows and grouting is then accomplished. The spacingof the injection holes is site specific, depending onsite soil characteristics, and is determined by thepenetration radius of the grout out from the holes.To form a continuous wall, grout injected from adja-cent holes should touch along the entire depth of thehole. When carried out properly, this process canproduce a curtain or wall that can be an effectivegroundwater barrier.

(2) Applicability. In general, grout curtain wallsare expensive and may not produce acceptable resultsfor most HTRW groundwater containment situations.Because of this, pressure grouting is used primarily toseal voids in porous or fractured rock, such as belowa slurry wall installation.

e. Deep soil mixing.

(1) Description. Deep soil mixing has been usedto install vertical barriers. An auger is rotated intothe soil while injecting various slurries, thus creatinga “column” of low-permeability soil. A continuouswall is thus created by overlapping the columns oftreated soil.

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(2) Applicability. In general, the advantages ofdeep soil mixing are the minimal handling of contam-inated soils during installation, and when compared toa vibratory beam wall, a thicker wall. Some concernswith the technique are wall continuity, (due to vari-ances in column overlap), and ensuring that the keylayer has been reached and penetrated.

f. Geomembrane walls.

(1) Description. One of the latest innovationsfrom the geosynthetic industry is the use of geomem-branes (Figure 4) made of high density polyethylene(HDPE) sheeting for vertical barrier walls. Geomem-branes may be installed directly into softer soils witha vibratory hammer, or for deeper installations, thegeomembranes are used to complement either S-B orC-B walls. When placed in a pre-excavated trench,the geomembrane panels may line one side of thetrench, installed either with a vibratory hammer ormounted on a steel frame and inserted into the trench.Another technique is to line both sides and the bot-tom of the trench by draping the geomembrane panelsinto the trench by using ballast material. The geo-membrane panels are typically 1.5 to 3 mm (60 to120 mils) thick, with each sheet interlocking to thepreceding panel.

Figure 4. Slurry trench with geomembrane liner

Depending on soil conditions, walls may be con-structed to a depth of 30 m (100 ft). Interlocks aresealed from leakage by the installation of a joint seal.

(2) Applicability. The use of geomembranes asa vertical barrier wall provides an excellent barrier togroundwater movement. Geomembranes installed asshallow sheet piles are not able to penetrate particu-larly stiff or rocky soils and, thus, are very dependenton site soil conditions and are limited in depth.

Geomembranes used in conjunction with slurry wallsprovide lower hydraulic conductivities for the com-pleted wall; however, the additional cost of thegeomembrane installation must be considered. It maybe necessary to run compatibility tests between sitecontaminants and the proposed sheeting materials.

g. SoilsawTM walls.

(1) Description. The SoilsawTM barrier system(Figure 5) can form a vertical barrier wall whichmight be best described as a “mixed in place” wall.Various slurries or grouts may be used to mix with insitu soils by means of a jetting pipe which has groutjets located at regular intervals along the bottom ofthe pipe. The jetting pipe is attached to a mechanicalcrawler machine which is able to reciprocate the pipealong its length through a stroke equal to the spacingof the grouting jets. The jetting pipe is supplied withhigh pressure (3,500 N/cm2 (5,000 psi)) slurry fromseparate mixing and pumping equipment.

Figure 5. Soilsaw TM barrier system

When the system is operating, the soil under thejetting pipe is liquefied along the entire length of thejetting pipe, and the beam sinks into the ground underits own weight. As the pipe reaches its workingdepth (approximately 45 deg), the crawler begins totravel at a rate which enables the jetting beam tomaintain the proper depth. Present equipment iscapable of forming a cutoff wall which is approxi-mately 300 mm (12 in.) wide.

(2) Applicability. Currently, the SoilsawTM

method is in the research and development phase.While not yet commercially available, it is planned tobe used commercially in the future. The SoilsawTM

method is best suited for soils which are fairly uni-form with occasional rocks and debris present. Small

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rocks or cobbles are incorporated into the wall, whilelarger rocks are pushed out into the surrounding soil.Various grouts, such as bentonite, cement/bentonite,cement/flyash/bentonite, may be utilized, dependingon the contaminant in the groundwater which must becontained. Currently, the working depth of the Soil-sawTM method is about 9 m (30 ft); however, it isexpected that larger equipment may be able to con-struct a wall up to 30 m (100 ft) in depth. Advan-tages of this system include no trench spoils tohandle, a continuous barrier wall, and high productionrates. Disadvantages of the system are relatively highmobilization costs, reduced efficiency in hard cohe-sive soils, inability to “key” into hard rock layers, andlarge rocks or boulders in the path of the wallinstallation.

h. Permeable reactive barriers.

(1) Description. One of the most recent innova-tions in vertical barriers are permeable reactive bar-riers (PRB). A PRB is unique in design whencompared to other vertical barriers since it involvesthe construction of a permeable reaction surfacewhich can be placed across the path of a groundwaterplume to permit groundwater to flow through the wallwhile degrading contaminants in situ or sorbing them.Contaminants can be degraded by chemical or biolog-ical processes or held in place by physical processessuch as sorption or cation exchange. A typical PRBmight be 15 to 60 m (50 to 200 ft) long, 1 to 5 m (3to 15 ft) wide and installed to depths of up to 15 m(50 ft). The actual depth of the wall will be thesaturated interval plus several feet to account forgroundwater fluctuations and hydraulic control. Thebottom of the wall ideally is keyed into an imperme-able layer to maintain hydraulic control. In someconfigurations, it is envisioned that the lateral hydrau-lic control would be achieved by using conventionalvertical barriers such as sheet pile walls. Sheet pilesor other vertical barriers could be tied into the PRBand used to funnel groundwater through the reactivearea which is referred to by some as a gate. In thismanner, all of the contaminated water of interest willflow through the passive treatment zone.

(a) Chemical. The most common chemicalreactions involve either redox changes or precipita-tion. An example of the redox reaction is the use ofgranular zero-valent iron to degrade halogenatedhydrocarbons. The zero valent iron is oxidized andreleases two electrons which appear to dehalogenatemany halogenated hydrocarbons (including, but not

limited to, perchlorethylene (PCE), trichlorethylene(TCE), and dichlorethylene (DCE)) which are com-monly found as contaminants in groundwater at haz-ardous and toxic waste (HTW) sites. There are otherforms of iron and other metals and materials that canalso be used in PRB for specific classes of contami-nants. The material emplaced in the central core ofthe trench is often composed of 50- to 100-percentgranular iron with the remainder being clean silicasand of similar grain size. In addition to the granulariron zone, there is often a zone on either side whichconsists of gravel or coarse sand to assist in maintain-ing hydraulic control.

(b) Biological. In these PRBs, the wall consistsof a material that will enhance biodegradation byindigenous species in the aquifer. This might involvethe introduction of a permeable substrate such as peator wood chips to remove nitrate contamination. Asthe water flows through the PRB, denitrificationreactions enhance the removal of the nitrates. Inother circumstances, one can add substances toenhance redox conditions or to add other requirednutrients for the bacteria such as adding oxygen ornitrate to enhance the biodegradation of benzene,toluene, ethylbenzene, and xylenes (BTEX)compounds.

(c) Physical. These processes include a numberof sorption reactions for both organic and inorganicchemicals as well as ion-exchange reactions. Theprincipal limitation of sorptive barriers is their finitecapacity and the possibility of breakthrough when thecapacity is exceeded. The reactive material wouldhave to be excavated and disposed of in a mannerconsistent with regulatory requirements. Dependingon the nature of the reactive material and the sorbedcontaminants, the material may be expensive to dis-pose of or require regeneration. Additional cleanmaterial would then have to be reinstalled. A soundtreatability study and knowledge of waste disposalrequirements are critical when proposing this kind ofbarrier.

(2) Applicability. Complete site characterizationdata is critical to the successful design and construc-tion of a PRB. It is critical to understand site geol-ogy, hydrogeology, and aquifer geochemistry beforeimplementing this technology. Basic hydrogeologicalinformation such as the groundwater flow directionand velocity must be supplemented by more detailabout seasonal fluctuations in head which, in turn,affect flow direction and velocity. Fluctuations in

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seasonal groundwater flow directions and velocitiesare critical to wall performance. It is imperative thatall of the contaminated water flows through, notaround, the treatment zone. The PRB has a fixedlocation, and changes in these parameters will limitits effectiveness if the residence time is decreased andpermits some of the contaminants to flow throughuntreated. Detailed geological data along the pro-posed PRB site is required to evaluate whetherhydraulic control is achievable. For most sites thetotal depth of the PRB is limited to approximately15 m (50 ft), and it is necessary to document thedepth to the aquitard which the wall will key into anddetermine the aquitard’s physical characteristics.Detailed site stratigraphy using a cone penetrometeror other continuous sampling technology is also nec-essary to show the potential effects of site hetero-geneity on contaminant migration with respect to thelocation chosen for the PRB.

(a) Clogging of the wall by precipitants or coat-ings may, after a time, decrease the permeability andrequire the replacement of the treatment media.There is no good database on wall performance inthis area, and it is impossible to predict with certaintyhow quickly a wall may need to be replaced. Geo-chemical data will assist in determining in situ redoxconditions and chemical parameters that might affectthe speed with which redox reactions take place andwhether or not the proposed reactions will be affectedby site chemistry. The major ions can also affect thespeed at which precipitation reactions will take placeand, thus, the time it takes for the reactive surface tobecome coated or clogged. Treatability and/or pilotstudies need to be carried out with site waters toassess the needed thickness of the barriers for eachsite. Each compound has a different degradation rate,and aquifer geochemistry may also affect the perfor-mance of the wall.

(b) The most important criteria for the design ofthe PRB is that the contaminants are destroyed orsorbed as they move passively through the treatmentzone. The exact method of emplacement will varydepending on the reactive substance to be emplacedand any requirements to maintain hydraulic control.A major consideration during emplacement is toensure that the barrier has no significant heterogenei-ties which may serve as a preferential flow path.Alternative materials and methods of emplacementare also being developed including cassettes or otherremovable devices to allow easy access to the react-ing agent. Because of the generally greater widths of

the PRBs as compared to slurry walls, there are addi-tional potential problems with excavating and main-taining trench stability.

B-5. Civil Design

Common civil design features for all vertical barrierwall projects which should be considered aredescribed below.

a. Site access routes. Both public and privateaccess routes to the site need to be investigated toensure they can handle the construction traffic. TheContractor should be required to maintain any accessroutes including post-construction rehabilitation, ifnecessary. Aggregate surfaced roads may need to bebuilt to allow access of construction vehicles or foroperation and maintenance of the barrier wall.Aggregate surfaced roads should be designed inaccordance with TM 5-822-12. Several U.S. ArmyCorps of Engineers guide specifications are availablefor the construction of roadways.

b. Decontamination facility. The Contractormust decontaminate all vehicles and equipment whichenter the exclusion zone or contamination reductionzone. The contract documents should address therequirements for a decontamination facility and forthe treatment and disposal of rinsate water. The finaldisposition of the decontamination facility should alsobe addressed. The Contractor should be required tosubmit a plan as part of the Site Health and SafetyPlan which describes vehicle, equipment, and person-nel decontamination procedures.

(1) Design criteria. The decontamination facilitytypically consists of 150 to 300 mm (6 to 12 in.) ofgranular material underlain by a protective geotextileand a geomembrane 1.0 mm (40 mils) in thickness.More elaborate designs may be used if the decontam-ination facility will be operated for a significantperiod of time. To minimize the volume of decon-tamination water, a temporary cover should be usedto shed rainfall when the facility is not in use.Rinsate water is collected by gravity into a polyethyl-ene or precast concrete storage tank which is typicallyabout 3 m3 (100 ft3) in volume. Treatment and dis-posal of the rinsate water and sediments should be inaccordance with all state and Federal regulations.Federal disposal regulations are described in 40 CFRParts 260-268. Rinsate water is typically disposed ofafter onsite treatment or is transported to an offsite

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treatment facility. If the decontamination water andsediments are defined as hazardous wastes and are tobe taken offsite, the Contractor must manifest thematerials in accordance with 49 CFR 100-180 and 40CFR 263. The materials must be transported by acertified hazardous waste hauler and the treatment,storage, and disposal facility (TSDF) must have theappropriate EPA and state permits. The Contractorshould submit the names of all haulers and TSDFfacilities proposed for use. Chemical test results,manifests, land disposal restriction (LDR) notificationforms, and certifications of final treatment/disposalshould also be submitted. See EP 200-1-2 for moredetailed information on this subject.

c. Security fencing. A chainlink security fenceis often used at the site boundary. The fence nor-mally has a standard single outrigger with threestrands of barbed-wire on the outrigger. The fencefabric should be a minimum of 1.8 m (6 ft) in height.USACE guide specification CEGS-02831 should beused in the contract documents to specify fencingrequirements.

B-6. Potential List of Drawings

Provided below is a list of potential drawings thatshould be included in the contract documents. Notall drawings will be applicable to every project.

Cover Sheet

Index of Drawings

Abbreviations

Legend

Vicinity Map

Location Map

Existing Site Conditions (including utilities)

Test Pit and Boring Location Plan

Contractor Access Plan

General Plan

Horizontal and Vertical Control

Removal Plan

Electrical Distribution Plan

Site Control Plan

Grading Plan

Vertical Barrier Wall Profiles

Vertical Barrier Wall Typical Sections

Vertical Barrier Wall Plan View Along Centerline

Erosion Control Plan (Temporary)

Slurry Wall Corner Details

Working Platform Cross Section

Traffic Crossing Cross Section

Wash-down Area Cross Sections and Details(Decon Pad)

Monitoring Well Details

Piezometer Details

Chain Link Fence Details

Chain Link Gate Details

Project Right-of-Way Map

Record of Borings (Geological Profile Sheets)

Borrow Area Grading Plan, Sections and SoilTest Data

New Utility Drawings

New Access Road Profiles and Sections

Dewatering Plan

Extraction Wells

Injection Wells

Extraction Trench

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Injection Trench

B-7. Potential List of Specification Sections

Provided below is a list of potential specificationsthat should be included in the contract documents.Not all specifications will be applicable to everyproject. U.S. Army Corps of Engineers guide specifi-cations for military construction are shown whenavailable. If no guide specification exists, experiencefrom previous sites or manufacturer specificationsshould be used by the design engineer to create aconstruction specification.

DIVISION 1 - GENERAL REQUIREMENTS

CEGS-01110 Safety, Health, and EmergencyResponse

CEGS-01300 Submittal Descriptions

CEGS-01305 Submittal Procedures

CEGS-01440 Contractor Quality Control

CEGS-01450 Chemical Data Quality Control

01XXX Summary of Work

01XXX Order of Work

01XXX Contractor’s Use of Site

01XXX Pre-construction and Pre-workConference

01XXX Progress Meetings

01XXX Special Clauses

01XXX Measurement and Payment

01XXX Special Project Features

01XXX Warranty of Construction

01XXX Construction General

01XXX Onsite Camera

01XXX Dust Control

01XXX Spill and Discharge Control Plan

01XXX Air Monitoring

01XXX Environmental Protection

01XXX Security

01XXX Regulatory Requirements

01XXX Decontamination and Disposal

01XXX Surveys for Record Drawings

01XXX Photographic Documentation

01XXX As-Built Drawings

01XXX Project Record Documents

01XXX Temporary Utilities and Controls

01XXX Support Facilities

01XXX Demobilization and Project CloseOut

01XXX Operation and Maintenance

DIVISION 2 - SITE WORK

CEGS-02110 Clearing and Grubbing

CEGS-02210 Grading

CEGS-02233 Graded-Crushed-Aggregate BaseCourse

CEGS-02234 Subbase Course

CEGS-02272 Separation/Filtration Geotextile

CWGS-02411 Metal Sheet Piling

CEGS-02444 Soil-Bentonite Slurry Trench forHTRW Projects

CEGS-02546 Aggregate Surface Course

CEGS-02671 Ground-Water Monitoring Wells

CEGS-02831 Chain Link Fence

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CEGS-02935 Turf

CWGS-03363 Concrete Cutoff Wall

CWGS-03365 Concrete for Concrete Cutoff Wall

02XXX Well Abandonment

02XXX Hazardous Material Excavationand Handling

02XXX Temporary Erosion and SedimentControls

02XXX Decontamination Facility

02XXX Contaminated Liquids Removal

02XXX Demobilization and Project CloseOut

02XXX Extraction Wells

02XXX Injection Wells

02XXX Extraction Trench

02XXX Injection Trench

B-8. Quantity Tabulation Sheet for CostEstimates

Provided below is a list of items that should be con-sidered when developing a cost estimate for a verticalbarrier wall. Not all items will be applicable to everyproject.

Description Unit

All Other Items Lump Sum

Site Preparation (Con- Lump Sumstruction Trailers,

Staging Areas, Parking Hectares (acres)Areas, etc.) SiteClearing and Grubbing

Description Unit

Decontamination Facility Lump Sum

Chain Link Fence Linear Meters (ft)

Access and PerimeterRoads and Ditches M3 (CY)

Access and Perimeter Metric Tons (tons)Road Surfacing

Utility Relocations Lump Sumand Extensions

Monitoring Wells Each

Monitoring Well EachAbandonment

Extraction Wells Each

Injection Wells Each

Extraction Trench M2 (SF)

Injection Trench M2 (SF)

Vertical Barrier Wall M2 (SF)

Piezometers Each

Random Fill M3 (CY)

Geotextile M2 (SY)

Turf Hectares (acres)

Temporary Erosion Lump SumControl Features

O&M Items Lump Sum

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APPENDIX C: VERTICAL BARRIER WALL DESIGN CHECKLIST

C-1. Vertical Barrier Wall Design Checklist

The purpose of this checklist is to prompt thedesigner or reviewer to consider all aspects of design.All major areas of design are covered by the check-list. However, the designer or reviewer must refer tothe narrative and other references for detailed designcriteria.

a. Predesign investigations.

(1) Field surveys and record searches.

• Have existing documents(Remedial Investigation,Feasibility Study, etc.)been reviewed? Y__N__N/A__

• Have recent and historicalaerial photographsbeen obtained? Y__N__N/A__

• Have design or as-builtdrawings of the existingsite conditions beenobtained? Y__N__N/A__

• Has current topographicmapping of the site, pre-ferably CADD generatedwith 0.3048-m (1-ft) con-tour intervals, beenobtained? Y__N__N/A__

• Does topographic mappingidentify all surfacefeatures (e.g., fences,trees, and buildings)? Y__N__N/A__

• Have existing monitoringwells, piezometers, etc.been surveyed and horizon-tal coordinates and verti-cal elevations determined? Y__N__N/A__

• Has horizontal and verticalcontrol been establishedand documented? Y__N__N/A__

• Have utilities beenresearched, identified,located, and mapped? Y__N__N/A__

• Have boundary surveys beenconducted for both the pro-ject site and impactedadjoining properties? Y__N__N/A__

• Has a property (deed)search of the site andadjoining property beenperformed? Y__N__N/A__

• Has access for offsitemonitoring been secured? Y__N__N/A__

(2) Geological investigations.

• Have the limits of cont-amination beendetermined? Y__N__N/A__

• Has material excavata-bility been evaluated? Y__N__N/A__

• Have groundwater condi-tions been evaluatedincluding water levels,flow directions, con-taminants present andgroundwater chemistry? Y__N__N/A__

• Has the depth and con-sistency of the aquicludebeen determined alongthe vertical barrierwall alignment? Y__N__N/A__

• Have the subsurface geo-logic conditions beencharacterized? Y__N__N/A__

(3) Vertical barrier wall selection.

• Have various verticalbarrier walls beenevaluated for use? Y__N__N/A__

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b. S-B slurry wall design.

(1) General.

• Do the contract documentsadequately specify materialand installation require-ments? See guide spec-ification CEGS-02444. Y__N__N/A__

(2) Pre-design issues.

• Have borings been takenalong the proposed align-ment of the slurry wall? Y__N__N/A__

• Have adequate borrowsources been identified? Y__N__N/A__

• Do the specificationsrequire all borrow mater-ials be tested forcontamination? Y__N__N/A__

(3) Compatibility/optimization testing.

• Have several types ofbentonite been obtained? Y__N__N/A__

• Has an adequate supply(40 L (10 gal) each) ofgroundwater and tap waterbeen obtained for the com-patibility testing? Y__N__N/A__

• Has an adequate supply(50 Kg (100 lbs)) of back-fill material been collected? Y__N__N/A__

• Is the proposed testing pro-gram adequate to optimizethe mix design and deter-mine the compatibility ofthe backfill mixture withthe contaminants presentat the site? Y__N__N/A__

(4) Geotechnical design.

• Does the slurry wall align-ment adequately surroundthe contaminated ground-water plume or hot spot? Y__N__N/A__

• Is the slurry wall align-ment clearly shown onthe drawings? Y__N__N/A__

• Is there an adequateaquiclude which the slurrywall will key into? Y__N__N/A__

• Is the depth of the slurrywall clearly shown onthe drawings? Y__N__N/A__

• Is the wall thicknessa minimum of 900 mm(36 in.)? Y__N__N/A__

• Is a work platform neededdue to the slope of theground surface (2 per-cent max slope)? Y__N__N/A__

• Is the work platform atleast 12 m (40 ft)wide? Y__N__N/A__

• Has trench stabilitybeen considered dur-ing design? Y__N__N/A__

• Is the site layout areaadequate for contractorstaging, material, andequipment storage? Y__N__N/A__

• Are stockpile locationsshown on the drawings? Y__N__N/A__

• Has a clay plug over thecompleted wall been ade-quately designed? Y__N__N/A__

• Have equipment crossingzones been adequatelydesigned? Y__N__N/A__

(5) Construction quality control.

• Are Contractor qualifica-tions outlined in thespecifications? Y__N__N/A__

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• Are requirements for ben-tonite, water, bentoniteslurry, and backfill materialsoutlined in thespecifications? Y__N__N/A__

• Are trench excavationrequirements outlinedin the specifications? Y__N__N/A__

• Are backfill placementrequirements outlinedin the specifications? Y__N__N/A__

• Are requirements for ini-tial backfill placementdescribed in thespecifications? Y__N__N/A__

• Are quality control testingrequirements described inthe specifications? Y__N__N/A__

• Will post-constructiontesting be performed onthe slurry wall? Y__N__N/A__

(6) Vegetative cover.

• Is a vegetative coverapplicable at this site? Y__N__N/A__

• Are locally adaptedperennial plantsspecified? Y__N__N/A__

(7) Instrumentation.

• Has instrumentation beenspecified to monitor ground-water levels or settlementof the slurry wall? Y__N__N/A__

(8) Groundwater monitoring and control.

• Is a groundwater extractionsystem to be utilized in con-junction with the slurrywall? Y__N__N/A__

• Have the regulatory require-ments for groundwater mon-itoring been defined? Y__N__N/A__

• Have existing wells beenevaluated for useas monitoring points? Y__N__N/A__

• Do the specificationsaddress existing monitoringwells that will be impactedby construction (i.e., aban-donment and extension)? Y__N__N/A__

• Do the contract documentsadequately specify material,installation, and monitoringrequirements? See guidespecification CEGS-02671,“Ground-Water Monitor-ing Wells?” Y__N__N/A__

(9) Final grading requirements.

• Has a final grading planbeen established? Y__N__N/A__

c. Civil design.

• Have site access routesbeen addressed in thecontract documents? Y__N__N/A__

• Are staging areas identifiedon contract documents? Y__N__N/A__

• Have phasing requirementsbeen addressed in the con-tract documents? Y__N__N/A__

• Have utility requirementsbeen specified? Y__N__N/A__

• Are decontamination paddesign, operation and dis-posal requirementsspecified? Y__N__N/A__

• Are security fence require-ments addressed in thecontract documents? Seeguide specification CEGS-02831, “Chain-LinkFence.” Y__N__N/A__

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• Are the limits of clearingand grubbing shown onthe drawings? Y__N__N/A__

• Has disposal of clearedand grubbed materialbeen addressed? Y__N__N/A__

• Have clearing and grub-bing been addressed in thecontract documents? Seeguide specification CEGS-02110, “Clearing andGrubbing.” Y__N__N/A__

d. Health and safety.

• Have health and safetyissues been addressed inthe contract documents?See guide specificationCEGS-01110, “Safety,Health and Emer-gency Response.” Y__N__N/A__

e. Chemistry.

• Have chemistry require-ments been adequatelyaddressed in the contractdocuments? See guide spe-cification CEGS-01450,“Contractor’s ChemicalQuality Control.” Y__N__N/A__

f. Operation and maintenance requirements.

• Have groundwater monitor-ing criteria been addressed? Y__N__N/A__

• Do the contract documentsadequately address mon-itoring and inspectionissues for the verticalbarrier wall? Y__N__N/A__

• Do the contract docu-ments adequately addressmaintenance and repairissues for the verticalbarrier wall? Y__N__N/A__

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