Design, Construction, and Performance of Seepage … Design Construction... · Design, Construction, and Performance of Seepage Barriers for Dams on Carbonate Foundations DONALD A

Design, Construction, and Performance of Seepage

Barriers for Dams on Carbonate Foundations

DONALD A. BRUCE1

Geosystems, L.P., P.O. Box 237, Venetia, PA 15367

TRENT L. DREESE

Gannett Fleming Inc., 207 Senate Avenue, Camp Hill, PA 17011

DOUGLAS M. HEENAN

Advanced Construction Techniques Ltd., 3935 Lloydtown Aurora Road, Kettleby,ON L0G 1J0, Canada

Key Terms: Dams, Design, Construction, Perfor-mance, Seepage Barriers, Carbonate Foundations

ABSTRACT

The design, construction, and performance of con-crete cut-offs and grout curtains as dam seepageremediation methods in carbonate foundations arereviewed. Recent experiences gained when attemptingto build concrete cut-offs through hard and highlypermeable rock masses have led the authors to developthe concept of a ‘‘composite cut-off.’’ A campaign ofhigh-quality drilling, permeability testing, and groutingis first conducted to pre-treat the very permeable and/orsolutioned zones, to seal the clean fissures, and toprovide an extremely detailed geological basis uponwhich to design the location and extent of thesubsequent concrete wall (if in fact needed). Bearingin mind that the average cost of a concrete wall is manytimes that of a grouted cut-off, and that there iscurrently a shortfall in industry capacity to constructthe former, the concept of a ‘‘composite wall’’ is logical,timely, and cost effective, as well as being sustainable.

INTRODUCTION

As documented by Weaver and Bruce (2007), groutcurtains have been used in the United States tocontrol seepage in rock masses under and arounddams of all types since the 1890s. For a variety ofunderstandable, if not always laudable reasons, thelong-term performance of many of these curtains hasnot been satisfactory, especially in lithologies con-taining soluble and/or erodible materials. Foundation

remediation in such instances traditionally hasinvolved re-grouting, often, of course, using the samemeans, methods, and materials with the same defectsthat were the underlying cause of the inadequacy inthe first place.

Disillusionment on the part of owners and engi-neers with the apparent inability of these traditionalgrouting practices to provide a product of acceptableefficiency and durability led to the chorus of‘‘grouting doesn’t work’’ voices in the industry fromthe mid-1970s onward. The fact that effective anddurable grout curtains were being installed success-fully elsewhere in the world, using different perspec-tives on design, construction, and contractor pro-curement processes, largely escaped the attention ofthe doubters who, for all their other and obviousqualities, exhibited technological xenophobia.

Partly as a result of the anti-grouting lobby, andequally in response to indisputable geological realitiesand challenges and building on technical advances in‘‘slurry wall’’ techniques, the concept and reality of‘‘positive cut-offs’’ became the mantra for majorembankment dam foundation rehabilitation in NorthAmerica from 1975 onward. Such walls, built throughand under existing dams by either the panel walltechnique, or large-diameter secant piles, consist ofsome type of concrete, ranging from high strength toplastic. In contrast to grout curtains, where well over90 percent of the cut-off is in fact the virgin, in siturock, these ‘‘positive’’ cut-offs constitute 100 percentpre-engineered material with well-defined properties.

Such ‘‘positive’’ walls are essential to provide long-term cut-off across karstic features, which containresidual, potentially erodible material: such materialsimply cannot be grouted with a degree of uniformityand confidence to ensure satisfactory long-termperformance. The list of successful projects executedto date in the United States is extremely impressive

1Corresponding author: phone: (724) 942-0570; fax: (724) 942-1911;email: [email protected].

Environmental & Engineering Geoscience, Vol. XVI, No. 3, August 2010, pp. 183–193 183

(Bruce et al., 2006; Bruce, 2007), and many have beeninstalled in carbonate terrains of varying degrees ofkarstification. To date, almost 7.5 million square feetof concrete cut-offs have been installed in 20 projects.

From the mid-1980s—albeit in Europe (Lombardi,2003)—a new wave of dam grouting concepts beganto emerge. Given that most of the leading NorthAmerican practitioners had close corporate and/orprofessional and personal links with this develop-ment, it is not surprising that their heretoforemoribund industry began to change. By the time ofthe seminal 2003 American Society of Civil Engi-neers (ASCE) grouting conference in New Orleans,the revolution in North American practice for damfoundation grouting had been clearly demonstrated(Walz et al., 2003; Wilson and Dreese, 2003). Theconcept of a quantitatively engineered grout curtainwas affirmed. Differences in opinion and philoso-phies with the great European practitioners such asEwert, or Lombardi, the architect of the GINmethod (i.e., grouting intensity number), were notnecessarily resolved: they were debated betweenequals, and the respective opinions were fairlyacknowledged.

It is therefore the case that, in North America,there is now expertise and experience of an unparal-leled level in both grout curtains and concrete cut-offwalls for dam remediation. This is particularlyserendipitous given that the dollar requirement forthe application of both technologies—in federal damsalone in the next 5 years—is of an order equivalent tothe aggregate of the preceding 40 years (Halpin,2007).

This paper presents a review of the current state-of-practice in each of these two technologies. The paperdescribes how these techniques can be combined inthe concept of a ‘‘composite cut-off,’’ which haspotentially extraordinary benefits to owners in thefinancial sense, while still assuring the highestverifiable standards of performance and durabilityin the field.

CONCRETE CUT-OFFS

Investigations, Design, Specifications, andContractor Procurement

Intensive, focused site investigations are essential asthe basis for cut-off design and contractor biddingpurposes. In particular, these investigations must notonly identify rock mass lithology, structure, abrasiv-ity, and strength (‘‘rippability’’), but also the potentialfor loss of slurry during panel excavation. This hasnot always been done, and cost and schedule havesuffered accordingly on certain major projects.

Special considerations have had to be made whendesigning cut-offs that must abut existing concretestructures, or that must be installed in very steep-sided valley sections, or that must toe in to especiallystrong rock.

‘‘Test sections’’ have proven to be extremelyvaluable, especially for contractors to refine theirmeans, methods, and quality-control systems. Suchprograms have also given the dam safety officials andowners the opportunity to gain confidence andunderstanding in the response of their dams to theinvasive surgery that constitutes cut-off wall con-struction. Furthermore, such programs have occa-sionally shown that the foreseen construction methodwas practically impossible (e.g., a hydromill at BeaverDam, AR) or that significant facilitation works wererequired (e.g., pre-grouting of the wall alignment atMississinewa Dam, IN, Clearwater Dam, MO, andWolf Creek Dam, KY).

Every project has involved a high degree of risk andcomplexity and has demanded superior levels ofcollaboration between designer and contractor. Thissituation has been best satisfied by procuring acontractor on the basis of ‘‘best value,’’ not ‘‘lowbid.’’ This involves the use of RFP’s (Requests forProposals) with a heavy emphasis on the technicalsubmittal and, in particular, on corporate experience,expertise, and resources, and the project-specificMethod Statement. These projects are essentiallybased on performance, as opposed to prescriptivespecifications. Partnering arrangements (which arepost-contract) have proven to be very useful to bothparties when entered into with confidence, enthusi-asm, and trust.

Construction and QA/QC

The specialty contractors have developed animpressive and responsive variety of equipment andtechniques to ensure cost-effective penetration andappropriate wall continuity in a wide range of groundconditions. More than one technique, e.g., clamshellfollowed by hydromill, has frequently been used onthe same project and especially where boulderyconditions have been encountered.

Cut-offs can be safely constructed with high lakelevels, provided that the slurry level in the trench canbe maintained a minimum of 3 ft above these levels.In particularly challenging geological conditions, thismay demand pre-treatment of the embankment (e.g.,Mud Mountain Dam, WA) or the rock mass(Mississinewa Dam, IN) to guard against massive,sudden slurry loss. For less severe geological condi-tions, contractors have developed a variety ofdefenses against slurry losses of smaller volume and

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rate by providing large slurry reserves, using floccu-lating agents and fillers in the slurry, or by limitingthe open-panel width.

Very tight verticality tolerances are necessary toensure continuity and especially in deeper cut-offs.Such tolerances have been not only difficult to satisfy,but also difficult to measure accurately (to #0.5percent of wall depth) and verify.

The deepest panel walls have been installed atWells Dam, WA (223 feet, clamshell) and at MudMountain Dam, WA (402 feet, hydromill). Thehydromill has proved to be the method of choicefor large cut-offs in fill, alluvial soils, and in rockmasses of unconfined compressive strengths less than10,000 psi (massive) to 20,000 psi (fissile, andtherefore, rippable).

Secant pile cut-offs are expensive and intricate tobuild. However, they are the only option in certainconditions (e.g., heavily karstified, but otherwise hardlimestone rock masses) that would otherwise defeatthe hydromill. The deepest such wall (albeit acomposite pile/panel wall) was the first—at WolfCreek, KY, in 1975. It reached a maximum of 280 feet.The most recent pure secant pile wall in carbonateterrain was constructed at Beaver Dam, AR, 1992–1994.

A wide range of backfill materials has been used,ranging from low strength plastic concrete toconventional high strength concrete. This is a criticaldesign decision.

The preparation and maintenance of a stable anddurable working platform has proven always to be abeneficial investment, and its value should not beunderestimated.

The highest standards of real-time quality assur-ance/quality control (QA/QC) and verification areessential to specify and implement. This applies toevery phase of the excavation process, and to each ofthe materials employed.

Enhancements have progressively been made incut-off excavation technology, especially to raiseproductivity (particularly in difficult geological con-ditions), to increase mechanical reliability, and toimprove the practicality and accuracy of deviationcontrol and measurement.

Potential Construction Issues with Cut-Offs

Satisfactory construction of positive cut-off wallsrequires experience, skill, and dedication to qualityin every aspect of the construction process,including site preparation, excavation, trench orhole cleaning, concrete mixing, and concreteplacement. A positive cut-off requires the elementsof the wall to be continuous and interconnected.

The following issues are possible concerns thatmust be taken into account in wall construction toprevent defects:

N Element deviation—Misalignment of the equip-ment or inability to control the excavation equip-ment can cause deviation of elements and can resultin a gap in the completed wall.

N Uncontrolled slurry loss—Cut-off walls throughexisting water-retaining structures are almostalways built to address seepage issues. Althoughbentonite slurries are proven in creating a filtercake in soils, the ability of bentonite slurries toform a filter cake in rock fractures is limited. As ageneral rule of thumb, if water is lost duringexploration drilling, one should assume thatslurry losses in rock will occur during excavation.If the rock mass is sufficiently permeable, un-controllable and complete slurry loss can occur.Slurry losses in embankments have also occurredon past projects due to hydrofracturing of sus-ceptible zones. This is a particularly sensitiveissue when excavating through epikarstic hori-zons, and major karstic features lower in theformation. In this regard, epikarst is defined asthe transition/interface zone between soil and theunderlying, more competent, if still karstified,rock. Epikarst typically contains very fracturedand solutioned conditions, and much residualmaterial and voided areas. Epikarst usually playsan extremely important role in the hydrogeologi-cal regime of many karst aquifers.

N Trench stability—The factors of safety of slurry-supported excavations in soil are not high. Move-ment of wedges into the trench or ‘‘squeeze in’’ ofsoft zones can occur.

N Concrete segregation—Mix design and construc-tion practices during backfill must be selected so asto prevent segregation or honeycombing within thecompleted wall.

N Soil or slurry inclusions—The occurrence of soil- orslurry-filled defects or inclusions in completed wallsis a known issue. These defects are not critical ifsmall or discontinuous, but they become verysignificant if they fully penetrate the width of thewall.

N Panel joint cleanliness—Imperfections or pervi-ous zones along the joints between elements are arecognized source of leakage through completedwalls. Cleaning of adjacent completed elementsby circulating fresh slurry is necessary to mini-mize the contamination of joints. In extremecases, mechanical cleaning with ‘‘brushes’’ isconducted.

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Performance

Surprisingly little has in fact been published to datedescribing the actual efficiency of cut-off walls aftertheir installation: most of the publications describedesign and construction and have usually beenwritten soon after construction by the contractorsthemselves. The research into this matter conductedby the Virginia Tech team of Rice and Duncan (Riceand Duncan, 2010a, 2010b) is, therefore, of particularsignificance. Although there is some published evidence(e.g., Davidson, 1990) that the walls have not alwaysfunctioned as well as anticipated, it can be reasonablyassumed that the majority of the remediations havebeen successful, provided that (1) the wall has beenextended laterally and vertically into competent,impermeable and non-erodible bedrock; (2) there isfull lateral continuity between panels with no claycontamination; and (3) the panels themselves containno concrete segregations or slurry/soil inclusions. Itmay also be stated that the capabilities of thetechnology of the day have not always been able tosatisfy the depth criterion. EM 1110-2-1901 publishedin 1986 by the USACE (U.S. Army Corps of Engineers)states that the experienced efficiency of cut-off wallscalculated based on head reduction across the wall was90 percent or better for properly constructed walls. Theterm ‘‘properly’’ is not defined, and no update to thisinformation has since been published.

There is also the case of the diaphragm wall at WolfCreek Dam, KY, the length and depth of which wererestricted by the technology and funds available at thetime (1975–1979). As a result, a new wall, deeper andlarger, is being built to finally cut off the flow, whichhas resumed through the deep, heavily karstifiedlimestones under and beyond the existing wall.

GROUT CURTAINS

Design

Design of grout curtains based on rules of thumbwithout consideration of the site geology is not anacceptable practice or standard of care. Contemporaryapproaches are based on the concept of a quantita-tively engineered grout curtain (QEGC), which pro-vides criteria for the maximum acceptable residualpermeability and minimum acceptable dimensions ofthe cut-off (Wilson and Dreese, 1998, 2003). Prereq-uisite geological investigations and other work re-quired to perform this quantitative design include:

N thorough geologic investigations identifying struc-ture, stratigraphy, weathering, solutioning, andpermeability of the foundation rock;

N establishment of project performance requirementsin terms of seepage quantities and seepage pressures(design requirements should consider dam safety,cost, and political acceptability or public perceptionas they relate to residual seepage);

N seepage analyses to determine the need for grout-ing, the horizontal and vertical limits of the cut-off,the width of the curtain, and the location of thecurtain;

N specifications written to require best practice forfield execution of every element of the work; and

N where relevant, the value of the lost water should becompared to the cost of more intensive grouting ina cost-benefit analysis.

Quantitative design of grouting requires that thecurtain be treated in seepage analyses as an engi-neered element. The specific geometry of the curtainin terms of depth and width must be included in themodel, and the achievable hydraulic conductivity ofthe curtain must also be assumed. Guidance onassigning grout curtain design parameters and per-forming seepage analyses for grout curtains is coveredin detail by Wilson and Dreese (2003). Moresubstantial and complete guidance on flow modelingof grouted cut-offs is included in the update toUSACE EM 1110-2-3506 issued in 2008.

Construction

Many aspects of the construction of QEGCs havealso changed greatly in the last 10 years or so, drivenby the goals of achieving improved operational speedand efficiency, satisfying lower residual permeabilitytargets, enhancing QA/QC, verification, and real-timecontrol, and assuring long-term durability andeffectiveness. Particularly important advances are asfollows:

N The traditional concepts of stage grouting (i.e.,up—or down—depending on the stability andpermeability of the rock mass) and closure (i.e.,primary-secondary-tertiary phases) still apply.However, construction in two initial rows, withthe holes in each inclined in opposite directions, hasbecome standard practice.

N Multi-component, balanced, cement-based groutsare used to provide high performance mixes, whichprovide superior stability and rheological anddurability properties. The use of ‘‘neat’’ cementgrouts with high water:cement ratios and perhapsnominal amounts of super-plasticizer or bentoniteis simply not acceptable (Chuaqui and Bruce,2003).



N The current state of the art in grouting monitoringand evaluation is a fully integrated system where allfield instruments are monitored in real timethrough a computer interface, all necessary calcu-lations are performed automatically, groutingquantity information is tabulated and summarizedelectronically, program analyses are conductedautomatically by the system using numerousvariables, and multiple, custom as-built groutingprofiles are automatically generated and main-tained in real time. This level of technologyprovides the most reliable and highest qualityproject records with minimal operator effort. Infact, the advent of such technology has been foundto substantially decrease grouting program costswhile providing unprecedented levels of assurancethat the design goal is being met (Dreese et al.,2003).

N Modern drilling recording instruments and bore-hole imaging technology allow for better investiga-tion and understanding of subsurface conditionsthan was previously possible. Measurement WhileDrilling (MWD) instrumentation provides addi-tional geological information during the drilling ofevery hole on a grouting project (Bruce and Davis,2005) and not only from a limited number of coredinvestigatory holes. Specific energy and otherrecorded data can be evaluated and compared tothe grouting data to extract as much information aspossible from every hole drilled. Each hole on agrouting project is thereby treated as an explorationhole, and the data gathered are utilized to increasethe understanding of subsurface conditions. After ahole has been drilled, borehole imaging can beperformed to obtain a ‘‘virtual core.’’ This equip-ment is especially useful for destructively drilledproduction holes where recovered core is notavailable for viewing and logging, and it providesinvaluable data such as in situ measurements offracture apertures and bedrock discontinuity ge-ometry. These are then utilized in designing ormodifying the grout methods and materials. Bore-hole images are mapped by qualified personnel, andthe data may be further analyzed using stereonetanalyses.

Verification and Performance

Successfully achieving a grouting project closure isa three-step process: achieving closure on individualstages and holes; achieving closure on individual lines;and achieving closure on the entire curtain. Properclosure on individual stages and holes is primarily afunction of the following six items: (1) drilling a

properly flushed hole, effectively washing the hole,and understanding the geology of the stages beinggrouted; (2) applying that knowledge, along with theresults of water-pressure testing, to determine techni-cally effective and cost-effective stage selection; (3)selecting appropriate starting mixes; (4) real-timemonitoring of the grouting and assessment of thecharacteristics of each grouting operation; (5) makinggood and informed decisions regarding when tochange grout mixes during injection within a stage;and (6) managing the hole to completion (i.e., refusalto further grout injection) within a reasonableamount of time. The key during grouting is togradually reduce the apparent Lugeon value of thestage to practically zero. The apparent Lugeon valueis calculated using grout as the test fluid, and takinginto account the apparent viscosity of the groutrelative to water.

Pumping large quantities of grout for an extendedperiod of time without any indication of achievingrefusal (i.e., reducing the apparent Lugeon value) isgenerally a waste of precious time and good grout.Unless a large cavity has been encountered, the groutbeing used in this case has a cohesion that is too lowand is simply traveling a great distance through asingle fracture. Grout mixes need to be designedproperly for economy and value, especially inkarstified conditions.

Each row of a grout curtain, and the completedcurtain, should be analyzed in detail. Each section ofthe grout curtain should be evaluated, and closureplots of pre-grouting permeability for each series ofholes in the section should be plotted. As groutingprogresses, the plots should show a continualdecrease in pre-grouting permeability for each suc-cessive series of holes. For example, the results for theexploratory holes and primary holes from the firstrow within a section represent the ‘‘natural perme-ability’’ of the formation. Secondary holes on eachrow should show a reduced permeability compared tothe primary holes due to the permeability reductionassociated with grouting of the primaries. Similarly,the pre-grouting permeability of tertiary holes shouldshow a marked decline relative to the secondaryholes, and so on.

In addition to performing the analyses describedpreviously, it is also necessary to review profilesindicating the geology, water testing, and groutingresults. Review of the profiles with the water Lugeonvalues displayed on each zone or stage givesconfirmation that the formation behavior is consis-tent with the grouting data, and permits rapidevaluation of any trends or problem areas requiringadditional attention. In addition, this review permitsidentification of specific holes, or stages within a hole,

Seepage Barriers


that behaved abnormally and that could be skewingthe results of the closure analysis. For example, theaverage pre-grouting permeability of tertiary holesthat appear on a closure analysis plot may be 10Lugeons, but that average may be caused by onetertiary hole that had an extraordinarily high reading:averages are interesting, but spatial distributions arecritical.

Review of the grout row profiles with the grouttakes displayed is also necessary along with compar-ison of the average grout takes compared to theaverage Lugeon values reported by the closureanalysis. Areas of abnormally high or low grouttakes in comparison to the Lugeon values should beidentified for further analysis. The grouting recordsfor these abnormal zones should be reviewed careful-ly, along with the pressure testing and groutingrecords from adjacent holes.

‘‘COMPOSITE’’ CUT-OFFS

Basic Premise

In recent years, there have been a number ofprojects, both completed and in planning, that havefeatured the construction of a concrete cut-off wallinstalled through the dam and into karstified carbon-ate bedrock. The basic premise of such a ‘‘positive’’cut-off is clear and logical: the presence of large clay-filled solution features in the bedrock will defeat theability of a grout curtain—even when designed andbuilt using best contemporary practices—to provide acut-off of acceptable efficiency and durability. This isparticularly important when permanent ‘‘walk-away’’solutions are required that must be robust, reliable,and durable. There is no question that rock-fissuregrouting techniques are incompatible with satisfyingthat goal in the presence of substantial clayey infillmaterials. However, the benefits of a concrete cut-offcome at a substantial financial premium over thoseprovided by a grout curtain. A typical industryaverage cost for a grouted cut-off is of the order of$25–$50 per square foot. The cost of a concrete cut-off is anywhere from 4 to 10 times this figure,depending on the technique (i.e., panel or secant), theground conditions, the depth of the cut-off, and thechallenges of the site logistics. Furthermore, theconstruction of a concrete cut-off wall through thetypical karstified limestone or dolomite rock mass willinvolve the excavation of the rock (which in the mainpart will be in fact very hard, impermeable, andcompetent with unconfined compressive strengthvalues in excess of 20,000 psi) and backfilling thatrelatively thin diaphragm with a material of strength5,000 psi or less. In effect, great effort and expense are

expended to provide a membrane through the greaterpart of the project, which is of lower strength than therock mass excavated to construct it.

Another practical factor that has often beenoverlooked historically is that construction of aconcrete cut-off wall may simply not be feasible inground conditions that permit the panel trench-stabilizing medium (i.e., bentonite or polymer slurry)or the drill flush medium (air or water) to be lost intothe formation: in extremis, either of these phenomenacould create a dam safety threat, let alone the loss ofvery expensive excavation or drilling equipment atdepth. The solution, not surprisingly, in suchsituations has been to suspend the wall constructionand to systematically and intensively pre-treat theformation by grouting.

In doing so, however, it has not been always thecase that the designer of the wall has appreciated that,in addition to this campaign of drilling, water-pressure testing, and grouting (constituting a facili-tating improvement to the rock mass), such work alsoconstitutes a most detailed site investigation—at veryclose centers—of the whole extent of the originallyforeseen concrete cut-off area. It is reasonable,therefore, to deduce that the data from these pre-treatment programs can be used to review the truerequired extent of the subsequent concrete wall, andthereby reduce overall project costs with soundengineering justification.

The concept may then be taken a stage further.Instead of drilling and grouting being conducted onlyas a remedial/facilitating operation under emergencyconditions, it can be specified as a rigorous designconcept to:

N precisely identify the location and extent of themajor karstic features that are actually required tobe cut-off with a concrete wall;

N pre-treat the ground, and especially the epikarst,to an intensity that bentonite slurry or drill flushwill not be suddenly lost during the concrete wallconstruction (a typical acceptance criterion is 10Lugeons); and

N grout, to a verified engineered standard, the rockmass that does not contain erodible material in itsfissures around and under the karstic features (atypical acceptance criterion is in the range of 1–3Lugeons).

By embracing these precepts, it is therefore logicalto propose the concept of a ‘‘composite cut-off’’: anexpensive concrete wall, where actually required forlong-term performance certitude, plus a contiguousand enveloping grout curtain to provide acceptablelevels of impermeability and durability in those



portions of the rock mass with minimal erodiblefissure infill material.

Illustrative Examples

With one eye on the immediate future requirementsof seepage remediation involving cut-offs underdams, it may be stated that karst is either stratigraph-ically driven, or structurally related. Figure 1 shows acase where the major horizon of concern for long-term seepage and erosion is limited to the 30 feet or soof epikarst; Figure 2 is the case where the seepage anderosion concern is in a certain deep stratigraphicmember; and Figure 3 shows the condition where thekarstification has developed along discrete, verticalstructural discontinuities. For the sake of illustration,it may be assumed that the final cut-off has to be1,000 feet long, the cost of drilling and grouting is $30per square foot, the concrete wall costs $120 persquare foot, and the maximum vertical extent of thecut-off is 110 feet since a shale aquiclude exists at100 feet below ground surface (b.g.s.). The dam itselfis ‘‘invisible’’ in this exercise.

In the configuration of Figure 1, the originaldesign featured a concrete cut-off wall extending10 feet into the aquiclude. The cost would thereforebe 1,000 feet 3 110 feet 3 $120 5 $13.2 million. Thiswould, of course, assume (or worse, ignore) thatconstruction of the wall through the epikarst wouldbe feasible without its pre-treatment by grouting.

Alternatively, if the entire alignment were to be pre-drilled and pre-grouted, it would be revealed that therewas no need to construct the wall deeper than, say35 feet. The total cost of this composite cut-off wouldtherefore be:

N drill and grout: 1,000 feet 3 110 feet 3 $30/squarefoot 5 $3.3 million

N concrete wall: 1,000 feet 3 35 feet 3 $120/squarefoot 5 $4.2 millionTOTAL $7.5 million

This represents a cost savings of $5.7 million on theoriginal estimate.

For the configuration of Figure 2, the cost of thepre-drilling and grouting would be the same, i.e., $3.3million. However, in this case, the concrete wallwould still have to be $13.2 million, since the criticalzone is at depth. The overall cost of the compositecut-off would therefore be $16.5 million. However,the pre-treatment in advance of the concrete wallwould assure that the wall could in fact be built in acost-effective, safe, and timely fashion, i.e., withoutinterruptions caused by massive slurry loss. Theoverall (high) project cost would simply be areflection of a uniquely challenging geological situa-tion, i.e., a continuous horizon of erodible material atdepth.

For the configuration of Figure 3, the pre-treat-ment cost would be the same (i.e., $3.3 million). It

Figure 1. Epikarst is found during pre-grouting to an average of 30 ft b.g.s. Therefore, the concrete cut-off is installed only to 35 ft belowground surface (b.g.s.), and the grouting provides the cut-off in the ‘‘clean’’ rock below.

Seepage Barriers


would result in the identification of three discretezones of structurally defined karst of combinedarea 3 3 80 feet 3 40 feet 5 9,600 square feet.Therefore, the cost of the concrete wall actuallyneeded to cut-off these features would be 9,600square feet 3 $120/square feet 5 $1,152,000. Thetotal cost of the composite wall is $3,300,000 +$1,152,000 5 $4.5 million, which would represent a

savings of $8.7 million on the original ‘‘full cut-off’’ cost estimate.

Thus, the investment in the pre-drilling and groutingprogram in this exercise generates significant savings inthe cases of Figures 1 and 3, whereas for the case ofFigure 2, it assures that the wall, which must be built tofull depth, can be installed without massive delays,difficulties, or—at worst—creating dam safety issues.

Figure 2. Heavily karstified horizons are found at depth during pre-drilling and grouting. Therefore, the concrete cut-off is required for thefull extent. The grouting has pre-treated the karstic horizons to permit safe concrete cut-off construction.

Figure 3. Discrete karstic features have been found during the drilling and grouting, structurally driven. Thus, individual concrete cut-offpanels can be installed, after drilling and grouting have confirmed the extent of these features and have pre-treated them to permit safeconcrete cut-off construction.



Recommendations for Grouting as Part of a‘‘Composite Cut-Off Wall’’

Site Investigation Assessment and Design

The most important elements are as follows:

N Research and utilize all the historical data (includ-ing original construction photographs) that mayhave bearing on the development of a tentativegeostructural model for the site. An excellentexample is provided by Spencer (2006) for WolfCreek Dam, KY.

N Conduct a new, thoughtful, and focused siteinvestigation to test the tentative geostructuralmodel and so provide prospective bidders withthe kinds of information they truly need to estimateconstruction productivity and to quantify otherconstruction risks.

N Develop an initial estimate of the extent of thecomposite cut-off and its contributory components,i.e., the concrete wall and the grout curtain.

N Assess the adequacy of the existing dam andfoundation instrumentation, and design and installadditional monitoring arrays as appropriate.Revise the reading frequency protocols as appro-priate, especially in the vicinity of constructionactivities.

Preparation of Contract Documents and ContractorProcurement Methods

Major recommendations are as follows:

N Draft a Performance Specification (as opposed toPrescriptive Specification), and clearly define themethods and techniques that are not acceptable.Performance goals must be explicitly defined,together with their means of verification.

N Procure the specialty contractor on the ‘‘bestvalue’’ basis, not ‘‘low bid.’’

N Mandate ‘‘partnering’’ as a minimum; favor‘‘alliancing’’ as the goal (Carter and Bruce, 2005).

N Perhaps separate general construction activities(e.g., office modifications, service relocation) intoa different contract, but always leave the design andconstruction of the working platform to thespecialist contractor.

Technical Aspects

The following items are particularly important, butthe list is not all-encompassing:

N If flush water has been lost during investigatorydrilling, slurry will certainly be lost during wall

excavation, without pre-treatment in those sameareas.

N The minimum pre-treatment intensity will featuretwo rows of inclined holes, one on either side of thesubsequent wall location. The rows may be 5 to10 feet apart, and the holes in each row willtypically close at 5- to 10-ft centers (i.e., after allsuccessive orders of holes are installed). Theinclination (typically 15 degrees off vertical) willbe oppositely orientated in each row.

N The curtain should be installed to at least 50 feetbelow and beyond the originally foreseen extent ofthe cut-off to ensure adequate coverage and toidentify unanticipated problems. The treatment isto be regarded as an investigatory tool, equally asmuch as a ground pre-treatment operation and as asealing of clean rock fissures.

N ‘‘Measurement while drilling’’ principles are tobe used, the philosophy being that every holedrilled in the formation (not just cored investi-gations) is a source of valuable geotechnicalinformation.

N Special attention must be paid to the epikarst,which will typically require special grouting meth-ods such as MPSP (multiple packer sleeve pipe)(Bruce and Gallavresi, 1988) descending stages, anddifferent grout mixes.

N A test section at least 500 feet long should beconducted and verified to allow finalization of theMethod Statement for the balance of the groutingwork. A residual permeability of 10 Lugeons or lessshould be sought in the area that is later to acceptthe cut-off, and 1–3 Lugeons in elevations belowthe future cut-off toe. Conversely, a falling headtest in vertical verification holes, using bentoniteslurry as the test fluid, is an appropriate test.Verification holes should be cored, and the holesobserved with a televiewer to demonstrate thethoroughness of the grouting.

N In terms of the details of execution, the principlespreviously detailed to create quantitatively engi-neered grout curtains should be adopted. Thus, onecan anticipate the use of stage water tests, balanced,modified, stable grouts, and computer collection,analysis, and display of injection data. Whendrilling the verification holes (at 25- to 100-ftcenters between the two grout rows), particular caremust be taken to ensure that no drill rods areabandoned within the alignment of the wall, sincethis steel will adversely impact subsequent wallexcavation techniques.

N Grouting pressures at refusal should be at leasttwice the foreseen maximum slurry pressure to beexerted during panel construction.

Seepage Barriers


Construction

Every project is different, and the following basicrecommendations must be supplemented on a case-by-case basis:

N The work must be conducted in accordance withthe contractor’s detailed Method Statement. Thisdocument, in turn, must be in compliance with theminimum requirements of the Performance Spec-ification unless otherwise modified during thebidding and negotiation process. At the sametime, modifications to the foreseen means andmethods can be anticipated on every project inresponse to unanticipated phenomena. Promptattention to, and resolution of, these challengesare essential.

N As noted already, special attention is merited to thedetails of the design and construction of theworking platform. The contractor’s site supportfacilities (e.g., workshop, offices, slurry storage andcleaning, concrete operations) can be completedand the utilities extended along the alignment(water, air, electricity, light, slurry) during thebuilding of the work platform.

N The test section should be established in astructurally non-critical area that does not containthe deepest extent of the foreseen concrete wall. Thetest section should, however, be integrated into thefinal works if it is proven to have acceptablequality.

N The concrete wall excavation equipment must haveadequate redundancy, and must be supported byappropriate repair/maintenance facilities. A varietyof equipment is usually necessary (clamshell,hydromill, chisels, backhoe) to best respond tovariable site conditions and construction sequences.Standard pre-installed mechanical features, such asthe autofeed facility on hydromills, must not bedisabled in an attempt to enhance productivity.

N Special protocols should be established to ensurethat the flow of real time construction data (e.g.,inclinometer readings from a hydromill) is regular,uncontaminated, and of verifiable provenance.

N The site laboratory must be capable of accuratelyand quickly conducting the whole range of materialtests required. In addition, the contractor’s techni-cal/quality manager, who is a vital component inany such project, must be fully conversant with allthe principles and details involved in the monitor-ing of the construction, and of the instrumentationof the dam itself. In particular, expertise with panelor pile verticality and continuity measurement isessential, as is an awareness of the significance ofpiezometric fluctuations or changes.

N Emergency response plans must be established tosatisfy any event that may compromise dam safety.

Assessment of Cut-Off Effectiveness

The protocols established for observations andinstrument readings during remediation must beextended after remediation, although usually at asomewhat reduced frequency. The data must bestudied and rationalized in real time so that theremediation can be verified as meeting the designintent. Alternatively, it may become apparent thatfurther work is necessary, a requirement thatbecomes clear only when the impact of the remedi-ation of the dam/foundation system is fully under-stood. Finally, owners and designers should publishthe results of these longer-term observations so thattheir peers elsewhere can be well briefed prior toengaging in their own programs of similar scope andcomplexity.

FINAL REMARKS

We arrive at an extraordinary and unprecedentedtime in the ongoing story of major dam rehabilitationin North America. Strengthened by decades ofoutstanding but hard won success and continuoustechnological developments, contractors who special-ize in constructing concrete cut-offs through andunder operational dams now have unprecedentedexpertise to offer to an industry craving their skillsand resources. Grouting specialists—both contractorsand consultants—have emerged to bring to the NorthAmerican market a unique perspective and feeling fortheir work that is unparalleled historically andgeographically. It is time to squash the false debateof ‘‘grouting versus concrete walls.’’ The obvious wayforward is to take the best from each camp: drill,water test, and grout (relatively cheaply) to preparethe ground for a concrete wall (relatively expensive),the extent of which is now properly defined. Then,build, in improved ground conditions, the definitiveconcrete wall only in those areas where the groutingcannot be expected to be effective in the long term.

Our dams must be repaired in a way that must beconceived to be ‘‘permanent.’’ However, the goalremains that we should ensure that our designs andimplementations are cost effective. Furthermore,there is simply insufficient industrial capacity in theUnited States to build the foreseen volume of cut-offssolely by concrete wall construction techniques in thetime frame available. The concept of the ‘‘compositecut-off’’ is therefore logical, timely, and the obviouschoice. The concept is wholly consistent with aphilosophy of sustainability.



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Seepage Barriers
