a study of the long-term performance on seepage barriers.pdf

8/11/2019 A STUDY OF THE LONG-TERM PERFORMANCE on seepage barriers.pdf

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The Role of Dams in the

21st Century

26th Annual USSD Conference

San Antonio, Texas, May 1-6, 2006


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On the CoverSalado Flood Retention Dam 15R, in San Antonio's McAllisterPark, was completed in October 2004. It was the final

in a series of 14 flood control dams along the Salado Creek watershed. The dam has a detention capacity of about

3,500 acre-feet, and allows slower release of accumulated rainfall, lessening the potential for erosion and flooding

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Copyright 2006 U.S. Society on Dams

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Library of Congress Control Number: 2006924170ISBN ISBN 1-884575-39-0

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Representing the United States as an active member of the International Commission onLarge Dams (ICOLD).


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Seepage Barriers in Dams 151

A STUDY OF THE LONG-TERM PERFORMANCE

OF SEEPAGE BARRIERS IN DAMS

John D. Rice, P.E.1 J. Michael Duncan, Ph.D.,P.E.

2

Matthew Sleep3 Richard R. Davidson, PE

4

ABSTRACT

It has usually been assumed that the installation of cutoff barriers (slurry walls, concretewalls, secant pile walls, jet grouted walls, deep soil mixed walls, and sheetpile walls)

results in permanent mitigation of seepage problems through embankment dams and

foundations. Over the past year, we have collected long-term performance data from alarge number of dams that have had seepage barriers in place for over 10 years. While

most of these dams appear to be performing as expected, some have not. The most

extreme example of unsatisfactory performance we have seen so far is Wolf Creek Damin Kentucky, where a concrete diaphragm wall was installed between 1975 and 1979.

Seepage problems at Wolf Creek Dam have redeveloped over the past 25 years to levelsequal to or exceeding those observed prior to installation of the wall. Most of the

seepage at Wolf Creek Dam appears to have developed beneath and around the wall andis thought to be the result of increased hydraulic gradients in these areas.

The mechanisms leading to unsatisfactory long-term performance of earth dam seepagebarriers can generally be attributed to the buildup of water pressure behind the wall and

the associated increase in hydraulic gradient beneath, around and through the wall. The

increased gradient can lead to internal erosion and piping in the dam and foundation.

This paper will include a description of the study, a brief summary of the performance ofthe seepage barriers we have studied, and a description of the Wolf Creek Dam case.

INTRODUCTION

This paper presents a summary of a research project currently being conducted in the

Department of Civil Engineering at Virginia Polytechnic Institute and State University

investigating the long-term performance of seepage barriers in dams. The inspiration forthis study was the recurrence of seepage problems that developed at Wolf Creek Dam in

Kentucky roughly two decades after a concrete diaphragm seepage barrier was

constructed. A literature search on the topic of long-term performance of seepagebarriers through the embankment and into the foundation revealed that, with a few

1Ph.D. Candidate, Department of Civil Engineering, Virginia Polytechnic Institute and State University, 19

Patton Hall, Blacksburg, Virginia 24061; [email protected] Distinguished Professor, Department of Civil Engineering, Virginia Polytechnic Institute andState University, 104 Patton Hall, Blacksburg, Virginia 240613Graduate Student, Department of Civil Engineering, Virginia Polytechnic Institute and State University,20 Patton Hall, Blacksburg, Virginia 240614Senior Principal and Vice President, URS Corporation, 8181 E Tufts Ave., Denver Colorado 80237


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152 The Role of Dams in the 21st Century

exceptions (Marsal and Resendiz 1971; Foster et al. 2000; Foster et al. 2000), little has

been written regarding the long-term performance of seepage barriers. Thus, with thecontinued seepage at Wolf Creek and the lack of a comprehensive study of the long-term

performance of seepage barriers, the need for this study was apparent.

Research Plan

We are presently collecting data on a large number of dams that have had seepage

barriers in service for over 10 years. This information includes original dam design andconstruction documentation, reports of seepage incidents, design basis or justification for

the seepage barrier, seepage barrier design and construction information, and most

importantly, long term performance data.

The collected data is being reviewed and analyzed to identify situations in which seepage

barriers are not performing as expected and also those cases where seepage barriers areperforming well. We also plan to perform analyses to further our understanding of the

mechanisms that lead to distress or unsatisfactory performance. The analyses will bedesigned to provide understanding of the conditions that can lead to deterioration of

performance over time. It is our eventual objective to develop guidelines for the designand assessment of seepage barriers, and for monitoring and instrumentation programs for

dams with seepage barriers.

BACKGROUND

Over the last century seepage barriers have been constructed in association with dams to

impede seepage through the dams and their foundations. As far back as 1910, concrete

seepage barriers were being constructed in dams and dam foundations in California andother parts of the U.S., the U.K. and Australia. These early concrete cutoff walls

extended to depths as deep as 50 feet, were hand excavated, and were shored withwooden supports and lagging (Leventon 1930). In the 1930s steel sheet piling began to

be used for seepage barrier construction in new dam construction, and for mitigation of

seepage problems in existing dams.

The use of deep cutoffs constructed using secant pile techniques began in the early

1960s and was followed by more advanced methods of constructing barriers usingvertical elements (Ressi di Cervia 1992). One of these advanced methods, consisting of

primary drilled elements with bi-concave secondary elements filling in the windows

between the primary elements, was used in construction of the seepage barrier in Wolf

Creek Dam (Couch 1977; USACE 2005), as discussed later in this paper.

Soil-bentonite and cement-bentonite slurry wall construction techniques were developed

mainly under the auspices of the United States Army Corps of Engineers (Ressi di Cervia1992). What is likely the first soil-bentonite slurry wall in a dam was constructed in 1952


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in McNary Dam on the Columbia River in Washington (USACE 1986). Development of

these techniques continued through the 1960s and 1970s.

In more recent years, additional techniques have been developed such as deep soil

mixing, hydrofraise-type cutters, and jet grouting, which allow for deeper barriers and

more difficult construction conditions. These techniques have been used extensivelyfrom the 1980s to present day to construct seepage barriers for both new dam

construction and mitigation of seepage problems in existing dams.

MECHANISMS LEADING TO DISTRESS

When studying the performance of seepage barriers it is helpful to look at the

mechanisms that act to affect the performance of the seepage barrier and the performance

of the entire dam. The mechanisms that we have identified can all be tied to a singlebasic factor, the buildup of hydraulic pressure behind the barrier and the resulting

increase in hydraulic gradient across and around the barrier. All seepage barriers, bydesign, are intended to impede the seepage flows through, beneath or around dams. Thatimpedance will lead to an increase of head upstream of the barrier and a general decrease

in head downstream, thus increasing the hydraulic gradient across the barrier.

The potential for differential water pressure forces to develop across a seepage barrier is

illustrated schematically on Figure 1. For simplicity and ease of discussion, we have

assumed a dam/seepage barrier system where the seepage barrier is 100 percent effective.

Thus, the hydraulic pressure distributions on the upstream and downstream sides of theseepage barrier are equal to the hydrostatic pressure distributions resulting from the full

head of the reservoir level and the full head of the tailwater level, respectively, as shown

in Figure 1a. The differential water pressure acting on the seepage barrier is a uniformpressure distribution with a magnitude equal to the difference in elevation of the reservoir

level and the tailrace multiplied by the unit weight of water, shown in Figure 1b. Thus,for a 100 percent effective dam/seepage barrier system, the hypothetical differential

pressure across the seepage barrier is over 6,000 psf for every 100 feet of elevation

difference between the reservoir pool and the tailwater level. Of course, no dam/seepagebarrier system is 100 percent effective because of their finite hydraulic conductivity,

joints and inherent defects. However, this hypothetical system does illustrate the

potential for large differential pressures across a seepage barrier.

A flow chart is presented in Figure 2 that illustrates how various modes of seepage flow

can develop due to hydraulic pressure buildup. The increased hydraulic gradient affects

the performance of the dam/barrier system by way of three general mechanismsdifferentiated by location of the seepage flow: through the foundation, at the edges of the

barrier, or through the barrier.

The first mechanism is flow through the foundation soils or bedrock below or around the

barrier. This seepage pathway is not dependant on the behavior of the barrier itself but is


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affected by the vertical and lateral extent of the barrier, foundation geometry and the flowcharacteristics of the foundation materials.

Figure 1. Differential Water Pressure Forces on an Ideal (100 percent effective) Seepage

Barrier

The second mechanism is seepage through construction defects at the boundaries of theseepage barrier. Such defects could be the result of uneven bedrock interfaces at the base

and sides of the barrier or difficult tie-ins with concrete structures.

The third mechanism is seepage through defects within theactual barrier. Within thiscategory there are two modes of development of the defects: those caused by construction

defects and those caused by barrier deterioration. Construction-related defects in thebarrier may consist of poorly constructed joints or voids due to concrete segregation or

soil intrusion. The severity of such defects may later be exacerbated bybackfilldeterioration, hydraulic fracturing, or wall deformations caused by high differential water

pressure.

A major design issue for soil bentonite cutoff walls is the significant differentialsettlement of the compressible wall backfill relative to the surrounding embankment fill

or foundation material. The settling backfill tends to hang up on the adjacent soilscreating lower vertical total stresses then would develop in a fully consolidated condition.

This phenomenon of arching of the wall backfill can then lead to hydraulic fracturingunder reservoir hydraulic loading. This was central to the design of the Manasquan Dam

cutoff wall described by Khoury et al (1992). Another key issue is the erodibility of thewall backfill material under the hydraulic loads. Erosion testing as described by Davidson

et al (1992) provides the basis to select the appropriate backfill for the dam seepageconditions.

It seems plausible that where a seepage barrier penetrates a soil layer that is significantly

more compressible than the surrounding soil, high stresses can be imposed on the barrierdue to differential lateral compression resulting in cracking of the wall. Another location

where high bending stresses may be imposed on a barrier is where the barrier isembedded into bedrock or a firm base materialthat is very dense compared to the


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Figure2.

FlowChartIllustratingM

echanismsofSeepageDistress


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overlying material. Cracks through the wall would be acted upon by the third

mechanism, the high gradient across the wall. It is worth noting that both high bendingstress locations noted above also have the potential to be locations with high

susceptibility for internal erosion.

All of the above seepage pathways upon which the elevated gradient could act have thepotential to develop into excessive seepage, provided the conditions exist for developing

internal erosion and piping. The general conditions required for internal erosion and

piping to occur are: (1) there must be adequate seepage velocity to dislodge and transportsoil particles and (2) there must be a seepage pathway that is capable of allowing passage

of the dislodged or suspended soil particles. If, however, a filter is present and there is

adequate capacity downstream of the wall through drainage or a pervious shell, thenpiping will not develop. The increased seepage velocity is a direct result of increased

gradient. Therefore, the critical factor regarding development of internal erosion and

piping is the potential for a seepage pathway capable of carrying soil particles. We havehypothesized several potential mechanisms by which these seepage pathways can

develop. These mechanisms are presented on the right side of Figure 2 and are discussedbelow.

In dams that are constructed on foundations of jointed bedrock, the combination of

increased hydraulic pressures and high hydraulic gradients may work together to develop

a seepage pathway through previously tight or filled joints if the infill is erodable. Inlocations where the overburden pressure of the reservoir water is not present (i.e. in the

downstream portion of the dam) increased hydraulic pressure will result in a decrease in

the effective stress acting on joint planes. Increased water pressure in joints can result indilation of the joint which will increase the hydraulic conductance of the joint.

Furthermore, if the joint has infilling and the conductance increase is sufficient todevelop velocities sufficient to initiate erosion of the infill material, the joint conductance

will tend to further increase as presented on Figure 3. In such a manner seepage

pathways may be developed through what was previously a low conductivity bedrockfoundation.

Dam/seepage barrier systems constructed on karstic limestone foundations may be

susceptible to developing seepage pathways sufficient for removal of soil particles.Solution cavities are generally interconnected and often infilled with soil. Because the

weight of the overburden is supported by the rock the infill material may consists of low-

density material deposited by water flow in the cavities. As a result the infill may besusceptible to erosion when the velocity of water flow increases due to the increase in

gradient imposed by construction of the seepage barrier as presented schematically in

Figures 4(a) and 4(b). Grouting in solutioned limestone is often ineffective in the longterm due to soil infilling in the solution cavities that precludes grout from completely

penetrating the voids. As shown in Figures 4(c) and 4(d), pressure will tend to build up

upstream of the grout curtain imposing a high gradient across the remaining soil in thecavity. This gradient may eventually result in a reopening of the void.


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Figure 3. Development of Seepage Pathway in Jointed Bedrock with Infilling

Grout Grout

(a) Initial conditions deposition of soil at low

flow velocity

(b) Post barrier construction s oil infill erodes

due to increased hydraulic gradient and velocity

(c) Post grouting upstream pressure develops

resulting in high gradient across infill

(d) Post grouting pre-grouting conductance is

restored due to erosion of infill

High Exit

Gradient

Soil Infill

Soil Infill

Soil Infill

Soil Infill

Solution Cavity Solution Cavity

Figure 4. Development of Seepage Pathway in Limestone Bedrock with Solution Voids


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Internal erosion can also develop within a soil mass due to loss of internal stability. We

have identified two ways in which this may occur due to the imposing of a gradient: (1) adegradation of the natural filtering ability of a soil and (2) through the process of

suffusion (or reorganization of soil particles). A seepage pathway can be developed by

degradation of the natural or man-made filtering capacity of the soil adjacent to the

eroding soil. Tomlinson and Vaid (2000) performed experiments using uniform glassspheres to model soils and filters that indicate that when the ratio of the D15of the

filtering spheres is between 8 and 12 times D85of the spheres representing the soil, the

soil was effectively filtered at relatively low gradients but the effectiveness of the filterdegraded with higher gradients. Similar research on the effect of gradient on filtering

ability of soil has been performed by others (Silveira 1965; Sherard and Dunnigan 1985;

Aberg 1993; Indraratna and Vafai 1997). While the filtering behavior of natural soilsmay differ from the limits described above, the results of these experiments represent

potential susceptibility of previously stable soils to be eroded when subjected to the very

high gradients often associated with seepage barriers.

The process of suffusion or lack of internal stability of the soil may also lead to thedevelopment of internal erosion. Suffusion is an internal reorganization of soil particles

whereby fine soil particles are redeposited in open graded layers or lenses within the soildeposit (Fell, Wan et al. 2003). If sufficient capacity is available in the open or gap

graded layers, soil can be redistributed and a seepage pathway capable of removing soil

particles developed.

There are documented cases of slurry trench construction inducing hydraulic fractures in

soil embankments (Bell and Sisley 1992; Davidson, Levallois et al. 1992; Erwin andGlenn 1992; Eckerlin 1993; Bravo Guillen 1995). Depending on the erodability of the

soil surrounding the fracture, such fractures may provide the seepage pathway for erosionof soil particles.

Some of these mechanisms have been observed in long-term performance of dams thathave been investigated as part of this study, while others are only hypothesized by the

authors of this paper or others. In the course of this study we will attempt to analyze and,

in some cases, model these mechanisms in order to gain further understanding of the

geotechnical processes and factors that affect their development.

RESULTS OF DATA COLLECTION AND REVIEW

At the time of this writing, long-term performance data has been collected from 26 dams

having seepage barriers in place for over 10 years. The long-term performance of each

dam based on the information reviewed is briefly summarized in Table 1. It is apparentfrom Table 1 that, while many of the dams are performing well, there are some where the

performance does not fully meet the design intent. A specific discussion on the

performance of Wolf Creek Dam and how it relates to the mechanisms previouslydiscussed in this paper is presented in the following section.


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Table 1. Summary of Seepage Barrier Performance

Dam Year

Completed

Dam Type Foundation Conditions Pre Seepage

Barrier DistressIndicators

Seepage Barrier

Type and YearCompleted

Other Seepage

Mitigation Measures

Beaver

- 1966

Homogeneous earth embankment -

Weathered limestone and calcareous

chert

Seepage exits,

muddy flows

Secant pile wall

- 1990, 1994

None

Cherry Flat -1932

Zoned earth embankment - Alluviumover various bedrock types (Franciscan

Complex)

None originalconstruction

Hand excavatedconcrete wall

- 1932

None

Clemson

Upper-1961

Homogeneous rolled fill with chimney

and blanket drain Alluvium over Granite/Gneiss

Excessive toe

seepage, boils

Concrete cutoff

wall (panel)-1983

Relief wells, sand be

interceptor trench

Clemson

Lower

-1961

Homogeneous rolled fill with chimney

and blanket drain

Alluvium over Granite/Gneiss

Excessive toe

seepage, boils

Concrete cutoff

wall (panel)

-1982

Relief wells, sand be

grout curtain

ComancheDike

-1964

Zoned earth embankment -Alluviumwith silty, clayey sands, and gravels

Seepage flows ontoprivate lands

After initialfilling slurry

trench wall

None

Crane Valley

- 1910

Combination rock fill and hydraulic fill

unknown

None original

construction

Formed concrete

core wall - 1910

None

El Capitan 1932

Hydraulic Fill Cemented gravel over decomposed

granite


Hand excavatedconcrete wall -

1932

None

Fontenelle- 1964

Zoned earth and rock fill Jointedsanstone/siltstone/shale

Rapid piezometerlevel rise, High

seepage rates

Concrete SlurryWall

- 1986

Grouting

Jackson Lake

- 1916

Zoned earth with concrete gravity

structure - Welded volcanic tuffdeposits overlain by gravels, sands, and

silts

Seismic upgrade Deep soil mixing

columns-1988

None


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Dam Year

Completed


Barrier Distress

Indicators

Seepage Barrier

Type and Year

Completed

Other Seepage

Mitigation Measures

Lake Wolford 1895, 1924

Combination rock fill and hydraulic fill Granite bedrock

Through seepage indam

Sheet piles-1931

None

Lower

Franklin

- 1922

Zoned earth embankment - Alluvium

over shale

None original

construction

Hand excavated

concrete wall

-1922

None

Manasquan

- 1989

Homogenous earth embankment Silty

Sand

None original

construction

Soil Bentonite

Slurry Wall

-1989

None

Meeks Cabin

- 1966,1977

Zoned earth with impervious core -

Glacial outwash gravels and glacial tills

Seepage exits

downstream and

upstream sinkholes

Plastic concrete

wall

- 1993, 1995

50 horizontal drains

1984

Mill Creek- 1941

Homogeneous embankment (silt) -Interbedded conglomerate and silt over

basalt

Reservoir leakage,downstream

seepage, sinkholesin reservoir

Concrete cutoffwall

-1981

Grouting interior drablanket of reservoir

area, relief wells,grouting -2001,2002

Mud Mountain- 1941

Zoned earth and rock fill - Andesiticvolcanic agglomerate

Core deteriorationdue to erosion into

rock joints

Concrete cutoffwall

-1990

Gravity grouting,recompression grout

Navajo

- 1958,1963

Zoned earth with impervious core -

Massive, flat-lying sandstone withinterbedded shale and siltstone

Seepage

approximately 600GPM

Concrete cutoff

wall-1987

None except to

investigate seepage

New Waddel

- 1987,1992

Zoned earth embankment with

impervious core - Alluvial materials of

loose sand, gravels, cobbles, and

boulders with silty fines

None Original

Construction

Concrete cutoff

wall

- 1987,1992

None except to

investigate seepage

Private #1

- 1952

Zoned compacted earth and rock fill -

Glacial till on lacustrine on granitic

gneiss

Sand boils, sink

holes

Concrete cutoff

wall

-1991

Seepage blanket (19

and relief wells (199


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Dam Year

Completed


Barrier Distress

Indicators

Seepage Barrier

Type and Year

Completed

Other Seepage

Mitigation Measures

Saylorville- 1970

Zoned earth embankment Glacialsands and gravels over shale


CementBentonite Slurry

Wall

- 1970

None

St. Stephen

- 1984

Zoned earth embankment

Shale/limestone/sand

None original

construction

Concrete slurry

wall

- 1984

None

Sulpher Creek

- 1990

Zoned earth embankment None original

construction

Cement

bentonite slurryWall

-1990

None

Twin Buttes- 1963

Zoned earth - Sandstone overlain byfluvial gravels and wind-blown clay

Heavy seepage Soil-cement-bentonite wall

-1996, 1999

Surface drains, reliefwells, grouting (1974

Virginia Smith

(Calamus)

- 1980, 1985

Zoned earth dam - Fine-grained

sandstone overlain by surficial deposits

of fine sands and silts

Constructed as part

of original design

Slurry trench

wall

- Original

construction

Toe drains and relief

wells (1985)

Walter F.

George- 1968

Zoned earth embankment

Limestone

Sinkholes, boils

high seepage rates,Erosion

Concrete

diaphragm-1981, 1985

Relief Wells 1963,

Bedrock Grouting

Wister

- 1949

Homogeneous earth embankment-Clay

and silt overburden overlying

Interbedded shale, sandstone andsiltstone

Embankment

piping (dispersive

soils)

Concrete cutoff

wall

-1991

Grouting, relief well

Wolf Creek- 1952 Homogeneous earth embankment andconcrete gravity - Alluvium on karsticlimestone

Sink holes,wet areas,muddy flows

Concretediaphragm-1975, 1979

Grouting of bedrockalluvium andembankment


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Wolf Creek Dam

Dam Seepage History

Construction of Wolf Creek Dam in Kentucky was completed in 1952. The dam is a

5,736 foot-long combination homogenous earth fill embankment and concrete gravitystructure (USACE 2005; Zoccola 2006). A cross section through the earth fillembankment is shown in Figure 5. The foundation conditions at the dam site consist of

an approximately 40-foot thick alluvial deposit that primarily rests on top of limestone ofthe Liepers and Catheys formations. The Liepers formation is approximately 100 feet

thick and contains an extensive interconnected system of solution cavities. The Catheysformation underlies the Liepers formation and has experienced a much lower degree of

solutioning activity.

Foundation preparation for the earth embankment portion of the dam was minimal(USACE 2005; Zoccola 2006). Most of the alluvium remains in place. A minimal cutoff

trench was constructed beneath the upstream slope of the dam (see Figure 5) primarily byremoving soil from a large solution feature and backfilling with compacted earth fill.

Construction techniques in the cutoff trench were such that poorly compacted fill wasplaced on both sides of a narrow central zone of compacted soil, and large caves and

solution voids branching off from the cutoff were left untreated. The concrete gravityportion of the dam is founded on bedrock near the contact between the Catheys and

Liepers formations.

In 1967 and 1968 the dam began exhibiting signs ofseepage related distress (USACE2005; Zoccola 2006). Wet areas in the downstream toe area and muddy flows into the

tailrace were observed in addition to three sinkholes extending from the ground surface tothe top of bedrock 40 feet below. An extensive grout curtain was installed in 1968 as an

emergency measure to control the seepage.

Figure 5. Cross Section of Earth Embankment Portion of Wolf Creek Dam (after

USACE 2005)


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Because the grouting was viewed as only a temporary fix of the seepage problems, aseepage barrier was installed between 1975 and 1979, extending from the dam crest into

the bedrock for a length of 2,239 feet. Limits of the barrier are shown on Figure 6. Thebarrier was constructed in a two-phase process using steel-encased drilled pier primary

elements that were connected with secondary bi-concave elements excavated using a

clam shell excavator guided by the steel casing of the primary elements (USACE 2005;Zoccola 2006).

Figure 6. Limits of Wolf Creek Dam Seepage Barrier

Although the board of consultants for the dam recommended the barrier be constructed

through the Leipers formation and embed into the top of the Catheys formation for thefull width of the earth embankment, only two small portions of the barrier, where the

solutioning was the worst, were constructed to this level. Furthermore, the barrierextended from the concrete gravity portion of the dam to a point about two-thirds of the

way to the right abutment, leaving the rightmost third of the dam having been treatedonly by grouting. The portion of the barrier not extended to the Catheys formation was

terminated in the upper portion of the Leipers formation. The remaining depth of theLeipers formation beneath the barrier was treated by the 1968 emergency grouting

supplemented by constructing a single line grout curtain by drilling from the base of eachof the primary elements.

Performance of Seepage Barrier

Over 300 piezometers have been installed in and around the dam since the late 1960s and

over 150 of these piezometers are still being monitored (AMEC 2004; USACE 2005).Piezometers downstream of the seepage barrier immediately after construction indicated

that, although the seepage through the dam was decreased, the piezometric levels werestill elevated above the tailrace elevations. In the time period from construction to

present, many piezometers have shown a steady increase in head, with several

piezometers showing acceleration in the rate of increase since the year 2000. A survey ofthe water temperature in piezometers indicates several zones where anomalously lowtemperatures exist, likely indicating a high conductance seepage pathway from the

reservoir (AMEC 2004). Two embankment piezometers near the contact between theembankment and concrete gravity portions of the dam have elevated levels indicating

possible leakage of the wall and possible hydrofracturing of the embankment.


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Following construction of the seepage barrier, approximately 10 percent of the secondary

elements of the wall were cored. A recent review of the logs of these cores performed byAMEC (2004) revealed several construction defects in the wall. In several of the cores

honeycombing of the concrete was observed and in a few of the cores, several feet of

crushed rock was observed at the base of the wall. Additional review of construction

records indicates that a high percentage of the 1-foot-diameter wooden chase balls usedin the concrete tremmie pipe were unaccounted for and probably remain embedded in the

secondary elements. These defects represent potential windows for seepage through the

barrier.

Sixteen inclinometers installed along the downsteam side of the seepage barrier have

been monitored since construction of the wall. These inclinometers indicate downstreamdeflection of the wall has occurred (USACE 2005). Such deflection has the potential to

crack the wall as well as to create voids along construction joints and to enlarge existing

construction defects.

In addition to the above distress indicators, the following observations were also noted: Seepage areas downstream of the dam have increased in number and size from

eight seepage areas in 1968 to 37 seepage areas in 2004.

Settlement of the dam crest has been monitored from1981 to 2004. In this timeperiod the area adjacent to the concrete gravity structure has settled 0.3 feet,

which does not seem to be an exceptionally large amount of settlement.

Six out of twelve recently drilled borings encountered zones of soft soil up to 16

feet thick in the embankment fill and alluvium. Many of these zones were located

directly above the bedrock interface in areas where high pieziometric levels wererecorded. The thickest of these zones was located close to where the maximum

crest settlements were recorded.

The cable tunnel located near the downstream toe of the embankment hasexperienced seepage and cracking since the mid 1980s.

Observations indicate that the amount of seepage has increased from theriverbank downstream of the dam and the bank is experiencing slope instability.

Assessment of Mechanisms Leading to Distress at Wolf Creek Dam

It seems clear from the discussion above that the seepage barrier and bedrock grouting

program are not performing as anticipated. Based on an assessment of the performanceindicators presented above, the deterioration of the seepage barrier performance over the

last 20 years is likely due to a combination of several mechanisms acting at a number of

locations. The USACE (2005) has stated that the seepage is likely from a combination ofthree sources: seepage under the barrier, seepage around the barrier, and seepage throughthe barrier.

In the second column of Figure 2 there are three areas where elevated hydraulic gradientscan act: through the foundation, at the barrier boundaries, and across the barrier. There is

strong evidence, in the form of high piezometer levels, wet areas downstream, and low


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water temperatures in piezometers, that there is increased seepage through the foundation

bedrock both under the barrier and around the end of the barrier.

There is also evidence of defects along the barrier boundaries that provide the

opportunity for development of seepage distress. As noted above, rock fragments were

detected at the base of some of the secondary barrier elements. It is also noted that thetie-in of the barrier with the concrete gravity structure was difficult due to the steeply

sloping interface between the concrete structure and the fill. Thus this area also

represents a potential location of construction related windows.

Finally, there are several potential seepage pathways across the barrier that could be acted

upon by the elevated hydraulic gradient. First, inclinometers have indicated deformationof the wall. This deformation could act to crack portions of the secondary elements of

the barrier that are not cased in steel and could act to open or widen the construction

joints between the primary and secondary elements. Also, as mentioned above, voids andhoneycombing of concrete were detected in cores of the secondary elements.

Based on the above discussion, it appears that there may be numerous mechanisms acting

to deteriorate the performance of the seepage barrier system at Wolf Creek Dam. For thisreason, it is difficult to quantify the effects of any one mechanism or to assess the risk

that each of the mechanisms would alone represent. However, it is the opinion of the

authors that, the preponderance of seepage is occurring beneath and around the wallthough areas not treated by seepage barrier construction.

SUMMARY

Seepage barriers have been used for years as part of the original design of dams or, inmore recent years, as a means of mitigating seepage problems in existing dams. While in

the past seepage barriers have been assumed to provide a permanent mitigation of

seepage problems, recent observations at Wolf Creek Dam and other dams presentlybeing studied indicate that, in certain situations, seepage barriers may be susceptible to

deteriorating performance in the long term. It is the identification and understanding of

these situations that is the goal of our research.

In this paper we identified several mechanisms that could lead to increased seepage

through or around seepage barriers. All of the mechanisms can be attributed to a single

basic factor that is characteristic of seepage barriers - the development of high hydraulicgradients in the soil and bedrock around and across seepage barriers.

The case study of Wolf Creek Dam has been examined in detail. It was concluded thatthere is evidence of numerous seepage distress mechanisms acting concurrently. The

mechanisms thought to contribute the most to the renewed seepage are believed to be

related to seepage around and below the existing seepage barrier.


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REFERENCES

Aberg, B. (1993). "Washout of grains from filtered sand and gravel materials." Journal of

Geotechnical Engineering 119(1 Jan): 36-53.

AMEC (2004). Seepage Analysis Wolf Creek Dam, Cumberland River, Jamestown,

Kentucky, AMEC Earth and Environmental, Inc., Nashville, Tennessee.

Bell, R. A. and J. L. Sisley (1992). "Quality control of slurry cutoff wall installations."

ASTM Special Technical Publication. Publ by ASTM, Philadelphia, PA, USA. n 1129:225-234.

Bravo Guillen, G. (1995). "Hydraulic fracturing in embankment dams on first filling ofthe reservoir." International Journal on Hydropower & Dams 2(4 Jul): 71-73.

Couch, F. B. (1977). Foundation Seepage Problems at Wolf Creek Dam. Ohio River

Valley Soils Seminar, Louisville, KY.

Davidson, R. R., J. Levallois, et al. (1992). "Seepage cutoff wall for Mud Mountain

Dam." ASTM Special Technical Publication. Publ by ASTM, Philadelphia, PA, USA. n

1129: 309-323.

Davidson, R. R,, Denise G., Findlay, B. and Robertson, R. (1992) "Design andConstruction of a Plastic Concrete Cutoff Wall for the Island Copper Mine," ASTM

Special Technical Publication. Publ by ASTM, Philadelphia, PA, USA. n 1129: 271 - 288

Eckerlin, R. D. (1993). Mud Mountain Dam concrete cutoff wall. Proceedings of

Geotechnical Practice in Dam Rehabilitation, Raleigh, NC, USA. , Publ by ASCE, New

York, NY, USA.

Erwin, E. D. and J. M. Glenn (1992). "Plastic concrete slurry wall for Wister Dam."ASTM Special Technical Publication. Publ by ASTM, Philadelphia, PA, USA. n 1129:

251-267.

Fell, R., C. F. Wan, et al. (2003). "Time for development of internal erosion and piping in

embankment dams." Journal of Geotechnical & Geoenvironmental Engineering 129(4April): 307-314.

Foster, M., R. Fell, Spannagle, M. (2000). "Method for assessing the relative likelihood

of failure of embankment dams by piping." Canadian Geotechnical Journal 37(5 Oct):

1025-1061.

Foster, M., R. Fell, Spannagle, M. (2000). "Statistics of embankment dam failures and

accidents." Canadian Geotechnical Journal 37(5 Oct): 1000-1024.

Indraratna, B. and F. Vafai (1997). "Analytical model for particle migration within basesoil-filter system." Journal of Geotechnical & Geoenvironmental Engineering 123(2

Feb): 100-108.


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