This paper is reproduced from the Proceedings of the 2014 United States Society on Dams (USSD) Annual Conference, with the permission of USSD. EMBANKMENT DAM SEEPAGE MODIFICATIONS CHOICES AND CONSIDERATIONS John W. France, PE, D.WRE 1 ABSTRACT Seepage modifications are among the most common dam safety risk reduction measures. Options for seepage modifications can be grouped into two categories: 1) seepage collection and control measures and 2) seepage cut off or reduction measures. Within each category there are a number of different alternatives and configurations available to dam engineers. This paper provides an overview of the wide range of seepage modification options and argues for careful consideration of the full range of options before selecting the best option for a particular dam. INTRODUCTION Seepage and internal erosion is one of the leading causes of embankment dam failure and incidents, and, consequently, seepage modifications are a common dam safety risk reduction measure. This paper discusses the options available for dam safety modifications and the factors to be considered in choosing an appropriate modification for a particular dam. This is an overview paper, which does not address design and construction details for the various available solutions. Rather, this paper describes the overall concepts of solutions and the general construction methods for some of the solutions. The reader is encouraged to consult other references for appropriate design and construction details. The objectives of an embankment dam seepage modification can include: preventing piping and internal erosion limiting pore pressures, uplift, and seepage forces preventing slope instability and surface sloughing preventing wet spots and surface erosion limiting loss of stored water. The first four objectives may be related to dam safety concerns, while the last objective, limiting loss of stored water, is an operational concern rather than a dam safety concern. Seepage modifications can be grouped into two different categories: 1) seepage cut off or reduction alternatives and 2) seepage collection and control alternatives. It has been the author s experience that engineers often think first of seepage cut off alternatives, 1 URS Corporation, 8181 East Costilla Place, Denver, CO, john.france@urs.com, 303-740-3812
and in some cases do not consider collection and control alternatives at all. It is suggested that in almost all cases seepage collection and control alternatives should be considered in evaluation of alternatives. Even in cases when seepage cut off is selected as the primary modification alternative, it may be appropriate to consider including a seepage collection and control system to provide redundancy and reliability to the solution. In modern design of zoned earthfill dams, both low permeability cores and filters to protect the cores are included. Philosophically, inclusive of both cut off and collection / control elements in a modification would be a similar approach to providing redundancy. Seepage collection and control systems offer two distinct advantages over cut off / reduction alternatives. First, the modification can be constructed at the locations where seepage has been observed, virtually assuring that the modifications are constructed in the correct location. In contrast, the locations of seepage cut offs are often based on interpretation of subsurface information, with the inherent risk that this information is inaccurate or incomplete and the cut off elements are not located correctly. Second, construction of the elements of a seepage control system can be observed directly, increasing the reliability of verification of satisfactory construction. Cut off elements are often constructed using subsurface techniques (e.g. drilling or slurry-filled excavations), which cannot be directly observed, making verification of satisfactory construction more challenging and potentially less reliable. Of course there are some potential disadvantages with seepage collection and control alternatives. Some alternatives, for example those requiring excavation into the embankment, may require lowering the reservoir to provide for safety during construction. Other alternatives may require excavation into the foundation at the downstream toe of the dam, which may require dewatering systems for construction. SEEPAGE COLLECTION AND CONTROL ALTERNATIVES Tools available for seepage collection and control include: filters to prevent internal erosion drains to collect and convey seepage flows relief wells to reduce uplift pressures. These tools can be combined in different configurations to address the characteristics of a particular dam. A few of the more common configurations are illustrated schematically and discussed below. In Figures 1 through 8, the speckled yellow layers and zones indicate filter materials, and the gray layers and zones indicate drain materials. The illustrations presented below are schematic and do present design details. For all of these solutions filter and drain materials would need to be specified to meet modern filter design criteria, and appropriate design details would need to be incorporated to provide for appropriate function and constructability.
Filter / Drain Blanket Figure 1 illustrates a filter / drainage blanket at the toe of a homogeneous dam. This concept is applicable to situations when seepage exits the face of the embankment and / or the toe area, and there is concern that the seepage could lead to piping or internal erosion of the soils at the exit point. It is a relatively low cost option to provide a filtered exit for the seepage. In this illustration, the layer placed adjacent to the embankment and foundation soil provides the filter function, and the outer gray-colored layer would be a filter-compatible drain layer (typically a gravel material) designed to safely convey the seepage. In most cases, a drain pipe would be installed within the drain layer to collect and measure the seepage. Although the illustration in Figure 1 shows a blanket at the toe of the embankment, this configuration could also be applied to abutments or abutment groins. Figure 1 Schematic Illustration of Filter / Drain Blanket Filter / Drain Blanket and Berm Figure 2 illustrates the filter / drainage blanket from Figure 1 with the addition of a covering berm. The berm can provide two functions. First, it can protect the gravel drain layer from contamination (e.g. with windblown dust) or damage (e,g. by vandalism). Second, it can provide weight to resist uplift forces that might develop within the filter / drain layers. If the gradation of the berm material is too fine to be filter-compatible with the drain layer, a filter or separator layer is appropriate between the berm material and the drain, to prevent contamination of the drain from infiltration through the berm. Since significant flow across this boundary is not expected, the separator does not need to be particularly clean (low in fines content). In some cases, the author has used an aggregate base course as a separator, instead of more expensive filter sand. A geotextile could also be considered at this interface.
Shallow Toe Drain Figure 2 Schematic Illustration of Filter / Drain Blanket and Berm Figure 3 illustrates a shallow toe drain, an option which is applicable if there is no seepage exiting the face of the dam and either limited seepage exiting the foundation or seepage generally confined to shallow foundation strata. As shown, this option typically includes a pipe within the drain zone to allow collection and measurement of seepage. If the drain is covered with finer material, a filter or separator would be needed between the drain and the cover material, as discussed above for the filter / drain and berm configuration. Construction of this alternative may require lowering of the reservoir and dewatering in advance of the excavation. Deep Toe Drain Figure 3 Schematic Illustration of Shallow Toe Drain Figure 4 illustrates a deep toe drain, an option which is applicable if there is no seepage exiting the face of the dam and either a large amount of seepage exiting the foundation or significant seepage in deeper foundation strata. The configuration is essentially the same as that for the shallow toe drain, except that it is larger and extends deeper into the foundation. Construction of this alternative would almost certainly require lowering of the reservoir and dewatering in advance of the excavation.
Toe Drain and Berm Figure 4 Schematic Illustration of Deep Toe Figure 5 illustrates a berm added to a toe drain alternative. The illustration is shown for the deep toe drain option, but it could also be used for the shallow toe drain option. The berm can provide the same two functions as described for the blanket filter / drain alternative: protection and uplift resistance. The illustration in Figure 5 shows a chimney drain extending up the slope between the embankment and the berm, which would provide filter protection for seepage exiting the downstream face of the dam below the top of the filter. Figure 5 Schematic Illustration of Deep Toe Drain and Berm Trench Toe Drain and Berm Figure 6 shows a trench toe drain and berm configuration. The trench toe drain would provide filter protection and seepage collection for the foundation layers. In most cases, groundwater levels in the foundation would be at or near the ground surface. In these cases, the trench would likely be constructed using slurry trench excavation methods. The trench excavation would be supported by degradable slurry (e.g. guar-based slurry). The trench would then be backfilled with a soil that is filter-compatible with the foundation soil, after which the backfilled trench would be treated to revert or degrade the slurry, restoring the permeability of the filter backfill. The advantage of this method is that the trench drain can be constructed without dewatering and with limited or no lowering of the reservoir. A disadvantage is that filter / drain function can be affected by construction methods. Although trench filter drains have been successfully installed using this method, there have been cases where the slurry has not been successfully reverted or degraded,
dramatically reducing the permeability of the trench. There also have been cases where segregation of the fill or low density of the fill has been reported. Careful selection of construction materials and proper construction methods are required for successful implementation of this method. Figure 6 Schematic Illustration of Trench Toe Drain and Berm Chimney Filter Overlay Figure 7 shows a chimney filter overlay configuration. In this configuration, any topsoil or vegetation is stripped from the downstream face of the embankment. The chimney filter is then placed on the embankment and covered with a protective layer of embankment material. This alternative provides filter protection and collection of seepage through the embankment. Depending on the depth of the toe trench, it may be possible to construct this alternative without significantly lowering the reservoir. In the illustration, the top of the chimney is shown near the embankment crest. Historically, chimney filters were designed to extend above the phreatic surface estimated from seepage analyses. More recently designers have begun to extend the tops of chimney filters to the normal storage pool or even to the maximum flood pool. The reason for the higher chimney elevations is to provide protection against seepage and internal erosion through potential defects (e.g. cracks or pervious zones) located relatively high in the embankment. Figure 7 Schematic Illustration of Chimney Filter Overlay
Internal Chimney Filter Figure 8 shows an internal chimney filter configuration. This approach is applicable if it is desired to lower the phreatic surface deep within the embankment shell or if there is filter incompatibility between the embankment core and the shell. To construct the configuration shown in Figure 7 the reservoir would be lowered to provide for safety during construction. The downstream section of the embankment would then be excavated. The filter would be placed, and then the downstream shell would be reconstructed and the reservoir raised. Relief Wells Figure 8 Schematic Illustration of Internal Chimney Filter Figure 6 illustrates a relief well installation. Relief wells are appropriate for relieving high piezometric pressures in a relatively pervious layer that is confined beneath a lower permeability layer. The pressure reduction provided by the relief wells helps prevent uplift and / or blowout of the confining layer. Relief wells require regular maintenance to maintain their effectiveness, because the relief wells foul and clog over time. In some cases, a filter drain trench, as illustrated above in Figure 6, may be a suitable alternative to relief wells, without the long term maintenance requirements. Low Permeability Confining Layer High Permeability Layer With High Pressures Figure 9 Schematic Illustration of Relief Wells
SEEPAGE CUT OFF ALTERNATIVES There are three general categories of seepage cut off or reduction alternatives: grouting, low permeability blankets, and barrier walls. The term cut off has been used in this paper in quotes, because it is rare, if ever, that seepage can truly be cut off completely. A more correct term is seepage reduction. Geologic and other subsurface conditions are such that it is unusual that seepage pathways can be completely blocked to effectively eliminate seepage. The three categories of seepage reduction alternatives are discussed below, followed by a discussion of different methods for constructing barrier walls. Grouting Figure 10 is a schematic illustration of grouting. Holes are drilled into the strata to be grouted. Grout is then injected into the target strata in a controlled manner. In the author s opinion, there are limited soils that are amenable to grouting as a seepage treatment. Permeation grouting, even with chemical grouts, requires soils that are sufficiently permeable for grout to penetrate. Caution is required for grouting in soils or soft rock to limit pressures and prevent hydrofracturing, which can damage soil or soft rock strata. In rock, grout can generally penetrate only into air and water filled spaces. As a result, when grout is injected into rock joints that are partially filled with soil, the grout will fill only the open parts of the joints, leaving the soil infilling in place. In some cases, this will reduce seepage in the short term, but over time the remaining soil is eroded out of the joints, resulting in an increase in seepage. In such cases, grouting can be only a temporary solution. Figure 10 Schematic Illustration of Grouting
Low Permeability Blankets Figure 11 illustrates a low permeability blanket option. In this illustration the blanket is placed on the reservoir floor and the upstream face of the embankment. The blanket lengthens the flow path for seepage through the foundations, reducing flow, downstream gradients, and foundation pressures. The illustration is applicable to a homogeneous embankment or a zoned dam with an upstream core, so that the blanket can be connected to the principal seepage barrier within the embankment. For a central core dam, the low permeability blanket would need to be connected to the central core to be effective. While this may be practical for a new dam, it would not likely be practical for a dam safety modification. An alternative for modification of a central core dam would be to extend the low permeability blanket up the upstream face to serve as a low permeability facing. Obviously, construction of a low permeability blanket for a seepage modification would require draining the reservoir for construction. Low permeability blankets can be constructed of compacted low permeability soil (clays, silts, clayey sands, etc.) or of geomembranes of various types. Barrier Walls Figure 11 Schematic Illustration of a Low Permeability Blanket Three potential barrier wall configurations are shown schematically in Figures 12 through 14. Figure 12 illustrates a barrier wall constructed through the centerline of the dam from the crest, while Figures 13 and 14 illustrate barrier walls constructed on the upstream slope or at the upstream toe, respectively. Upstream slope or upstream toe locations would require partial or total lowering of the reservoir for construction as a modification. The upstream slope or upstream toe locations are most suitable to homogeneous embankments or upstream core embankments in which the barrier wall can easily be connected to the seepage barrier in the embankment. However, these wall locations could be used for central core dams, by constructing a low permeability blanket on the upstream slope above the top of the barrier wall. The blanket would essentially serve as an upstream low permeability facing.
In all of the installations the barrier wall reduces seepage quantities, gradients, and piezometric pressures downstream of the wall. Figure 12 Schematic Illustration of a Centerline Barrier Wall Figure 13 Schematic Illustration of an Upstream Slope Barrier Wall Figure 14 Schematic Illustration of an Upstream Toe Barrier Wall Barrier Wall Construction Methods There are a variety of methods available for construction of barrier walls, with advantages and disadvantages of each. The methods can generally be divided into four
broad categories: continuous walls, mixed-in-place walls, element walls, and jet grout walls. Each of these categories is discussed below. Continuous Barrier Walls are most often constructed using backhoes to excavate continuous, slurry supported trenches, as illustrated in Figure 15. After excavation the trench is backfilled with one of three types of backfill: soil bentonite (SB), cement bentonite (CB), and soil cement bentonite (SCB). For SB and SCB, the backfill is mixed on the ground surface, and then placed in the trench, displacing the slurry. SB backfill is developed by mixing the excavated soil with a specified amount of bentonite to produce a low permeability backfill. SCB backfill is produced by mixing the excavated soil with specified amounts of bentonite and cement to produce a low permeability backfill with cementitious properties. The cementitious properties of the SCB can provide some increased erosion resistance, however, it should be recognized that the with the addition of cement the SCB backfill sets, and care must be taken in construction to assure that the backfill does not set prematurely causing problems in placing subsequent backfill and compromising the integrity of the wall. For the CB alternative, the cement bentonite is used as the slurry and the backfill is created in the trench as the CB slurry sets. The CB is mixed in plants at the surface and placed in the trench during excavation. Advantages of continuous barrier wall construction include relatively simple construction and the lack of joints, increasing the likely of wall continuity. A barrier wall constructed with this methodology will almost certainly be continuous, unless there are trench wall collapses or backfill inconsistency. Long stick backhoes, such as that shown in Figure 15, can excavate continuous barrier walls to depths of as much as 80 to 90 feet. Figure 15 Construction of a Continuous Method Cement Bentonite Barrier Wall
Mixed-In-Place Walls can be constructed with several different types of equipment, all of which mix soil in place with cement to create cemented soil barrier walls. Three different types of equipment are illustrated in Figures 16 and 17. Figure 16 shows a deep soil mixing (DSM) device comprised of multiple augers which mix soil in place while injecting cement into the mixture. The augers are used repeated in a line to create a barrier wall. Figure 16 Deep Soil Mixing (DSM) Wall Construction Figure 17 shows two different pieces of equipment used for mix-in-place walls. The device shown on the left is a cutter soil mix (CSM) device, which is comprised of cutter wheels at the bottom of the device. The cutter is advanced into the ground and cement is injected as the cutter wheels mix the in place soil to create the barrier wall. As with the DSM device, the CSM device is used repeatedly in a line to construct a barrier wall. The device shown on the right is a trench cutting remixing deep (TRD) device, which is like a giant trenching machine which mixes soil in place with the cement. The TRD is advanced along the barrier wall alignment to construct the wall.
Figure 17 A Cutter Soil Mixing (CSM) Device (Left) and a Trench Cutting Remixing Deep (TRD) Wall Device (right) Construction with both the DSM and CSM devices involves joint connections between each subsequent advance of the device. Care must also be taken to maintain alignment of the resulting barrier wall panels to be confident that gaps are not created between adjacent panels. The TRD is a continuous construction method without joints, which may increase the likelihood of continuity; however shrinkage during curing may create small cracks in the wall after curing. The mixed-in-place methods are generally limited to somewhat more than 100 feet depth. Element Barrier Walls can also be constructed with a number of types of equipment that are used to construct either rectangular panels or circular secant piles. Figure 18 shows a clam shell excavator (left) and a hydromill (right), both of which can be used to excavate rectangular (in plan) panels. Figure 19 shows a Wirth downhole drill, which was used at Wolf Creek Dam to construct circular (in plan) secant piles. Augers and drills with and without casing can be used to construct secant piles.
Figure 18 A Clam Shell Excavator (left) and a Hydromill (right) Figure 19 A Wirth Downhole Drill for Secant Pile construction Element barrier walls can be constructed in a variety of configurations, some of which are illustrated in Figures 20 through 23. In almost all cases, element walls are constructed in sequences of primary and secondary elements. Figure 20 shows two primary and one secondary rectangular barrier wall panel elements. The primary panels would be excavated and backfilled with concrete first. Then the secondary panel would be excavated between and connected to both of the two primary panels. The panels are
normally excavated with slurry support and backfilled with concrete placed from the bottom using tremie methods to displace the slurry. P = Primary Element S = Secondary Element Figure 20 Plan Illustration of a Panel Barrier Wall Installation Figure 20 illustrates a barrier wall constructed of secant pile elements. As for the panel wall, the secant pile wall is constructed with a series of primary and secondary elements, with the primary elements placed before the secondary elements. The secant piles can be constructed with slurry filled holes or with cased holes. Cement backfill is placed from the bottom, using tremie methods. P = Primary Element S = Secondary Element Figure 21 Plan Illustration of a Secant Pile Barrier Wall Installation Figure 21 illustrates a combined barrier wall constructed of secant pile primary elements and panel secondary elements. Potential advantages of this configuration include fewer joints than an all secant pile wall and more reliable joints than and all panel wall. P = Primary Element S = Secondary Element Figure 22 Plan Illustration of a Combined Barrier Wall Installation Challenges for element wall construction include assuring element alignment and assuring joint quality. Advantages of element walls are the substantial depths that can be reached with these walls, and the ability to advance the walls into hard rock with the right equipment. Element walls have been constructed to depths of as much as 400 feet and depths exceeding 150 feet are not uncommon. The recently completed barrier wall at the U.S. Army Corps of Engineers Wolf Creek Dam, KY (Santillan and Salas, 2012) set a new standard for element alignment. Using directionally drilled pilot holes, secant piles at Wolf Creek dam were consistently drilled to depths up to 275 feet and more than 100 feet into very hard limestone to within about three inches of target locations.
Jet Grout Barrier Walls The jet grouting process is illustrated in Figure 23. A hole is advanced with drill rods. When the drill hole reaches the target elevation, a nozzle or monitor is used to cut the soil and inject cement grout. The process is continued as the monitor is retracted to create a jet grout mass. The monitor can be a single, double, or triple fluid device. The single fluid monitor uses grout to both cut the soil and create the mass. The double fluid monitor uses a combination of grout and air to cut the soil and create the mass. The triple fluid monitor uses air and water to cut the soil and grout to create the mass. A jet grout barrier wall is constructed by installing jet grout columns adjacent to and in contact with each other. With a skilled operator jet grout can be used to create barrier walls around existing subsurface features. An advantage of jet grouting is that it can be used to construct jet grout masses at depth without extending them all the way to the surface. After the jet mass is extended to the desired top elevation, the drill hole above that elevation can simply be filled with grout, rather than jetted. 1. Drilling 2. Jetting from bottom-up 3. Column completed Figure 23 Jet Grouting Process
CONSIDERATIONS From the preceding discussions, it is clear that there is a wide range of alternatives available for seepage modifications. When developing seepage modifications, the engineer should consider this wide range of alternatives in light of the specific project and site conditions. Factors that will enter into the determination of the best alternative for a particular project include cost, constructability, technical effectiveness, and operational impacts. The significance of the loss of water due to leakage and seepage can be a consideration in comparing collection / control alternatives with cut off / reduction alternatives. If water is pumped or diverted into the reservoir and the leakage impacts the quantity of water available from the project, solutions that reduce leakage may be weighted more heavily. On the other hand, if the leakage does not significantly impact water availability, reducing leakage is not of great importance. For example, for the Washakie Dam project discussed below, the project is located on the main stem of a river and the reservoir fills completely and the spillway operates almost every year. Sufficient water is available to meet irrigation demands almost every year, despite the substantial foundation leakage. Consequently, reducing leakage was not a significant consideration in selecting a modification alternative. Site conditions may significantly impact technical effectiveness. For example, consider the effectiveness of a partially penetrating barrier wall, such as that shown in Figure 24. As shown, the wall penetrates only part of the way through a pervious foundation layer and seepage will occur beneath the wall. Cedegren evaluated the effectiveness of such walls, as shown in Figure 25 (Cedergren, 1989). As illustrated in that figure, the a cut off needs to penetrate almost fully through a pervious layer to significantly reduce the amount of seepage. The wall must extend about 80 percent of the way through the layer to reduce the seepage by 50 percent, and a wall that extends 90 percent of the way through the layer only reduces seepage by just over 60 percent. Figure 24 A Partially Penetrating Barrier Wall
The influence of site conditions can also be illustrated through consideration of two example dams: Washakie Dam in Wyoming and Wolf Creek Dam in Kentucky. Washakie Dam is an embankment dam founded on glacial soils on the Wind River Reservation in Wyoming. The dam had a long history of significant seepage with some evidence of internal erosion. When seepage modifications were evaluated, both barrier walls and seepage collection and control alternatives were considered. In this case, barrier walls were eliminated from consideration because of several site conditions. First, the glacial foundations were very complex and the exact pathways for seepage were not well known. The glacial foundations included pervious layers and lenses intermixed with low permeability deposits. Confidence in the ability to construct a barrier wall in the right location was low. Second, there was no low permeability layer within practical depths, so the cutoff would be partial, at best. Lastly, the glacial deposits included large boulders, which would have made construction of a barrier wall very difficult, if not impractical. A downstream, deep toe trench and chimney filter seepage collection and control system was selected and successfully constructed at Washakie Dam (France, 2002 and France, 2005). Figure 25 Flow Net Analysis of Partially Penetrating Cutoff (from Cedergren, 1989)
Wolf Creek Dam is a homogeneous embankment dam constructed on karstic limestone foundations in south central Kentucky. The dam has a long history of seepage and internal erosion problems related to the karstic foundations (Zoccola, Haskins, and Jackson, 2009). In this case, the seepage that causes the problems is occurring through a network of joints and karst features in the foundation rock. These features are open to partially open in many locations. Consequently, there is no practical way to construct a reliable downstream filter and collection system, and such systems were eliminated from further consideration. A deep element barrier wall was selected and successfully constructed (Santillan and Salas, 2012). From the above discussion, it is clear that the engineer should enter into a seepage modification project with an open mind, not with a predilection toward either cut off solutions or seepage collection / control solutions. The full range of alternatives should be considered and evaluated in light of the specific site and project constraints. CLOSURE There is a wide range of potential solutions to seepage and internal erosion problems, which can be grouped into two broad categories: 1) seepage collection and control alternatives and 2) cut off or seepage reduction alternatives. There is no one size fits all preferred solution to all seepage issues. Each dam is unique and the seepage solution should be tailored to the specific project and site conditions for that dam. The engineer should not begin the evaluation with a basis toward either category of solution. Rather, the full range of solutions should be considered in light of cost, constructability, technical effectiveness, and operational impacts. It has been the author s experience that seepage issues can often be addressed with collection / control solutions more reliably and at less cost than with cut off / seepage reduction solutions. REFERENCES Cedergren, Harry R., 1989, Seepage, Drainage, and Flow Nets, Third Edition, John Wiley and Sons, Inc., New York, NY. France, John W., 2002, Seepage Control in Glacial Foundations - A Lesson in Humility Dam Safety 2002, National Conference of the Association of State Dam Safety Officials, Tampa, Florida, September 2002. France, John W., 2005, Washakie Dam Safety Modifications, Wyoming - A Case Study In Seepage Collection And Control In Glacial Foundations, H2GEO, Denver, CO, A Geotechnical Practice Publication, pgs. 28-42, American Society of Civil Engineers, Reston, VA 20191-4400. Santillan, Alberto Fabio and Pierre Salas, 2012, Wolf Creek Dam Foundation Remediation An Innovative Successful Solution, (2012), Dam Safety 2012, National
Conference of the Association of State Dam Safety Officials, Denver, CO, September 2012. Zoccola, Michael F., Tommy A. Haskins, and Daphne M. Jackson, 2009, Seepage, Piping, And Remediation in a Karst Foundation At Wolf Creek Dam, Managing Our Water Retention Systems: Proceedings of the 29th Annual Meeting and Conference of the U.S. Society on Dams, Nashville, Tennessee, April 2009.