Safety of Existing Dams: Evaluation and Improvement (1983)

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EMBANKMENT DAMS 213 7 Embankment Dams TYPES OF DAMS AND FOUNDATIONS Embankment-type dams have been classified in a number of different ways, but various authorities have not always been in agreement on terminology. Classification generally recognizes (1) the predominant material comprising the embankment, either earth or rock; (2) the method by which the materials were placed in the embankment; and (3) the geometric configuration or internal zoning of the cross-section. A classification modifier is often included to denote the purpose or use of the dam, such as diversion clam, storage dam, cofferdam, tailings dam, afterbay dam, etc. A formal, rigid classification is less important than an understanding of the performance characteristics and purposes of the zones and components forming the total dam. Embankment dams are constructed of natural materials obtained from borrows and quarries and from waste materials obtained from mining and milling operations. The two primary types are the earthfill dam, an embankment dam in which more than one-half of the total volume is formed by compacted or sluiced fine-grained material, and the rockfill dam in which more than one-half of the total volume is formed by compacted or dumped pervious natural or quarried stone. Earthfill Dams Homogeneous Earthfill Dams Homogenous earthfill dams are composed of materials having essentially the same physical properties throughout the cross-section. Modern homo 

EMBANKMENT DAMS 214 geneous dams usually incorporate some form of drainage zones or other elements for controlling internal saturation and seepage forces; however, many small older dams do not have these provisions. Rock toes, horizontal blanket drains, vertical and inclined chimney drains, line drains, and finger drains or a combination of these various forms have been used for these purposes. The drainage facilities are composed of pervious sand, gravel, or rock fragments separated from direct contact with the main body of the dam by properly graded filter zones to prevent migration of fine-grained soils into the drain elements and to reduce rapidly the hydraulic gradient of the seepage flow. Hydraulic Fill Dams Hydraulic fill dams are constructed of materials that are conveyed into their final position in the dam by suspension in flowing water. Originally this sluicing was assumed to sort out and deposit the coarser materials near the faces of the dam and the finer materials near the center of the cross-section. With few exceptions, dams of this type have not been constructed in the United States since about 1940 mainly because the development of large, efficient earth- moving machines has made other types of embankment dams more economical and because the seepage and structural performance of these other types are more predictable (Jansen et al. 1976). The experience record during the period 1920-1940 demonstrates the unreliability of the theory of idealized grading and sorting into pervious shells and impervious cores and the propensity for failure during construction. The vulnerability of hydraulic fill dams to accidents and failures from long- duration seismic ground motions was vividly demonstrated during the 1971 San Fernando, California, earthquake. Consequently, many old hydraulic fill dams in California have either been replaced or extremely modified and strengthened. Others, after site-specific investigations, have been declared safe and are in service. Hydraulic fill dams and earthquakes are not confined to California. Although they are no longer favored in the United States, a substantial number of hydraulic fill dams are in service in the United States and require surveillance and safety evaluation (Leps et al. 1978). Zoned Earthfill Dams Zoned earthfill dams are composed of an impervious zone or core of fine- grained soils located within the interior of the cross-section and supported by outer zones or shells of more pervious sand, gravel, cobbles, or rock fragments. Transition zones of intermediate permeability are frequently in 

EMBANKMENT DAMS 215 cluded between the core and the shells for economic utilization of all materials that must be excavated for the project and to prevent intermingling or transport of materials at the zone interfaces. Various configurations and positions of the core zone have been used. The zone may be centered on the dam axis with positive slopes or it may have a vertical or overhanging downstream slope. The selection of the various shapes is controlled by the properties and quantities of the available construction materials and the stability and seepage control objectives of the design. Diaphragm Earth Dams Diaphragm earth dams consist of a pervious or semipervious embankment together with an impermeable barrier formed by a thin membrane or wall. The diaphragm may be positioned in the embankment along the axis or on the upstream face of the embankment. The stability of the dam is supplied by the mass of the embankment, and the water retention capability is supplied by the diaphragm. Cement concrete, asphaltic concrete, and steel plate have been used for diaphragms. Stonewall-Earth Dams Stonewall-earth dams are composed of rubble-masonry walls and an earth filling. This type of dam is generally quite old—100 or more years—and of modest height. Some have only a downstream wall, in which case the upstream face of the earth filling is sloped. Others have both an upstream and a downstream wall that retains the interposed filling. The exposed surfaces of the walls are usually vertical or near vertical. The filled surfaces are sometimes battered or sloped. The walls are usually dry rubble but may occasionally be mortared. Rockfill Dams Faced Rockfill Dams Faced rockfill dams consist of a pervious rock embankment with an impermeable membrane on the upstream face. The rock mass provides stability and the membrane, or facing, retains the water. Older faced rockfill dams were constructed by dumping the rock in relatively high lifts or tips. Sometimes the rock was sluiced in an attempt to reduce settlement by washing the rock fines and spalls into the interstices of the mass and creating direct contact between the larger blocks of rock. An upstream narrow zone of 

EMBANKMENT DAMS 216 derrick-placed stone was commonly used to create a uniform surface to support the facing and to reduce the amount of movement and distress in the facing. Since about 1960 the construction procedures for faced rockfill dams have been improved considerably, through the efforts of J. B. Cooke and others, resulting in less embankment settlement and less damage to the facing. A large percentage of the rock is placed and compacted in horizontal lifts. A special zone of selected small-size rock is used to support the face. The main body of the embankment is zoned with the rock sizes in the zones increasing toward the downstream face. All but the zones of larger-sized rock are compacted by vibratory rollers or rubber-tired compactors. The largest-size rock is usually dumped in lifts of moderate height. The facings consist of reinforced portland cement concrete, asphaltic concrete, reinforced or unreinforced gunite or shotcrete, and timber. Different thicknesses, joint details, and spacing for concrete facings have been developed over the years. Newer dams of this type have been constructed with thinner slabs, reduced amount of reinforcement, minimum joint spacing, and closed vertical construction joints. Horizontal joints have been limited to those required for construction purposes. A zone of compacted fine-grained soil has been placed over the lower elevations of the facing when the dam site is V-shaped or where there is an inner gorge (Davis and Sorenson 1969). Impervious Core Rockfill Dams Impervious core rockfill dams consist of an interior impervious zone or element supported by zones of dumped or compacted rock. The interior element controls the retention of the water and is usually a compacted impervious soil protected by filter or thin transition zones. A few old dams have thin vertical concrete core walls located on the central dam axis. Depending on the position and configuration of the core, these dams are usually classified as central core, inclined core, or sloping core rockfills, and each has its own stability, seepage control, construction advantages, and site compatibility characteristics. The composition and construction of the filter and transition zones are especially critical in this type of dam because of the relative thinness of the core and the magnitude of the hydraulic gradient. Rockfilled Crib Dams Rockfilled crib dams consist of a framework of interlocked timbers or concrete prismatic bars that confine rock blocks and fragments. The water facing and overpour surfaces are usually timber fastened to the crib members. 

EMBANKMENT DAMS 217 Construction of this type of dam was common around the turn of the century, and some were later modified by the construction of concrete superstructures. The stability characteristics of a crib dam resemble those of a concrete gravity section; however, it is listed here because of its rock composition. Timber crib dams are sometimes constructed for diversion purposes. Foundations Embankment dams can be constructed on foundations that would be unsuitable for concrete dams. The foundation requirements for earthfill dams are less stringent than those for rockfill dams (Engineering Foundation 1974). Foundations for embankment dams must provide stable support under all conditions of saturation and loading without undergoing excessive deformation or settlement. The foundation must also provide sufficient resistance to leakage where excessive loss of water would be uneconomic. Foundations are extremely variable in their geologic, topographical, strength, and water retention characteristics. Each is unique and is an integral part of a dam. During design and construction the foundation characteristics can be modified and improved by such treatments as excavation, shaping, curtain and consolidation grouting, blanketing, densification, installation of sheet piling, prewetting, etc. These various forms of treatment are primarily for the purposes of (1) strengthening, (2) safely controlling seepage and leakage, and (3) limiting the influence of the foundation on embankment deformations. However, for an existing dam one can only evaluate the effectiveness of the treatment from the construction record and observable performance. Based on strength and resistance to seepage and leakage, foundations can be typified as (1) rock, (2) sand and gravel, and (3) silt and clay or a combination thereof. Earthfill dams have been adapted to all three of these types of foundations. Types 2 and 3 have generally been determined unsuitable for faced rockfill dams. Type 3 has been determined unsuitable for impervious core rockfill dams without extensive foundation excavation and treatment. The foundation types have been treated in a variety of ways depending on the designers' versatility and objectives and the type and configuration of the dam. The foundations of many existing dams will not have received any special treatment and present safety concerns. Where treatment was afforded, it varies under the different zones of the dam, depending on the intended functions of the zones and the foundation type. Foundations have been treated for seepage control by (1) earth back-filled cutoffs, (2) concrete or sheet piling cutoff walls, (3) slurry walls, (4) grout curtains, (5) vertical drains, (6) relief wells, and (7) impervious earth 

EMBANKMENT DAMS 218 blankets or a combination of these methods (Wilson and Marsal 1979). Foundations have been treated for strengthening by (1) excavating weak materials and formations; (2) consolidation grouting; (3) prewetting collapsible soils; (4) installing vertical drains to accelerate consolidation and accompanying strength gain during embankment placement; and (5) to a limited extent, vibratory densification. Foundations containing saturated, fine, cohesionless sand of low density are suspect, especially in regions of higher seismicity, because of the tendency of the sand to collapse and liquefy during long-duration ground shaking from earthquakes. Many foundations of this type have probably received no treatment for such a condition. DEFECTS AND REMEDIES It has been emphasized that dam failures are usually caused by a complex chain of events that involves one or more defects and that failure can be averted by properly identifying and remedying the defects. For embankment dams the major nonhydraulic defects causing failure ultimately involve slope or foundation structural instability and/or slope or foundation seepage instability. Closely associated defects are excessive settlement, slope erosion, malfunctioning drains, problems at the abutment or foundation/embankment interface, and/or excessive vegetation and rodent activity. Equally important threats to the overall structural or seepage stability of the dam are defects in appurtenant structures, such as spillways and conduits, and associated outlet works, such as gates, hoists, and valves. The following sections include discussions of common defects that can cause partial or total failure of the dam, indicators of these defects, possible causes of each defect, effects on the dam, methods of investigating the defects, and potential remedial measures, with brief examples of actual applications on existing dams. Table 7-1 is an evaluation matrix for embankment dams that briefly summarizes these discussions. In applying these and other remedies the complex interrelationships between the dam and its foundation, appurtenant structures, and reservoir margin must be considered. Furthermore, extreme caution must be exercised to avoid creating a new defect in the process of remedying an existing one. Slope and Foundation Instability Instability of embankment dams or their foundations may occur as a result of (1) extended periods of high reservoir level that result in high pore pres 

EMBANKMENT DAMS 219 sures within the embankment, (2) rapid drawdown of the reservoir from a high level, (3) earthquake shaking, or (4) deterioration of effectiveness of drains and other factors. Each of these conditions deserves careful attention when dam safety is evaluated. Unless an embankment shows signs of instability or high pore pressures during normal operations, there is no way to determine by inspection whether it will be stable under the above described loading conditions. Evaluating the stability of an embankment for such conditions requires determination of the strengths of the embankment and foundation materials and comparison of these strengths with the stresses that result from the loading. Failures of dams due to extended periods of seepage at high pool have occurred in at least one large dam and in a number of smaller dams. If a dam is found to be unstable for steady seepage at high pool level, the most common remedy is to install drains, relief wells, or other seepage control measures to reduce the magnitudes of the pore pressures within the embankment and/or its foundation. Rapid drawdown has caused instability in the upstream slopes of many dams, including Pilacritos Dam and San Luis Dam in California and many others. Rapid drawdown slides in embankments ordinarily do not have any significant potential for loss of impoundment, because they usually involve sliding at a depth of only a few feet in the upstream slope and do not extend through the top of the dam. In the ease of San Luis Dam, however, the sliding was considerably deeper, extending into a layer of highly plastic clay in the foundation. Although the slide at San Luis Dam did not extend through the top, deep-seated failures of this type do have a potential for doing so, and this possibility needs to be considered when stability during drawdown is evaluated. Stability during drawdown can be improved by flattening the upstream slope or by adding a layer of free-draining material to blanket the upstream slope. Earthquakes have caused instability and complete failures in dams built of loose, cohesionless (sandy or silty) soils and dams built on foundations containing such soils, which can "liquefy" or lose all strength under cyclic loading. Examples include Sheffield Dam and Lower San Fernando Dam in California. Sheffield Dam failed completely as a result of liquefaction of loose sands in the foundation, and the entire reservoir was released. Lower San Fernando Dam suffered a deep-seated upstream slide that extended through the top of the dam and lowered the top to within about 3 feet of reservoir level. The reservoir was lowered as quickly as possible, and complete failure was avoided by a narrow margin. Usually when a dam is found to be unsafe during an earthquake because of a liquefiable foundation, the remedy is to build another, more stable 

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EMBANKMENT DAMS 224 Defect Possible Possible Effects Potential Indicators Causes Remedial Measures F5. Obvious Lack of log Overtopping Establish Obstruction visual booms or of dam or maintenance/ indicators trash racks spillway inspection Improper walls program sizing Damage to Trash racks Unanticipated spillway and log booms trash burden structures Modify sizing Remove source of obstructing material (G) Spillways and outlets G1. Faulty Normally, Settlement Loss of Realign and gates and gates and Corrosion control of replace as hoists hoists are Initial spillway necessary operated to misalignment release Provide test working Vandalism Could result protection order in inability against Visual to drain tampering inspection reservoir to and vandalism for prevent corrosion structural or and to seepage ensure that failure all Could result components in are present overtopping and in good failure order G2. Obvious Inadequate Loss of Establish Obstruction visual trash rack, log outlet maintenance/ indicators booms capacity inspection Collapse (see could result program above in Trash races, possible overtopping log booms causes) failure of Vandalism dam (see above effects) G3. Obvious Poor design Loss of Replace or Inaccessible visual Bridge control of strengthen gate controls indicators overload reservoir structural Pier releases component displacement Loss of Counteract from earth control of earth load loads spillway Convert dry Unstable gate releases tower to wet tower tower Inundated or Provide blocked roads alternate access 

EMBANKMENT DAMS 225 Defect Possible Possible Effects Potential Indicators Causes Remedial Measures G4. Obvious Lack of Structural Security Project visual barriers damage fences insecurity indicators Infrequent Interference Intrusion from Rocks in site with flow alarms tampering stilling basin visitations by Inoperable Periodic site and Missing or owner controls visits vandalism damaged could Putting locks control stem preclude on wheels, etc. emergency emergency spillway controls releases or usually not opening advisable bottom drain, resulting in dam failure (H) Outlet works H1. Visual Misaligned Piping of Sleeve Leaking inspection or pipe due to embankment conduit and conduit water leaks settlement or soil, may grout annulus into pipe poor result in Grout entire Visual placement sudden and conduit and inspection for Inappropriate complete provide new soil deposits in connections failure of dam outlet pipe Improper Loss of Grout outside TV inspection bedding storage perimeter of for above if Improper Loss of conduit pipe not backfill conduit Move control accessible to compaction gate to direct Corrosion of upstream end inspection pipe to Sinkholes in Electrolytic depressurize embankment loss of pipe For eroding surface over or metal pipe, remove near alignment Erosion of source of of conduit conduit eroding Discharge Uplift of drop material and/ temperature/ inlet or pave chemistry Vibration bottom of pipe measurements (water impact Provide (rarely or water additional conclusive) hammer) anchorage if Pipe thickness Downstream needed to measurements control only resist uplift (ultrasonic) Modify hydraulically to eliminate vibration Provide cathodic protection 

EMBANKMENT DAMS 226 Defect Possible Possible Effects Potential Indicators Causes Remedial Measures H2. Piping May be Inadequate Possible Install along difficult to compaction sudden and filtered drain conduit detect of backfill complete around outlet Seepage/soil Improper failure of dam of conduit deposits at bedding Loss of Grout outside downstream Leaks in storage perimeter of end of pipe conduit (see Loss of conduit if Sinkholes in above conduit significant embankment possible loss of surface over causes) embankment or near soil is alignment of thought to conduit have occurred Deformation Provide of conduit cathodic due to loss of protection soil support Voids between pipe and soil at outlet (I) Reservoir margin I1. Mass Indications of Complex Obstruction Removal of movement cracking, array of of emergency unstable (landslides) creep, possible spillway material distortion of geologic Damage to Rock bolting reservoir factors outlet works Buttresses margin Earthquake Wave Other triggering damage to classical Rainfall dam landslide triggering In severe treatment Reservoir cases, techniques rise damage (movement) of embankment I2. Seepage Loss of Complex Loss of Blanket on from margin storage array of storage reservoir of reservoir Emergence of possible In rare cases, bottom seepage geologic failure of In rare cases, downstream factors reservoir grouting of reservoir margin by In rare eases, piping provide filtered drain to prevent piping NOTE: This table considers only the major nonhydraulic considerations for existing embankment dams. It is not meant to be comprehensive. Also, the complex interrelationships between the embankment, foundation, appurtenant structures, and reservoir margin must be kept in mind. 

EMBANKMENT DAMS 227 dam downstream, abandoning the original embankment. If an embankment itself is potentially unstable during an earthquake, it may be replaced, as was the ease with Upper San Leandro Dam (Gordon et al. 1973), or it may be possible to strengthen the dam with a buttress fill, as was done for San Pablo and Henshaw dams. In the case of Henshaw Dam in southern California, the reservoir level was lowered and a reinforced rockfill embankment was constructed to buttress the downstream slope of the dam and to retard outflow from the reservoir in ease the old dam failed. A foundation that contains loose liquefiable soils may be densified by deep compaction techniques, such as vibroflotation, compaction piles, or heavy tamping, or it may be excavated and replaced. Drainage to reduce liquefaction susceptibility has been considered in some cases. Dams built of cohesive soils on stable foundations have been found to perform quite well during earthquakes, and they pose a much smaller hazard than do dams constructed of or founded on loose, liquefiable, cohesion-less soils (Seed et al. 1977). Even though they may remain stable during an earthquake, cohesive soil embankments may suffer permanent deformations as a consequence of earthquake shaking, which may take the form of bulging of the slopes, bodily movement of the dam, and possibly settlement of the top. Causes of Slope Instability During Operation Once an embankment dam has been constructed to full height and has been stable for a period of time, there are (at least) three conditions that may result in subsequent instability: 1. Rapid drawdown of the reservoir may result in instability of the upstream slope. There are no reported eases where this type of failure led to loss of impoundment. 2. High reservoir levels for extended periods may result in high pore pressures in the embankment and its foundation and in instability in the downstream slope. One such case has been reported where this type of failure led to loss of impoundment, although the failure might also have been due to seepage erosion and piping (ASCE/USCOLD 1975). 3. Earthquakes subject the embankment to transient forces and may also result in loss of strength in loose cohesionless soils. Methods of Assessing Stability of Existing Dams The fact that a dam has been subjected to the most severe conditions (draw- down, sustained high reservoir level, or earthquake) expected during its life 

EMBANKMENT DAMS 228 without suffering failure or accident is the strongest possible indicator of its ability to withstand similar conditions in the future. Even then, a careful examination of the dam, searching for signs of actual or incipient instability, is an absolutely essential part of any evaluation of stability for an existing dam. If a dam has not been subjected to the most severe conditions expected, evaluation of its safety requires collection of data to estimate the strength properties of the embankment and the water pressures to be expected in the embankment during the event. These data can be obtained through examination of as-built drawings, construction records, and test results for record samples, if available, combined with field explorations, field tests, and laboratory tests. Existing piezometer records can be evaluated and new piezometers installed if necessary to obtain needed information. In a case where investigations indicate that design assumptions were conservative or correct with regard to strength and pore pressures, additional stability analyses are not necessary. If the investigations indicate lower strength or higher pore pressure than used in design analyses, additional analyses will be needed to determine if the stability of the embankment will be adequate. Analyses used to determine the stability of embankment dams are discussed in the Stability Analyses section. Remedies Some earthfill dams were constructed with unreinforced concrete face linings under the assumption that the embankment was much more pervious than the lining and would thus result in a low phreatic surface in the embankment. These linings have cracked and admitted water in sufficient quantities into the embankment so that the raised phreatic surface reduces the embankment stability unacceptably. The design concept is no longer accepted as valid. In some cases the stability has been restored to the desired degree by installing a filter and drain blanket beneath a new portland cement or asphaltic concrete lining so that seepage through the lining no longer saturates the embankment. In another case a terminal storage reservoir was completely lined with a compacted impervious clay lining, which was protected from weathering by a 3-inch asphaltic concrete covering. The impervious earth lining was used as protection against seepage losses and instability in the geologic formations comprising the reservoir bowl. The purpose of the asphaltic covering was to preserve water quality and to aid in periodic cleaning of the reservoir. Large areas of the lining failed by sliding due to reservoir drawdown. The slide material was removed and a 

EMBANKMENT DAMS 229 system of line drains and toe collector drains installed in the formation. The system was drained through an existing drain outfall serving the upstream impervious zone of the earthfill dam. The clay blanket and asphaltic covering were then replaced. Excessive Settlement Settlement due to consolidation or compression of embankment and foundation soils usually continues at a slowly decreasing rate after construction of an embankment, and this action may lower the top below its design elevation. Such a condition can be detected readily by level surveys of the dam crest or by settlement observations. Usually dams are constructed with excess freeboard or ''camber,'' in order that some amount of settlement may occur without lowering the top below design level. If excessive settlement occurs and the top is too low, it can be raised by adding fill or a concrete parapet wall to the top of the dam. In extreme cases the foundation soils may be so weak and compressible that it is not possible to raise the top to the desired level simply by adding fill, because the embankment would be unstable. This was the case at Mohicanville Dike No. 2 (appurtenant to Mohicanville Dam in Ohio), which was constructed on soft peat and clay and had been about 12 feet below its design level for many years. Methods considered to raise the dike to design elevation were (1) stage construction to raise the embankment slowly, while the foundation soils gained strength through consolidation; (2) construction of a concrete parapet wall atop the dike, together with a concrete diaphragm wall cutoff; and (3) use of geofabric reinforcing in the embankment to permit construction of a less massive dike imposing smaller loads on the foundation, together with an upstream seepage cutoff. Transverse vertical cracks in homogenous earthfill dams have resulted from differential settlement caused by foundation profile irregularities and by consolidation upon saturation of the embankment or of a granular foundation. Nonplastic soils placed at below-optimum water content are especially vulnerable because they are brittle and highly erodible. One successful remedy has been the placement of a flexible, impervious asphaltic membrane on the upstream face together with a shallow trench at the upstream edge of the dam crest backfilled with a compacted mixture of soil and bentonite. The backfilled trench serves to intercept the wider, more defined cracks that are usually confined to shallow depths, while the membrane serves to seal off any narrow or incipient cracks that might extend more deeply. The embankment face is shallowly stripped, rolled, and bladed. A heavy asphaltic impregnated hemp mesh is anchored and a sprayed asphaltic emulsion is applied, followed by a thin sand coating and 

EMBANKMENT DAMS 230 a second penetration coat of sprayed asphalt. A 2-foot normal thickness protective blanket of fine-grained soil is then placed over the membrane commencing at the bottom of the slope. Older dumped rockfill dams have characteristically settled relatively large amounts, especially from the initial application of the water load during first filling. Where the facing is reinforced concrete, this settlement has often cracked the slabs, closed and spalled the central vertical joints and the horizontal joints, torn waterstops, and opened the terminal vertical joints and any perimetric joints. This disruption of the facing has usually been most severe in the lower elevations and has resulted in leakage that in some instances has been very large (hundreds of second-feet). Fortunately, this type of embankment dam is very resistant to large leakage flows as long as the foundation is nonerodible. Where there is concern for safety or economic loss of water or where circumstances are psychologically disturbing, the leakage can be reduced and controlled by placing a compacted berm of a well-graded clay-silt-sand-gravel mixture on the facing to an intermediate height. If the reservoir cannot be taken out of service, a gravel-clay-bentonite slurry can be deposited under water on the face with a wheeled enclosed skip cart. The skip cart travels on the face and is remotely opened when at the discharge position. The conventional tremie method is suitable when the required volume of the mixture is smaller. Slope Erosion Embankment dams, particularly if they are relatively old or did not have proper slope protection measures incorporated into their original construction, are subject to slope deterioration from erosion of both their upstream and downstream faces. Such erosion does not necessarily lead to catastrophic dam failure or even a major safety problem. However, if it continues to occur and corrective measures are not taken, serious consequences could develop because the embankment cross section would continue to be reduced, often at the most critical elevations. As noted in Chapter 5, the potential for rapid erosion of dispersive clays calls for special care in investigation where the presence of such clays is a possibility. The erosional problems most commonly encountered are those created by wave action on the upstream dam slope and by improperly controlled runoff from precipitation on the dam top and/or downstream slope. Areas of contact between the embankment and abutments and the embankment and appurtenant structures are especially vulnerable. However, if the public has access to a particular site for recreation purposes, erosion of both upstream and downstream slopes and the dam crest can be aggravated by 

EMBANKMENT DAMS 231 repeated foot or vehicular traffic, causing rutting and loosening of soils, which become susceptible to further erosion during succeeding runoff from rainfall or snowmelt. Downstream slope erosion is readily recognizable at any given time in the form of shallow or deep gullying caused by runoff concentrations on the slope or at the embankment-abutment contact. On the other hand, upstream slope erosion may or may not be readily recognizable, depending on the level of the reservoir as related to the eroded area. Improperly protected upstream slopes can erode rapidly and severely from periods of heavy wave action, but it is possible that such erosion could temporarily go undetected, particularly if the reservoir is rising, until the water level later drops to expose the damaged area. This latter type of erosion usually results in combined "benching" and oversteepening of portions of the upstream slope and could lead to more serious slope instability if not corrected in time. Remedies Upstream slopes severely benched by erosion can be restored by removal of loose surface materials, slope grading, and replacement with compacted fill. A cushion or bedding layer of properly graded sand and gravel or small rock is then placed on the restored slope and covered with a layer of sound, durable riprap (graded rockfill), properly sized and dimensioned to suit the wave conditions expected to be encountered. As a substitute for large, graded rockfill, which may not be readily available in the proper size and soundness, commercially available gabions or slope protection mattresses can be placed on the embankment face and filled with smaller rock to provide equivalent protection. If erosion of the underlying embankment is a concern, a suitable geotextile filter fabric can first be placed on the slope to prevent migration of the fine soil particles from beneath the gabions. The general characteristics and uses of geotextiles and allied products are briefly described in the section Seepage and Piping. Fabric forms have been successfully used for erosion-controlling revetments on embankment, excavated, or natural slopes instead of the more expensive or unavailable conventional rock riprap (Lamberton 1980). The revetments are made with a two-layer fabric woven together at tie points to form a quilt-like envelope. The tie points can be spaced on grids from 5 to 10 inches on centers to create "pillows" analogous to stone of different sizes. An open weave is used at the tie points to join the layers together and to form weep holes for relieving hydrostatic pressure behind the revetment. 

EMBANKMENT DAMS 232 The revetment is installed by first anchoring the upper edge in a trench along the top of the slope with mortar injected into the end of the fabric envelope. The fabric is then rolled down the slope and pumped full of mortar, expanding the envelope into a pattern of individual nodules or pillows. The mortar may be either a sand-cement or a pea gravel-sand-cement mix. Although the upper layer of fabric may be gradually degraded by ultraviolet radiation and abraded by erosion, the lower layer is bonded to the mortar and provides both a filter to retain the underlying soil and flexible tensile reinforcement to help hold the mortar pillows in position. For some subsoil conditions a filter layer may be required below the fabric. Other common means of upstream slope protection in areas where rockfill is not readily available are asphalt or portland cement concrete and soil cement. Each type of protection requires its own unique design and bedding conditions. In some smaller less important storage structures, under certain conditions of reservoir size, operating conditions, dam face slope, and protection from prevailing winds, grass slope vegetation has been used as a relatively inexpensive erosion protection measure. However, this latter means must be viewed as a less reliable long-term approach and would normally require more frequent maintenance. Other, less common and usually more expensive means of upstream slope protection, such as commercially manufactured liner fabrics or sheeting, are also available and are certainly worth considering under the proper site conditions and environment. Pacific Gas and Electric Co. has had good experience with a number of installations of a gunite membrane. This is also highly effective as a rodent inhibitor. Downstream slope erosion, in the form of gullying, can be repaired and its recurrence prevented by excavating the eroded areas to provide working room, refilling and compacting the eroded and excavated areas (placement of impervious materials over large areas on the downstream slope should be avoided), then placing a protective layer of angular rock or stream gravels and cobbles on the slope. Often, vegetative slope protection, in coordination with proper slope drainage, will provide a relatively inexpensive means of slope protection. A system of concrete-or asphalt-lined surface drains, either preformed or east in place, can be installed on narrow berms and used in conjunction with a protective cover of planted grasses to provide the required level of protection. Maintenance, in the form of initial watering and periodic mowing, is often necessary with this type of slope cover. The means of erosion protection discussed above for both upstream and downstream embankment slopes can be used equally as effectively on native soil slopes adjacent to the dam embankment if erosion of such slopes threatens to undermine the dam embankment or create a more serious 

EMBANKMENT DAMS 233 slope stability problem related to the dam abutment or associated hydraulic structures. Any surface erosion that seems to be resulting from water seeping through the dam embankment or abutment may be indicative of a much more serious dam stability problem and should be quickly and thoroughly checked by a qualified engineer to determine the need for further study and remedial action. Seepage and Piping All dams, regardless of type, leak to one degree or another. Seepage may be through the foundation or the embankment, along the foundation- embankment interface, or in any combination of those paths. The seepage volumes may be substantial or barely noticeable. The water may be transporting suspended or dissolved solids. In some cases the seepage may be entirely harmless; in others, it may be extremely serious and immediate treatment becomes imperative. Whether such leakage can or will lead to serious stability problems or will require expensive repairs depends on many factors, some of which may be related to the original embankment design (for example, inadequate seepage cutoff measures) and some of which may be related to other factors, such as undetected foundation conditions or improper construction procedures or control. Whatever the cause, a developing seepage problem is often quite evident from visual inspection and/or changes in piezometer readings (if such devices have been installed in the dam) and, if not given proper, timely attention, can lead to serious and expensive problems or even catastrophic failure of the dam. Where treatment is necessary, in the interest of safety, various remedies are available. The choice of remedy is controlled by many factors, including the quantity, path, pattern, and gradient of the seepage flow; the configuration of the dam; its zoning; the characteristics of the foundation formations and geologic structure; the engineering properties of the materials composing the embankment; the foundation treatment and materials placement procedures during construction; and the financial feasibility of comparative costs. It is not possible and, in many instances, is undesirable to stop the seepage completely. Instead, the objective is to control the forces and actions of the water that would otherwise adversely affect the stability and integrity of the dam-foundation unit. The major forces and actions are softening, saturating, solutioning, pressuring, internal erosion, and transporting. These factors may occur in various combinations to produce abnormal conditions not contemplated in the design of the structure and which cannot be 

EMBANKMENT DAMS 234 adequately resisted. For example, saturation and pressure may reduce the strength of the materials in an embankment zone sufficiently to cause a slide. Or solutioning and transporting may remove materials from a foundation formation, causing its collapse and disruption of the embankment. Almost all of these forces and actions are created in some way by the hydraulic gradient of the water as it travels into, through, and out of the dam and foundation. Accordingly, the fundamental achievement of a remedy must be the control of the hydraulic gradient within tolerable limits. The hydraulic gradient can be controlled by barriers of low permeability, adjacent zones of increasing permeability in the direction of flow, lengthened travel paths that increase friction losses, and forced flow through anisotropic foundation formations in the direction of lesser permeability. All the remedies for seepage employ one or more of these techniques. Piping, a form of internal embankment erosion, is caused by the progressive movement of soil particles from unprotected exits due to uncontrolled seepage emerging from an abutment or embankment slope. Piping occurrences are a very common cause of dam failures. Areas around and adjacent to conduits are particularly susceptible to piping because of the difficulty in properly compacting fill around these conduits. A near-failure of Daggs Dam in Arizona in 1973 was attributed to a long- term piping problem associated with a damaged low-level outlet conduit. Repairs included the removal and replacement of the damaged conduit and surrounding embankment material as well as other measures to improve the dam's performance. Piping problems have also developed at dams with many years of satisfactory performance due to solution of soluble materials, such as gypsum or limestone, within the dam foundation. Similar problems in both embankments and foundations have resulted from animal burrows and rotted tree roots. Embankment cracking due to differential settlement can also provide paths for uncontrolled seepage and progressive internal erosion. In fact, abnormal seepage through either the dam or foundation, from whatever cause, even if it does not cause a piping problem, can lead to high hydrostatic uplift pressures or unanticipated uplift pressure distributions. These pressures can, in turn, lead to the formation of boils and springs in, or downstream, of the dam. Also, through the reduction of shearing resistance, these excessive hydrostatic pressures can cause the failure of slopes and abutments. Pervious foundation seams, adversely oriented bedding planes, and open-jointed foundation rock can all provide paths for uncontrolled seepage. If the seepage path is through erodible material or dispersive clays, rapid failure can develop from a small initial seep. 

EMBANKMENT DAMS 235 How can seepage problems with potential serious consequences be placed under satisfactory control? There are a number of remedial measures available for such problems, depending on the nature and location of the seepage and on how rapidly the situation is recognized and given the attention it deserves. Examples of successful remedial measures for these more common defects are given below. It must be recognized, however, that in each specific ease the details will differ and that remedial construction must be adapted to the actual conditions. Embankment Seepage Corrective Measures Treatment for control of hydraulic gradients in the embankment varies with the type of dam. The need for such treatment is rare for rockfill dams because of the mass and high permeability of the downstream zones and resistance to turbulent flows. The treatment for zoned earthfill dams will usually be limited to added zones of higher permeability at the downstream face because the impervious zones are buried and unreachable, although the addition of barriers of low permeability within the upper elevations of an existing impervious zone may sometimes be feasible. Seepage through so-called homogeneous earth dams, where permeability is relatively high or where leakage may concentrate through anomalous regions or transverse cracks, can be controlled by treatment either, or both, upstream or downstream. Upstream treatment may consist of placing a compacted, more impervious zone on the stripped upstream face of the existing dam. The reservoir must be emptied. If the presence of the impervious blanket on the upstream slope presents a stability problem during reservoir drawdown, the slope can be flattened by adding a pervious zone surmounting the added impervious zone. If the reservoir cannot be emptied, in some cases a layer of bentonite pellets placed underwater may be effective. If the defect includes excessive seepage through the foundation or along the interface with the dam (often the result of inadequate foundation preparation originally), the new impervious zone can be extended in the form of a cutoff trench or a slurry trench excavated in the bedrock formation across the valley section and into the abutments along the upstream toe of the dam or in the form of an upstream blanket. Time must be allowed for accumulated silt deposits to dry unless excavating by dragline is possible. Partial cutoffs in stratified alluvial formations can reduce the hydraulic gradient by forcing the flow across the strata in the direction of lesser permeability. If seepage emerges uncontrolled along the toe or over the lower portion of the downstream face, a berm or mildly sloping zone of sand and gravel 

EMBANKMENT DAMS 236 or cobbles and rock fragments may be added to that face. The grading of the materials positioned immediately against the dam and abutments must be much more pervious than the material upstream and must also prevent movement (piping) of fines from the dam or foundation. If pervious material of the requisite grading is scarce or costly, the main body of the added mass can be comprised of other types of materials, if they are enveloped by pervious materials at all interfaces. The effect of the berm on the stability of the downstream slope must be considered. While it will normally enhance stability, it may not. If the seepage is largely concentrated along the toe or groins, a drain pipe of clay tile, sewer tile, asbestos-bonded corrugated metal pipe (CMP), or PVC pipe, successively enveloped by gravel and by sand or filter fabric, can be installed in a trench excavated into the foundation along the toe of the dam. If the drain can be safely installed on an alignment upstream of the toe, it will be more effective, especially for slope stability. Transverse cracking in homogeneous dams can be repaired if the causative forces have stabilized or have attenuated with time. One method was discussed above. When the cracks are limited to the higher elevations in a dam, as they usually are, a narrow trench can be excavated from the top of the dam and backfilled with impervious plastic soils. The reservoir may have to be drawn down or even emptied during repair. The strength of the backfill materials must be adequate; otherwise a critical failure plane may be induced by the backfilled trench. Reinforced plastic fabrics, anchored or buried along their perimeters and placed on a smooth prepared surface on the upstream slope and covered by a protective element, can also be considered. Excessive leakage caused by disruption of the concrete face elements of a rockfill dam can be reduced or eliminated by selective removal and replacement of damaged panels, if the waterstops from adjacent panels are serviceable. If the embankment is still settling at a significant rate, the repair process will have to be repeated several times. The damaged panels can be covered with courses of redwood tongue and groove planking for increased flexibility during the active settlement period. Anchored butyl rubber sheets have been used successfully on the surface of the panels to waterstop the panel joints. A leaky rockfill dam can be modified to include an inclined earth core by using the existing dam for the downstream shell and constructing transition zones, filter zones, impervious zones, and shell elements upstream. The opportunity for improved control of foundation seepage, if necessary, is available in such an alteration. The upper sections of embankments that are riddled with tree roots or rodent holes can be restored by complete removal of the infested portions 

EMBANKMENT DAMS 237 and by replacement with compacted fill securely bonded to the unaffected portions. Foundation Seepage Corrective Measures Seepage through foundations can be controlled by grouting, blanketing, new cutoffs, drainage, and pressure relief wells. Usually the exact nature of the problem will have to be investigated and defined before the most appropriate means of treatment can be identified. Possible remedies follow. • A grout curtain can be installed beneath the impervious zone of an embankment dam by drilling through the dam. Care must be used to avoid hydraulic fracturing of susceptible fills with the drilling fluid. Injection of grout between the foundation surface and the base of the embankment should be done carefully. Different techniques are available. Portland cement, bentonite, and chemical grout mixes are the three most common types for seepage cutoff applications. • An impervious blanket of compacted earth or a commercially available liner can be placed on the floor of the reservoir. The blanket must be joined to the impervious element of the dam and to the abutments and must terminate in a satisfactory manner. • The construction of a new cutoff and an impervious facing was described earlier in this chapter. A new cutoff can also be formed in alluvial deposits with a slurry wall. The wall must be joined to the impervious element of the dam. A horizontal impervious zone (blanket) can sometimes be used. Driven sheet piling cutoffs have been used with limited success. • Embankment toe drains and drain blankets also were described earlier. The toe drain or part of the blanket drain can also be installed at depth in the foundation for dual service. • Pressure relief wells or trenches backfilled with drain rock and filter material can be drilled or excavated along or beyond the toe of an embankment dam to control the escape gradients of seepage flowing through the foundation. • In rock abutment formations, both grouting and drainage curtains can be formed by holes drilled from tunnels or galleries. Use of Synthetic Fabrics Geotextiles, or synthetic fabrics, are gaining acceptance in various kinds of construction. These materials may be manufactured from fiberglass, nylon, polyester, polyethylene, polypropylene, or polyvinylchloride and may be woven or nonwoven. Some are available either reinforced or nonrein 

EMBANKMENT DAMS 238 forced for different exposure and installation or service requirements. The predominant polymers used in civil works are polyester and polypropylene (Koerner and Welsh 1980; Timblin and Frobel 1982). Although not strictly meeting the definition of a geotextile, closely allied products composed of similar synthetic materials have been developed and are being used for impermeability and for surface protection. Impermeability prevents saturation and uneconomic water loss by controlling seepage and leakage. Surface protection provides erosion control on slopes and bed and bank protection in water conveying channels. These products are the impervious flexible membrane liners and fabric forms manufactured from synthetics, including polyvinylchloride (PVC), chlorinated polyethylene (CPE), and chlorosulfonated polyethylene (CSP). Some are available either reinforced or nonreinforced for different exposure and installation or service requirements. Current purposes served by geotextiles in construction are (1) filtering, (2) drainage, (3) separation of dissimilar materials or zones, and (4) reinforcement. Filtration arrests the movement of finer soil particles from a protected layer or zone of soil of lesser permeability to one of greater permeability. Drainage promotes the controlled passage of water in the plane of the geotextile either vertically, horizontally, or inclined to an outfall or drain line. Separation prevents the intermixing of adjacent layers or zones of materials of dissimilar grain sizes. Reinforcement helps stabilize a fill or embankment by supplying tensile strength to the mass. In the improvement of existing dams, these fabrics have a potential for drainage and filter protection that has yet to be developed fully. They can be used as a boundary membrane beneath riprap or gabions to avoid washing of the underlying embankment or natural slope. In many cases drainage of some kind will be found to be the solution of a deficiency at an earthfill. Sands and gravels have been used commonly to create the necessary filters and drains. Such materials may be in short supply and/or may require expensive washing and screening. Geotextiles may offer some benefits in such cases. Although for various reasons they may not be acceptable as complete substitutes for natural materials, their potential for cost saving can be considered. Since their overall record of performance worldwide is relatively short, the ability of the fabrics to endure and to retain their capabilities for a long time is not yet known. However, especially in remedial work on existing dams, where they may not have to be buried deeply and irretrievably, geotextiles may have suitable applications. From the descriptions of the functions of geotextiles and these allied products it can be seen that they can have applications in the correction of defects in existing dams and their foundations. Following are two examples. 

EMBANKMENT DAMS 239 1. In 1979 a low earthfill dam located in Martin County, Florida, used as a cooling water reservoir, failed from water seepage through the foundation (see Figure 7-1). This foundation defect was corrected by an upstream bentonite slurry wall and by a downstream geotextile drainage system. The slurry wall was formed in a 14-foot-deep trench, 12 inches wide, joined to the impervious dam by an impervious blanket. The drainage system was constructed by first trimming back the downstream face and excavating a trapezoidal trench into the foundation along the toe of the steepened slope. A layer of sand was spread over the trench surfaces and part way up the slope and was covered with a layer of filter fabric. A 12-inch layer of gravel was spread on the fabric, and a 12-inch perforated bituminous pipe was placed in the trench and covered with additional gravel. A second layer of fabric was placed over the gravel on the slope and in the trench. The entire drain system was then covered with compacted fill to a new, flatter downstream slope. The 12-inch drain pipe is connected to a series of round concrete sumps that collect drain flows for discharge by a float-controlled pump system (Civil Engineering- ASCE 1981). This example demonstrates the application of several remedial measures for seepage control both in a foundation and in an embankment—the slurry wall and the connecting impervious blanket upstream; the inclined embankment drain and foundation trench downstream. Two basic functions of geotextiles are also demonstrated—filtration for the first layer and separation for the second layer. Also demonstrated is the increased slope stability from the flatter, modified slope in addition to that obtained by the elimination of embankment pore pressures downstream of the inclined drain. 2. The 11,530 acre-foot upper reservoir at Mr. Elbert Forebay, Colorado, (USCOLD 1981) of a large pumped-storage power facility was created by closing the open margins of a topographic depression with a 90-foot-high earthfill dam and a small dike at the power plant intake-outlet located directly above the valley in which the power plant is situated. The reservoir margin on the valley side is a series of lateral moraines in which there are old landslide scarps. The reservoir bowl was originally lined with a 5-foot-thick compacted earth blanket. The blanket extends to an elevation 3 feet above the maximum reservoir water surface, including the side of the forebay formed by the moraines. Soon after the first partial filling of the forebay, water levels in the piezometers and observation wells placed along the valley side began rising significantly. From a study of the situation it appeared that enough water might possibly seep through the blanket into the morainal formations and adversely affect the stability of the valley side. A flexible membrane liner was installed over the entire 290 acres of the reservoir bowl. A 45-millimeter, reinforced, chlorinated polyethylene liner 

EMBANKMENT DAMS Figure 7-1 Geotextiles (filter fabric) used to encapsulate sand and gravel filter in repair of dam for cooling water reservoir in Indiantown, Florida. Source: Civil Engineering-ASCE (1981). 240 

EMBANKMENT DAMS 241 was used. Riprap and other slope protection material were first removed from the earth blanket. The top 2 feet of the blanket were excavated and screened to obtain minus 1-inch material for the membrane subgrade and earth cover. The reservoir slopes were trimmed to 3:1 or flatter to facilitate placement of the earth cover over the liner and for stability. The bedding for the liner was placed and compacted to obtain a smooth surface with two passes of a pneumatic-tired roller followed by two passes of a vibratory steel roller. The liner was anchored by planting it in a 1-foot-wide by 2.5-foot-deep trench at the top of the reservoir side slopes. The liner was unfolded and positioned manually by work crews. All field seams were overlapped, chemically cleaned, and then sealed with solvent adhesives. The liner is protected from weathering, vandalism, animal traffic, and ice action with an 18-inch earth cover. The earth cover is protected from erosion on the slide slopes by coarse gravel and bedded riprap. It was unnecessary to line the upstream face of the earthfill dam so the membrane liner was terminated by planting it in an anchor trench in the dam embankment along the toe of the slope (USCOLD 1981). Slurry Walls Excessive seepage in a dam may be remedied by installing a trench or slot filled with an impervious material along the dam's axis, working from or near the top. In some instances it may be desirable and feasible to extend the seepage barrier into the foundation to remedy foundation seepage problems. Caution must be used in employing a slurry wall or membrane for controlling seepage in a dam. The wall obviously must not be placed downstream from the centerline, since uplift pressures will increase upstream from the wall. Furthermore, the wall may introduce a definite plane of weakness through the dam that could result in structural failure of the dam, especially if the phreatic surface is not significantly lowered by the barrier. Another consideration is cracking (and, possibly, significant movement) of the soils along the wall as the transition from at-rest to active earth pressures occurs during and after wall installation. Cracking of the wall itself can occur if the backfill is not sufficiently plastic. The reservoir should be drained prior to wall installation. Embankment and foundation slurry wall installations must be closely monitored, refilling the reservoir should be done slowly, and piezometers should be installed to monitor seepage pressures. Three basic techniques exist for installing slurry walls. 1. The ''trench'' method uses a backhoe, dragline, clamshell, or similar equipment for excavating a relatively wide (1.5- to 2-foot) trench. During 

EMBANKMENT DAMS 242 excavation the trench is kept filled with a bentonite-cement slurry to support the walls of the trench, so the excavation is done "in the wet." After excavation the slurry is left in place to form a relatively impermeable seepage barrier. This wall type is relatively simple to construct. Problems can include difficulties with quality control, controlling depth of excavation, segregation of materials in the slurry, and the necessity of disposing of excavated materials that may be semiliquid in form (due to mixing with the slurry). A variation of the bentonite-filled slurry trench method consists of excavating panels in a bentonite slurry, using piles spaced a few feet apart as guides for the excavating tools, then tremieing concrete into each panel to construct a concrete diaphragm wall. 2. The "ditch" method uses a rapid-acting ditching machine that has multiple rotating buckets (normally 10 to 12 inches wide) that feed the excavated material onto a conveyor belt for loading and disposal. This method also employs a bentonite-cement slurry to support the excavating process and to form the relatively impermeable barrier. Quality control and disposal problems may be reduced by this method as compared with the trench method. 3. The "injection" method uses pressure jetting a bentonite-cement or other impervious slurry through ports in a beam. Multiple contiguous insertions of the jetting beam are made along a line to form a relatively impermeable thin membrane. A successful development of this concept uses a vibrating pile driver and an H-beam with jetting pipes welded inside the flanges; each insertion of the vibrating and jetting H-beam partially overlaps the previous insertion to ensure continuity and alignment of the seepage cutoff wall. Injection is continuous during insertion and extraction. Quality control is exercised at the slurry batch plant and by monitoring the pump pressures during jetting. The width of the membrane is sufficient to control seepage. Barriers of this type have been installed to depths of over 100 feet, and some have been installed through relatively dense and gravelly materials. Where the seepage water is chemically reactive with bentonite-cement (such as storage lagoons for some chemical-processing facilities), chemically resistant slurries have been developed and used. This technique would not be suitable where large boulders exist or where hard rock layers have to be penetrated. Following are several examples of slurry walls in dams. • Trenching Method 1. Razaza Dam, Iraq. A slurry trench wall was installed in 1969 in this 30- foot-high dam and through about 30 feet of aeolian and fluvial foundation soils to remedy excessive seepage problems. The trench was excavated 

EMBANKMENT DAMS 243 with a kelly mounted hydraulic grab bucket to form a wall about 20 inches wide, up to 80 feet deep, and 8,200 feet long. The slurry used consisted of cement (C/W = 0.10), bentonite, and local sands and silts, mixed to a paste with a water content of about 35 to 40%. After slurry wall installation and reservoir refilling, piezometers indicated a 90% seepage head loss across the slurry wall (Japan Dam Foundation 1977). Other examples of successful remedial use of bentonite-cement slurry trenches are at the Eberlaste Dam, Austria; Kranji Dam, Singapore; Laguna Dam and others in Mexico (Japan Dam Foundation 1977). 2. Wolf Creek Dam, Kentucky. Construction was completed in 1951 on the 259-foot-high, 3,940-foot-long homogeneous earth embankment portion of this dam (flanking a concrete gravity overflow section). The foundation includes up to 40 feet of alluvial soils overlying limestone. In 1967 muddy seepage discharge and sinkholes developed, caused by piping of cavity-filling soils in the limestone foundation and collapse of overlying fill. In 1968-1970 a remedial limestone grouting program beneath a 250-foot-long section adjacent to the concrete gravity section was successful in treating that immediate area, but it was decided that a positive cutoff in the foundation was needed for an additional 2,000 feet beneath the embankment section. A 2-foot wide, 260-foot- deep concrete panel wall was installed by a modified slurry trench technique. The lower 100 feet of the wall had to penetrate cavernous limestone. The wall was constructed by sinking steel casings on 4.5-foot centers, excavating between the casings in panels using a specially designed clamshell bucket working in a bentonite slurry mix, then tremieing portland cement concrete (3/4- inch maximum aggregate size, 6- to 8-inch slump, 3,000 psi compressive strength) into the panels. A sequence of five stepped easing sizes (26 to 51 inches) had to be used because of the depth and rock conditions. Rock excavation was accomplished with percussion chisels, expandable biconcave chisels, and clamshell buckets (USCOLD Newsletter 1977). This remedial work was completed in 1979 and is reported to be performing satisfactorily (Fetzer 1979). Similar concrete panel construction is being done at the Waiter F. George Dam in Alabama, Clemson Lower Diversion Dam in South Carolina, and at other locations. At the Clemson project, panels 20 feet long and up to 85 feet deep are being employed to intercept seepage in alluvial sands and gravel deposits in the foundation. At the Walter F. George dam the panels are extended up to 100 feet deep, into pervious limestones underlying the dam. • Martin County Power Plant Cooling Pond Dike, Florida. A piping failure of a portion of this 20-mile-long dam in 1979 was caused by excessive foundation seepage. Remedial work included installation of a 10-inch 

EMBANKMENT DAMS 244 wide bentonite-cement partial cutoff by using a rapid ditching machine to excavate a slurry wall about 20 miles long. The slurry ditch was installed at the upstream toe of the dike, 14 feet into permeable foundation sands, down to a porous shell layer, and was tied into an upstream blanket. The slurry ditch is thought to be effective in reducing foundation seepage problems. • Martin-Marietta Sodyeco Chemical Plant Waste Lagoon Dike, North Carolina. A 3- to 6-inch-wide vibrated beam bentonite-cement slurry wall about 2,500 feet long was installed to depths of 25 feet through permeable alluvial foundation sands and gravels to intercept anticipated foundation seepage. The vibrating beam also penetrated underlying residual micaceous sandy silts having standard penetration resistances of 20 to over 100 blows per foot. Penetration refusal depths for the jetting vibrating beam closely corresponded to power auger refusal depths of the site investigation test borings. At the time of this report, insufficient head has been put on this wall to determine the effectiveness of the cutoff. Vibrated beam slurry walls have been put in numerous dam foundations in the United States and in Europe, prior to dam construction, with good performance. At chemical waste lagoons near Romulus, Michigan, and Richmond, California, chemically resistant asphaltic slurry walls have been successfully installed using this technique. Repair of Timber Facings A number of older rockfill dams were originally faced with tongue and grooved dimensioned lumber secured on timber sleepers set in vertical chases in the upstream slope of the rockfill that was usually hand or derrick placed. These facings gradually rotted or sometimes were quickly destroyed by fire when the reservoir stages were low. The water retention capability has been restored by removal of any facing remnants, except the sleepers, followed by the application of a thin (3 to 4 inches) gunite membrane facing. The membrane is reinforced for temperature with steel mesh or by an orthogonal system of reinforcing bars. The membrane is locally thickened and more heavily reinforced as a beam horizontally across the sleeper locations. The membrane is joined to the existing concrete of the foundation cutoff by shooting the gunite into the cleaned-out grooves in which the original timber facing had been installed. If the existing foundation cutoff is unservicable or ineffective, a new cutoff slab doweled to bedrock might be feasible. Of course the reservoir must be fully emptied or locally cofferdammed to expose the lowest elevations of the top of the cutoff. If the reservoir stage can be suitably controlled for the inflows anticipated during the selected 

EMBANKMENT DAMS 245 construction season, the facing remnants can be removed and the new facing installed by working from barges or rafts floating on the falling and rising reservoir stages. Because the embankment has already undergone most of the time- dependent settlement due to its own weight and the settlement from initial water-load application, any residual settlement will be very small. Consequently, the facing will not be significantly stressed except by temperature and possibly earthquake. Several dams in California owned by the Pacific Gas and Electric Company and by the Nevada Irrigation District have been successfully restored in this manner and are serving satisfactorily many years later, one for as long as 52 years. Overtopping In the engineering of permanent embankment dams, overtopping has been strictly avoided, for sound reasons. While the advantages of earthfills are well known, their vulnerability to erosion unless properly safeguarded is basic. There is also ample record of the dislodging of rockfills subjected to overspill. An almost inviolable rule in the design of these structures has been to keep them free of superimposed conveyance structures, whether they be spillways, fish ladders, or pipelines. Any such facility that would obscure the embankment from inspection or conceal underlying adverse conditions has been regarded as objectionable. These rules are well founded, and any deviation from them should not be taken lightly. However, confronted with the current reality of numerous dams of this type that are inadequately protected from flood damage and considering the insufficiency of funds for immediate full-scale increase in spillway capacity by traditional methods, the engineer may be obligated to weigh the merits of passing water over the embankment, at least as a temporary expedient. Remedial Measures Various ways have been proposed to protect embankments during overtop- ping. Principal among these have been either armoring or reinforcing the embankment. Also, the use and/or contribution of parapets have been proposed to prevent the overtopping. Protective Armor. Pravidets and Slissky (1981) have discussed the comparative effectiveness of riprap, packed edge-to-edge concrete cubes, and precast reinforced concrete slabs. Their work on a test spillway chute at the 

EMBANKMENT DAMS 246 Dnieper power station and their experience at a full-scale facility at the Dniester power development have demonstrated that important cost savings are obtainable by installation of precast slabs. These are reinforced wedge-shaped concrete members with drain holes, laid on the slope in shingle fashion. The Dnieper test section was 46 feet wide and 118 feet long, installed in the spillway at the dam on a 6.5:1 slope. Dimensions of an individual slab or panel were 9.8 by 6.6 by 2.3 feet. Tests reportedly were conducted with a drop of 115 feet, unit discharge of about 640 cfs per foot of crest, and mean velocities as high as 75 feet per second. At the Dniester project the crest and downstream face of an embankment cofferdam 23 feet high and 820 feet long were covered with concrete slabs fastened together, each of these having dimensions of about 33 feet by 15 feet by 20 inches thick. This overflow facility was reported to have passed flood peaks with unit discharges as high as 140 cfs per foot. Both of these structures performed without significant problems. The stability of this protective armor is enhanced by its multistepped profile and the favorable hydrodynamic pressure on the concrete surface. The drain holes and an underlying filter drain contribute substantially to stability and seepage control. A stepped toe is provided at the toe of the overflow structure for erosion protection and to ensure proper entry of the jet into the tailwater, with an unsubmerged hydraulic jump. Advocates of this method of embankment protection believe that it is amenable to construction under a range of site and weather conditions. The slabs can be repaired readily. Reinforcement. The experience already accumulated in the steel reinforcement of rockfill diversion dams shows the way to possible applications to other embankment barriers. Certainly, even in the case of comparatively resistant rockfills, extreme care must be exercised in designing for overtopping. Most critical is the protection of the downstream slope and especially its toe, which will be exposed to the potentially destructive velocities of both surface and seepage flow. In projects where the effectiveness of reinforced rockfill diversion dams has been demonstrated, essential elements have been (1) a membrane or zone to ensure relative impermeability, (2) safeguards for erosion protection of the top of dam and the upper part of the upstream slope, (3) containment of the rockfill on the downstream slope, and (4) anchorage of the steel reinforcement to prevent bursting of the contained fill by seepage pressures. Various designs have been adopted for such reinforcement. The most successful are those that provide complete enclosure of the surface rockfill units by steel mesh that is deeply anchored in the dam, commonly by hori 

EMBANKMENT DAMS 247 zontal steel bars tied to anchor plates. Side anchorage into the abutment is also important. The gabions, or gabion-like elements, may be of large size and may therefore require internal rib-bars and tie-bars to maintain the geometry of the steel-mesh cage. In some cases, also, lean concrete has been placed in the toe of each armoring unit for shape retention. The rock material enclosed in the cage may have a range of gradations (e.g., 3 to 6 inches) depending on what is produced by rock excavations at the site. Obviously, the confined stones must be large enough so that they will not be washed through the mesh. The use of roller-compacted concrete might also be a solution. Parapets. Although, in the most widely accepted practice, parapets on embankment dams are not intended to have water stored against them, such encroachment has happened on many projects and has in some cases become an approved part of the operating regimen during extreme floods. The addition of parapets and the changes in their purpose over a period of years may be part of the evolution of a project as conditions and demands change. Among the important changes may be the updating of hydrologic data or the development or application of different techniques for hydro-logic analysis since the project was placed into operation. Parapets originally designed as ornamental features or for residual free- board may have only marginal structural capability. This could include (1) comparatively thin, sometimes dry masonry, walls with only a modest layer of mortar added to the upstream face; (2) thin gunite walls placed on steel mesh against a single vertical form; or (3) timber walls. Any of these must be analyzed carefully for structural adequacy against the surcharge water load. This may not be easy if knowledge is incomplete regarding the parapets materials and details, including bonding, anchorage, reinforcement, and tying to the adjoining structural elements. Not infrequently strengthening is found to be necessary. The importance of a competent parapet cannot be overemphasized if it is depended upon to support stored water, even temporarily, and if the top and downstream slope of the embankment are vulnerable to erosion. Malfunctioning Drains Adequate embankment and foundation drainage is one of the most important aspects of maintaining a stable embankment dam. If a proper, well- functioning drainage system is originally incorporated into an embankment dam, it is quite possible that this system will continue to perform adequately throughout the entire service life of the structure. However, it is also possible that a drainage system that performed satisfactorily during 

EMBANKMENT DAMS 248 the early life of a dam may become plugged or broken, or otherwise malfunction, thereby creating excessive uplift (internal hydrostatic pressures) within the dam and foundation. It is also possible that an outward appearance of drain malfunction can be created not by changes in the drains themselves but by deterioration of the dam embankment or foundation, permitting increased seepage that the drainage system was not designed to handle. On the other hand, silt deposits in the reservoir may reduce the seepage appearing at foundation drain outlets. It is extremely important, therefore, that any situation that appears to involve a deterioration of drainage capacity be thoroughly evaluated by an experienced engineer before corrective measures are defined and implemented. One excellent means of inexpensively providing information that can be highly useful for diagnosing a potentially deteriorating drainage system is to channel to one or two locations and regularly measure visible seepage flows emerging from the dam toe area and any installed drains. This should be done over as wide a range in reservoir levels as possible, so that a relationship can be established between reservoir level and anticipated drainage flow rate. Any significant changes in the flows defined by this relationship may be cause for further investigation. Drain water samples can also be taken for chemical testing if it is suspected that chemical or bacterial reactions are involved in changing drain flows. If flow reductions are found to be occurring, backflushing of the drains can sometimes alleviate the problem. If flow increases are occurring, the water chemistry testing might indicate foundation solution activity. Other corrective measures for malfunctioning drains or inadequate drainage can assume a number of forms. Excessive uplift resulting from inadequate control of seepage can be reduced to acceptable levels. If there are foundation drains and formed drains in the dam that have become plugged with chemical deposits, they can sometimes be reamed and their effectiveness restored if they are accessible from drain galleries or from the top of the dam. New foundation drains, both vertical and horizontal, can be drilled. If water losses are excessive, the foundation can be regrouted from galleries, if they exist, or from the top of the dam, but usually the more effective way to reduce uplift is by the addition of drainage. Foundation-Embankment Interface The foundation-embankment interface is a critical area from the principal standpoints of both overall stability and seepage prevention and control. A poor bond between embankment and foundation can lead to piping by creating a favored seepage path along the contact. Improper or incomplete 

EMBANKMENT DAMS 249 treatment of foundation joints and fractures, alone or coupled with an inadequate filter relationship between the embankment and the joints, can also lead to embankment piping and eventual collapse from internal erosion. Inadequate stripping of loose or otherwise undesirable foundation materials can create a weak plane along which major embankment instability could occur. Where one or more grout curtains are constructed beneath a dam, it is imperative that continuity of the impervious dam zone and the grout curtain be maintained. This cannot be achieved without proper treatment of the foundation- embankment interface. Once a dam embankment is constructed, it is obviously rather difficult, and sometimes impossible, to correct construction deficiencies along the foundation- embankment interface without first removing the embankment material from the problem area. As discussed earlier, problems such as incomplete treatment of foundation fractures and joints can sometimes be remedied by grouting through the dam embankment if the deficient areas are discovered and their extent defined, and they can be corrected before a major failure occurs. The physical features of a defect at the foundation-embankment interface usually are not directly observable because they are hidden by the dam. The presence of the defect characteristics must, therefore, be deduced from indirect as well as direct evidence, obtained instrumentally or from drilling cores and logs and a study of visual manifestations, such as dissolved solids in seepage water or movements in the dam itself. For this type of problem, evaluation by an experienced engineer is essential, but even if the problem is properly defined, the cost of its solution may be very high. Trees and Brush Trees and brush are frequently allowed to grow on the slopes and tops of embankment dams. These forms of vegetation should be removed, especially for small dams, for the following reasons: • Potential for loss of freeboard and breaching if trees on the top are blown over during high-water conditions. • Potential dangerous loss of dam cross section if trees on or near the slopes are blown over. • Potential initiation of leakage by piping if trees die and root systems rot to become channels for flow. • Obstruction of visibility and access to hamper observation and maintenance of embankments. 

EMBANKMENT DAMS 250 Each of these potential problems is considered in the following paragraphs and recommended general criteria for removal are included. As a general rule, trees and brush should be removed from, and their growth prevented on, dam and dike embankments. Vegetation on slopes should consist of grass and should be cut at least annually so that there can be effective monitoring for animal burrows and seepage. Trees and brush adjacent to embankment slopes should be cut back at least far enough to permit such observation and to allow access to the toes of the slopes by maintenance equipment. Tree root systems will vary with soil type and groundwater conditions. The following general comments, subject to further consideration at each site, are offered for general guidance: • Root systems of usual tree types do not grow into the zone of saturation, i.e., below the steady-state phreatic surface. One result is that trees in swamp areas are shallow rooted and easily blown over. • The spread of root systems is generally comparable to the spread of the branches but will vary with tree type and soil conditions. • Root system penetration tends to be as follows: Pine: typical mat depth of 1 to 2 feet, maximum of 2.5 to 3 feet. Softwoods: generally shallow rooted. Oak: both deep and shallow rooted, typically 2 to 5 feet maximum mat depth (in glacial till likely to be 1 to 3 feet, more typical in loose- to-medium-compact, fine sand). Maple: 10 to 20% shallower than oak, typically 1 to 2.5 feet for major part of mat. Ash: relatively deep rooted but less dense mat than oak. Birch: relatively shallow rooted, typically 1 to 2 feet maximum mat. Criteria for removal of individual trees and stumps should also consider the potential for damage due to the root systems. Living or dead trees whose uprooted root systems could endanger a dam or dike should be cut. This would include trees that could damage upstream slope protection and trees on a crest where uprooting could leave less than a 10- or 19-foot width of undamaged embankment. Trees on or near a downstream slope should be removed if their root systems can penetrate significantly into the minimum necessary embankment cross section. Stump and major root removal should also be based on potential for damage. Major root systems in the top of the dam or within a minimum embankment section offer some potential for embankment damage by decay and should probably be removed. However, the decay will generally be to humus rather than to a void. There is some possibility of inside root de 

EMBANKMENT DAMS 251 cay with attendant potential for water flow, more so with hardwoods, and it would be prudent to remove major roots that traverse the top of a dam. Stumps and root systems that are not in critical locations can be left to rot in place unless removal is necessary for slope grading. The soil will gradually close in. In connection with tree work on dam and dike embankments, the following additional comments are offered: • The integrity of trees that remain in place should be considered—their root systems may be damaged by adjacent work, or they may be more exposed to wind. • Cutting shallow roots to limit growth will not work without a barrier. Cutting will stimulate additional growth. • Vegetation must be reestablished in work areas to protect embankments from erosion. • Backfill material after stump or root removal should have characteristics similar to the embankment material at that location. Rodents and Other Burrowing Animals The burrowing of holes in earthfill dams by rodents is a widespread maintenance problem. This problem is known or suspected to have caused several failures of small dams. The animals that have caused the most problems are beavers, muskrats, groundhogs, foxes, and moles. Beavers and muskrats cause the largest problem because they operate below the water level. They sometimes burrow holes below the water from the lakeside all the way through the dam. Frequent visual inspections of the earthfill embankment should be made to detect the presence of animals or the holes they have made. If the presence of these animals is detected in the vicinity of the dam area, the animals should be eradicated by either trapping, shooting, or with poison. If they have made holes that are carrying or could carry water through the dam, these holes should be immediately repaired by excavating and recompaction or by filling with a thick slurry grout. STABILITY ANALYSES The stability of an embankment dam, in conjunction with its foundation, must be evaluated from a number of different standpoints, as can be appreciated from the preceding discussions concerning the many potential defects that can create unstable conditions within the structure-foundation system. Among the various methods of stability analyses available to the 

EMBANKMENT DAMS 252 engineer is the conventional analysis of slope stability, which, although it is a valuable tool in the assessment of embankment adequacy, must be performed and used with experienced judgment or it can produce completely misleading results that could lead to erroneous, even disastrous, conclusions regarding the safety of a dam. This is true because, as with most other types of numerical analysis, the final results are only as valid as the data used as input to the computations. In the case of earth dams and their foundations, the input data themselves are often subject to fairly wide ranges of interpretation simply because the engineer is working with native materials that have been altered in varying degrees by the forces of nature. Especially in the ease of the analysis of stability of existing embankment dams, the exploration, sampling, laboratory testing, and materials properties evaluation program always has physical and economic constraints that limit the extent of knowledge that can be gained about the important physical properties of any given structure. For this reason the physical properties data that must be input to a numerical stability analysis are always subject to varying uncertainties that must be put in their proper perspective for each individual case. It is in this critical area that experience, as well as engineering judgment, are critical to the performance of the numerical analysis and evaluation of results. Even when the results of an analysis appear favorable, they cannot be viewed in a vacuum but must be integrated with all the other information available on the safety of the particular structure, thus becoming an important part, but still only a part, of the overall stability evaluation. Methods of Slope Stability Analysis Various methods of slope and foundation stability analyses are available. The more common ones are two-dimensional and are based on limiting equilibrium. These analyses are known by a variety of titles, including slip circle, Swedish circle, Fellenius method, method of slices, and sliding block. There are differences in assumptions and force resolutions in the different methods. When forces representing earthquake effects are included, the analysis is often termed pseudostatic. An analysis is made by assuming some form and location of failure surface, such as a circular are, compound curved surface, or a series of connected plane surfaces. The configuration and positioning of the surface depend on the kind of embankment dam, the internal zoning, and the foundation's geologic structure. For example, connected plane surfaces are often used for an inclined or sloping core rockfill dam. Also, the trial failure 

EMBANKMENT DAMS 253 surfaces are positioned judgmentally to pass through weaker or more highly stressed regions. Thus, a plane surface may be positioned in a confined fluvial foundation susceptible to high pore pressure. The most critical surface is defined as the one having the least computed factor of safety. The factor of safety is considered to be the ratio of forces or moments resisting the movement of the mass above the surface being considered to the forces or moments tending to cause movements. Both embankment slopes are analyzed for the specific service conditions expected. Allied analyses are used during stability studies to determine seepage patterns and amounts, pore pressures, uplift forces, hydraulic gradients, and escape gradients in the embankment zones and the foundation by the application of the principles of flow through porous media and the graphical or mathematical modeling of flow nets (Cedergren 1967). It is beyond the scope of this report to present the details of the many numerical methods available to analyze the stability of an embankment dam foundation system. These methods are discussed in great detail, with examples, in university textbooks for fundamentals; professional engineering society publications, such as the journals of the American Society of Civil Engineers, which consider practical, specific applications; and design manuals, monographs, handbooks, and standards of federal and state agencies engaged in the design of earth dams, such as the U.S. Army Corps of Engineers and the U.S. Bureau of Reclamation. The reader is referred to the references in this chapter for details of the subjects discussed in the following sections. Loading Conditions Dams that have been stable for a period of time may become unstable when subjected to more severe loading conditions. Conditions that should be considered in the stability analyses of dams are listed in Table 7-2. These include (1) steady seepage with the highest pool level that may persist for a significant period, (2) rapid drawdown from normal pool to lower pool elevations, and (3) earthquake loading conditions. If a dam has not been subjected to the most severe loading conditions expected, its safety can be evaluated by measuring the strengths of the materials of which it is built and by performing analyses to compare these strengths to the stresses in the dam. For modern dams, where factors of safety as shown in Table 7-2 were evaluated during design, sufficient information may already be available such that only a review is needed to establish the adequacy of the embankment and its foundation. As-built drawings, construction records, tests and 

EMBANKMENT DAMS 254 record samples, and performance records of piezometric levels and movements can be used to confirm or refute the suitability of design assumptions and evaluations. TABLE 7-2 Loading Conditions, Required Factors of Safety, and Shear Strength for Evaluations for Embankment Dams Case Loading Condition Required Factor of Shear Strength for Safetya Evaluationb 1 Steady seepage at high 1.5 S strength pool level 2 Rapid drawdown from 1.2 Minimum composite pool level of R and S 3 Earthquake reservoir at 1.0 R tests with cyclic high pool for loading during shear downstream slope: reservoir at intermediate pool for upstream slope aRatio of available shear strength to shear stress, required for stable equilibrium. bTerminology from U.S. Army Corps of Engineers. R = total stress shear strength from consolidated-undrained shear tests; S = effective stress shear strength from drained or consolidated undrained shear tests. SOURCE: U.S. Army Corps of Engineers. For older dams, where factors of safety as shown in Table 7-2 have not been calculated, evaluating safety will require collecting data to estimate the strength properties of the embankment and the pore pressures within it, and performing stability analyses. Steady Seepage Conditions The highest reservoir level that may persist over a significant period of time constitutes the most severe conditions of steady seepage, resulting in the lowest factor of safety for the downstream slope. A knowledge of water pressures within the various zones in a dam and its foundation is essential for a stability analysis. Field data may be obtained from observation wells and piezometers, as described in Chapter 10. Thereafter, pore water pressure throughout the embankment can be predicted from seepage analyses, providing the information needed for an effective stress analysis of slope stability. Rapid Drawdown Condition Rapid drawdown subjects the upstream slopes of dams to severe loading by quickly reducing the stabilizing effect of the water acting against the slope 

EMBANKMENT DAMS 255 without significant reduction of the pore water pressures within the soils forming the upstream embankment. Rapid drawdown slides occur in soils of low permeability, which do not drain freely. Generally, they are shallow slides within the upstream slope that pose no significant threat of loss of impoundment. In some eases (notably the slide at San Luis Dam) rapid drawdown slides extend into the foundation, and such deeper slides may pose a hazard for loss of impoundment if the slide cuts through the top of the dam. Earthquake Condition Earthquake accelerations impose forces on embankments and their foundations; these forces are superimposed on the static forces. As a result, embankment dams may suffer a number of kinds of damage during earthquakes. According to Seed et al. (1977), these include disruption by fault movement, loss of freeboard owing to fault movement, slope failures induced by ground shaking, liquefaction of the foundation or embankment material, loss of freeboard due to slope failures, loss of freeboard owing to compaction of embankment materials, sliding of the dam on weak foundation soils or rock, piping through cracks induced by shaking, overtopping by earthquake- generated waves in the reservoir, and overtopping by waves caused by earthquake-induced landslides or rockfalls into the reservoir. The type of damage is highly dependent on the type of soil acted on by the earthquake forces. For example, embankment fill material that contains significant amounts of clay is able to withstand short-lived increases in load without a catastrophic failure; however, such embankments may suffer some slumping and permanent deformation. Cohesionless soils that are saturated may suffer dramatic loss of shearing resistance when subjected to cyclic loading. In the extreme case, saturated cohesionless materials may assume the properties of a dense viscous liquid. This liquefied state may persist for several minutes under the earthquake motion and cause the embankment fill and/or the foundation to flow as a liquid. The most severe failures of embankment dams during earthquakes have occurred as a result of this liquefaction of loose sandy soils. To evaluate the possible effects of earthquakes on embankment dams, two possibilities must be considered: (1) the fault motion in the foundation can disrupt the embankment or cause loss of freeboard and (2) there may be some form of damage caused by the ground shaking. In the first case the dam must be able to absorb cracks and shears without suffering damaging piping or erosion; it must have an adequate amount of freeboard prior to the earthquake; and the dam designer must accurately estimate the potential magnitude, location, and direction of the fault movement during the 

EMBANKMENT DAMS 256 earthquake. The second effect can be sliding within the embankment or the foundation material, settlement due to compaction of the soil materials, and cracking and/or subsequent erosion of the embankment materials. To survive this type of earthquake motion the dam must be designed with adequate density of soils, the construction process must have had adequate quality control, and finally the intensity of the earthquake shaking must have been properly estimated by the designer. Experience with embankment dams during earthquakes has shown a marked difference in performance dependent on the type of material of which the dam is built and the quality of construction. The performance of nearly 150 dams during earthquakes has shown that hydraulic- fill dams and dams built on loose materials frequently suffered severe damage. On the other hand, dams built of clay soils on stable foundations performed very well, although many were subjected to very strong shaking (Seed et al. 1977). Shear Strength Evaluation Soil strengths for stability analyses are most often evaluated through laboratory triaxial or direct shear tests. To provide useful information, the tests must be performed under conditions corresponding to those in the field (drained or undrained, static or cyclic loading), and the samples must be representative of the soils in the field with respect to density and water content. For most cohesive soils it is possible to obtain ''undisturbed'' samples for testing that retain essentially the same properties as in the field. It is possible to sample cohesionless soils only by very expensive and elaborate procedures requiring highly sophisticated equipment and procedures, and it is common to estimate the in situ relative densities of such soils based on the results of static or dynamic penetration tests. The shearing resistance of cohesionless soils may be evaluated by performing laboratory tests on samples compacted to the in situ relative density, by correlations between shearing resistance and relative density for similar soils, or by large-scale field direct shear tests. Large triaxial shear testing equipment developed in the past 30 years has enabled more accurate determination of strengths of rockfills. Friction angles vary widely, depending on characteristics of the rock in the fill. Confining pressure is an important parameter (Leps 1970). Seismic Analyses Pseudostatic methods of analysis, in which dynamic earthquake loads are represented by static loads, can be used to assess the stability of dams built 

EMBANKMENT DAMS 257 of cohesive soils on stable foundations (Makdisi and Seed 1977). Pseudo-static analyses do not provide a suitable means for evaluating stability of dams built of or built on loose cohesionless materials, because they do not provide a means for including the potential these materials have for strength loss under cyclic loading. To evaluate the stability of loose cohesionless materials, more realistic dynamic analyses should be used, in conjunction with special laboratory tests to evaluate soil strength under cyclic loading. Although they generally perform well during earthquakes, dams of cohesive soils on stable foundations may suffer some permanent deformation and loss of freeboard due to earthquake shaking. These deformations may be estimated using a simplified procedure suggested by Makdisi and Seed (1977), or they may be analyzed in greater detail through dynamic finite element analyses of embankment and foundation response to seismic loading. Factors of Safety Typical factors of safety for the loading conditions discussed previously are shown in Table 7-2. These are the minimum values required for dams under the jurisdiction of the U.S. Army Corps of Engineers and thus represent standards of practice that find wide application, even though they may not be universally accepted by all agencies and for all circumstances. REFERENCES ASCE/USCOLD (1975) Lessons from Dam Incidents, USA, American Society of Civil Engineers, New York. Cedergren, H. R. (1967) Seepage, Drainage, and Flow Nets, John Wiley & Sons, New York. Civil Engineering-ASCE (1981) Dike Safety Upgraded with Millions of Square Feet of Fabric, January. Davis, C. V., and Sorenson, K. E. (1969) Handbook of Applied Hydraulics . Section 18 by John Lowe III, Embankment Dams, and Section 19 by I. C. Steele and J. B. Cooke, Concrete- Face Rock-Fill Dams. Fetzer, C. A. (1979) Wolf Creek Dam, Remedial Work Engineering Concepts, Actions and Results, Transactions of ICOLD Congress, New Delhi. Gordon, B. B., Dayton, D. J., and Sadigh, K. (1973) Seismic Stability of Upper San Leandro Dam, ASCE, San Francisco. Jansen, R. B., Dukleth, G. W., and Barrett, K. G. (1976) Problems of Hydraulic Fill Dams, Transactions of ICOLD Congress, Mexico. Japan Dam Foundation, Tokyo (1977) "Use of Slurry Trench Cut-Off Walls in Construction and Repair of Earth Dams," in World Dam Today '77, 576 pp. Koerner, R. M., and Welsh, J. P. (1980) Construction and Geotechnical Engineering Using Synthetic Fabrics, John Wiley & Sons, New York. 

EMBANKMENT DAMS 258 Lamberton, B. (1980) "Fabric Forms for Erosion Control and Pile Jacketing," Concrete Construction Magazine, May. Leps, T. M., Strassburger, A. G., and Meehan, R. L. (1978) "Seismic Stability of Hydraulic Fill Dams," Water Power and Dam Construction , October/November. Leps, T. M. (1970) "Review of Shearing Strength of Rockfill," Journal, ASCE Soil Mechanics and Foundations Division, July. Makdisi, F. I., and Seed, H. B. (1977) A Simplified Procedure for Estimating Earthquake-Induced Deformations in Dams and Embankments , EERC, University of California. Pravidets, Y. P., and Slissky, S. M. (1981) "Passing Floodwaters Over Embankment Dams," Water Power and Dam Construction, July. Proceedings of Engineering Foundation Conference (1974) "Foundation for Dams," Asilomar, Calif. Seed, H. B., Makdisi, F. I., and DeAlba, P. (1977) The Performance of Earth Dams During Earthquakes, Report No. UCB/EERC-77/20, Earthquake Engineering Research Center, University of California, Berkeley. Sowers, G. F. (1962) Earth and Rockfill Dam Engineering, Asia Publishing House. Timblin, L. O., Jr., and Frobel, R. K. (1982) Geotextiles—A State-of-the-Art Review, paper presented to USCOLD annual meeting. U.S. Army Corps of Engineers (1982) National Program for Inspection of Nonfederal Dams. Final Report to Congress, May 1982 (contains ER 1110-2-106, September 26, 1979). U.S. Bureau of Reclamation (1974) Design of Small Dams, Water Resources, Technical Publication, Government Printing Office, Washington, D.C. United States Committee on Large Dams (1981) "Mr. Elbert Forebay Reservoir," USCOLD Newsletter, March. Wilson, S. D., and Marsal, R. J. (1979) Current Trends in Design and Construction of Embankment Dams, ICOLD/ASCE, New York. Recommended Reading ASCE, Soil Mechanics and Foundation Division (1969) Stability and Performance of Slopes and Embankments, New York. Cortright, C. J. (1970) "Reevaluation and Reconstruction of California Dams," Journal of the Power Division, ASCE, January. D'Appolonia, D. J. (1980) "Soil-Bentonite Slurry Trench Cutoffs," Journal of the Geotechnical Engineering Division, ASCE, April. Golze, A. R., et al. (1977) Handbook of Dam Engineering, Van Nostrand Reinhold Company , New York. Hollingworth, F., and Druyts, F. H. W. M. (1982) Filter Cloth Partially Replaces and Supplements Filter Materials for Protection of Poor Quality Core Material in Rockfill Dam, Transactions, ICOLD. USCOLD Newsletter (1977) "Diaphram Wall for Wolf Creek Dam," July.