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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 termi- nology. Classification generally recognizes (l) 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 con- figuration 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 dam, storage dam, coffer dam, 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 em- bankment 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- 213

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214 SAFETY OF EXISTING DAMS 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 fin- ger 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 embank- ment dams more economical and because the seepage and structural per- formance of these other types are more predictable (Jensen et al. 1976~. The experience record during the period 1920-1940 demonstrates the unre- liability 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 hy- draulic fill dams in California have either been replaced or extremely mod- ified 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 (tops 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 frag- ments. Transition zones of intermediate permeability are frequently in-

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Embankment Dams 215 ctuded between the core and the shells for economic utilization of all mate- rials 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 vari- ous 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 sup- plied by the diaphragm. Cement concrete, asphaltic concrete, and steel plate have been used for diaphragms. Stone~vall-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 up- stream 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 imper- meable 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. Some- times the rock was sluiced in an attempt to reduce settlement by washing the rock fines and spells into the interstices of the mass and creating direct contact between the larger blocks of rock. An upstream narrow zone of

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216 SAFETY OF EXISTING DAMS 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 I. B. Cooke and others, resulting in less embankment settlement and less damage to the fac- ing. 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 "unite or shotcrete, and timber. Dif- ferent 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, re- duced amount of reinforcement, minimum joint spacing, and closed verti- cal construction joints. Horizontal joints have been limited to those re- quired 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 ele- ment supported by zones of dumped or compacted rock. The interior ele- ment controls the retention of the water and is usually a compacted imper- vious soil protected by filter or thin transition zones. A few old dams have thin vertical concrete core walls located on the central dam axis. Depend- ing on the position and configuration of the core, these dams are usually classified as central core, inclined core, or sloping core rockfilis, 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 con- crete prismatic bars that confine rock blocks and fragments. The water fac- ing and overpour surfaces are usually timber fastened to the crib members.

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Embankment Dams 217 Construction of this type of dam was common around the turn of the cen- tury, and some were later modified by the construction of concrete super- structures. The stability characteristics of a crib dam resemble those of a concrete gravity section; however, it is listed here because of its rock com- position. Timber crib dams are sometimes constructed for diversion pur- poses. Foundations Embankment dams can be constructed on foundations that would be un- suitable 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 un- der all conditions of saturation and loading without undergoing excessive deformation or settlement. The foundation must also provide sufficient re- sistance 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 inte- gral part of a dam. During design and construction the foundation charac- teristics can be modified and improved by such treatments as excavation, shaping, curtain and consolidation grouting, blanketing, densification, in- stallation 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 combi- nation 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 imper- vious core rockfill dams without extensive foundation excavation and treat- ment. 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

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218 SAFETY OF EXISTING DAMS 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 col- lapsible 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 liq- uefy Luring long-duration ground shaking from earthquakes. Many foun- dations of this type have probably received no treatment for such a condi- tion. 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 embank- ment dams the major nonhydraulic defects causing failure ultimately in- volve 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 founda- tion/embankment interface, and/or excessive vegetation and rodent activ- ity. Equally important threats to the overall structural or seepage stability of the dam are defects in appurtenant structures, such as spillways and con- duits, 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 de- fects, and potential remedial measures, with brief examples of actual appli- cations on existing dams. Table 7-1 is an evaluation matrix for embank- ment dams that briefly summarizes these discussions. In applying these and other remedies the complex interrelationships be- tween the clam and its foundation, appurtenant structures, and reservoir margin must be considered. Furthermore, extreme caution must be exer- cised 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-

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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 atten- tion 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 de- termination 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 em- bankment and/or its foundation. Rapid drawdown has caused instability in the upstream slopes of many dams, including Pilarcitos 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 case 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 com- plete 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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248 SAFETY OF EXISTING DAMS the early life of a dam may become plugged or broken, or otherwise mal- function, thereby creating excessive uplift (internal hydrostatic pressures) within the dam and foundation. It is also possible that an outward appear- ance 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 be- fore 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 rela- tionship can be established between reservoir level and anticipated drain- age flow rate. Any significant changes in the flows defined by this relation- ship 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. Founciation-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 cre- ating a favored seepage path along the contact. Improper or incomplete

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Embankment Dams 249 treatment of foundation joints and fractures, alone or coupled with an in- adequate filter relationship between the embankment and the joints, can also lead to embankment piping and eventual collapse from internal ero- sion. Inadequate stripping of loose or otherwise undesirable foundation materials can create a weak plane along which major embankment insta- bility 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 cur- tain 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 embank- ment 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 inter- face usually are not directly observable because they are hidden by the dam. The presence of the defect characteristics must, therefore, be de- duced 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, espe- cially 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 main- tenance of embankments.

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250 SAFETY OF EXISTING DAMS 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 satura- tion, 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 12-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 de- cay 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-

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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 fol- lowing additional comments are offered: The integrity of trees that remain in place should be consideredtheir root systems may be damaged by adjacent work, or they may be more ex- posed 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 embank- ments from erosion. Backfill material after stump or root removal should have characteris- tics 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 main- tenance problem. This problem is known or suspected to have caused sev- eral failures of small dams. The animals that have caused the most prob- lems 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 pres- ence 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 recompac- tion 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 appre- ciated from the preceding discussions concerning the many potential de- fects that can create unstable conditions within the structure-foundation system. Among the various methods of stability analyses available to the

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252 SAFETY OF EXISTING DAMS engineer is the conventional analysis of slope stability, which, although it is a valuable too] in the assessment of embankment adequacy, must be per- formed and used with experienced judgment or it can produce completely misleading results that could lead to erroneous, even disastrous, conclu- sions 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 them- selves are often subject to fairly wide ranges of interpretation simply be- cause the engineer is working with native materials that have been altered in varying degrees by the forces of nature. Especially in the case of the analysis of stability of existing embankment dams, the exploration, sampling, laboratory testing, and materials proper- ties evaluation program always has physical and economic constraints that limit the extent of knowledge that can be gained about the important phys- ical properties of any given structure. For this reason the physical proper- ties 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 ap- pear 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 dif- ferent 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 sur- face, such as a circular arc, compound curved surface, or a series of con- nected plane surfaces. The configuration and positioning of the surface de- pend 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

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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 con- fined 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 an- alyzed 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 graphi- cal 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 engineer- ing 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 fol- lowing 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 embank- ment and its foundation. As-built drawings, construction records, tests and

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254 SAFETY OF EXISTING DAMS TABLE 7-2 Loading Conditions, Required Factors of Safety, and Shear Strength for Evaluations for Embankment Dams Case Loading Condition Required Factor of Safetya Shear Strength for Evaluationb 1 2 Steady seepage at high pool level Rapid drawdown from pool level Earthquake reservoir at high pool for downstream slope; reservoir at intermediate pool for upstream slope 1.5 1.2 1.0 S strength Minimum composite of R and S R tests with cyclic loading during shear 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 con- solidated-undra~ned shear tests; S = effective stress shear strength from drained or consoli- dated undrained shear tests. SOURCE: U.S. Army Corps of Engineers. record samples, and performance records of piezometric levels and move- ments can be used to confirm or refute the suitability of design assumptions and evaluations. 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 pres- sure 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

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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 shal- low slides within the upstream slope that pose no significant threat of loss of impoundment. In some cases (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 foun- dations; these forces are superimposed on the static forces. As a result, em- bankment dams may suffer a number of kinds of damage during earth- quakes. According to Seed et al. (1977), these include disruption by fault movement, loss of freeboard owing to fault movement, slope failures in- duced 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 founda- tion 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 sat- urated 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 em- bankment 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 poten- tial magnitude, location, and direction of the fault movement during the

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256 SAFETY OF EXISTING DAMS earthquake. The second effect can be sliding within the embankment or the foundation material, settlement due to compaction of the soil materi- als, 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 embank- ment dams during earthquakes has shown a marked difference in perfor- mance dependent on the type of material of which the dam is built and the quality of construction. The performance of nearly 150 dams during earth- quakes has shown that hydraulic-fill dams and dams built on loose materi- als 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 labo- ratory biaxial 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 possi- ble to sample cohesionless soils only by very expensive and elaborate proce- dures 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 co- hesionless 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 biaxial shear testing equipment developed in the past 30 years has enabled more accurate determination of strengths of rockfills. Friction an- gles vary widely, depending on characteristics of the rock in the fill. Con- fining pressure is an important parameter (tops 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

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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 cohe- sionless materials, more realistic dynamic analyses should be used, in con- junction with special laboratory tests to evaluate soil strength under cy- clic loading. Although they generally perform well during earthquakes, dams of co- hesive soils on stable foundations may suffer some permanent deforma- tion 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 seis- mic 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 un- der 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 Engi- neers, 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, Con- crete-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., lDukleth, 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.

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258 SAFETY OF EXISTING DAMS Lamberton, B. (1980) "Fabric Forms for Erosion Control and Pile Jacketing," Concrete Con- struction 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," Asilo- mar, 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) GeotextilesA State-of-the-Art Review, pa- per 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 Publi- cation, Government Printing Office, Washington, D.C. United States Committee on Large Dams (1981) "Mt. Elbert Forebay Reservoir," USCOLD Newsletter, March. Wilson, S. D,. and Marsal, R. J. (1979) Current Trends in Design and Construction of Em- bankment 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 Geotechni- cal 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 Supple- ments Filter Materials for Protection of Poor Quality Core Material in Rockfill Dam, Trans- actions, ICOLD. USCOLD Newsletter (1977) "Diaphram Wall for Wolf Creek Dam," July.