National Academies Press: OpenBook

Safety of Existing Dams: Evaluation and Improvement (1983)

Chapter: 5 Geologic And Seismological Considerations

« Previous: 4 Hydrologic and Hydraulic Considerations
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 132
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 133
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 134
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 135
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 136
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 137
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 138
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 139
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 140
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 141
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 142
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 143
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 144
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 145
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 146
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 147
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 148
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 149
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 150
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 151
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 152
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 153
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 154
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 155
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 156
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 157
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 158
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 159
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 160
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 161
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 162
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 163
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 164
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 165
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 166
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 167
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 168
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 169
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 170
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 171
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 172
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 173
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 174
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 175
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 176
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 177
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 178
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 179
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 180
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 181
Suggested Citation:"5 Geologic And Seismological Considerations." National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, DC: The National Academies Press. doi: 10.17226/289.
×
Page 182

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 132 5 Geologic And Seismological Considerations GENERAL GEOLOGIC CONSIDERATIONS Defective foundation and reservoir conditions have perhaps been responsible for a majority of dam failures and accidents (see Chapter 2). This chapter will discuss the major geologic features and rock types that may contribute to the development of serious conditions at a dam site. In the 5 billion or so years of the earth's life, many changes have taken place in the rocks forming the earth's crust. These changes are continuing. The result is a great variety in the type of rock from place to place and also great differences in the quality of the rock. Engineers and geologists, after much experience with dam site geology, frequently associate certain defects with each class of rock. While rocks differ greatly in the kind and size of their mineral constituents, the broadest general classification of rock is based on the way in which they are formed: igneous, sedimentary, and metamorphic rocks. Coincidentally, certain types of defects in dam foundations often seem to have some correlation with each of these broad classifications. Rock Classification Igneous Rocks Igneous rocks are formed by the solidification of molten rock or lava. If solidification takes place slowly at great depths in the earth, the rocks are called plutonic or intrusive rocks and are composed of masses of crystalline particles. Depending on the relative amounts and sizes of these constitu

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 133 ents, these rocks will have various names, such as granite, diabase, or gab-bro. When the molten rock is expelled from the interior to the surface of the earth, it cools quickly, the crystal sizes are generally very small, and the dissolved gases in the rock expand. These rocks are called volcanic or extrusive rocks. These include basalt, rhyolite, obsidian, and pumice. A common characteristic of plutonic rocks is their large crystal sizes and the interlocking of grains of different minerals. As the rock cools, it shrinks. When the internal tensile stresses resulting from this shrinkage become greater than its strength, the rock cracks and develops a regular pattern of joints. At the surface, weather processes break the rock down almost completely into residual soil. Volcanic rocks have their own special problems. The material may be ejected explosively in the form of rocks, molten bombs, or small, dust-like particles or may flow like a dense liquid, such as lava. Deposits of the ejected material, such as pumice, are apt to be porous, with easy permeability and erodibility to flow of water. A reservoir rim or dam foundation containing such ejecta might require extensive (and often very difficult) grouting before a satisfactory reduction of seepage can be achieved. The rock mass, with large or small open bubbles from expanded gases, tends to be weak and needs careful study to determine if it has sufficient strength for heavily loaded structures. Volcanic rock often tends to break down easily by weathering to leave a weak residue. Each lava flow lasts only a relatively short time. Therefore, a deep deposit of lava may be made up of many individual flows. Since the surface of each flow tends to deteriorate, the mass is frequently characterized by interfaces of altered material and sometimes volcanic ash; these latter materials may be mechanically weak and very permeable to water flow. Washing and grouting often improve such foundation materials. Sometimes the flowing mass cools and hardens on the surface, and the included gases may escape while the liquid interiors simply run out and, in either case, can leave a hollow pipe of considerable size and length. Careful exploration is needed in regions of volcanic deposits for assurance that reservoir leakage can be held to an acceptable minimum. Sedimentary Rock Rock at the earth's surface is continually being broken down not only by tectonic forces but also by the process of weathering. Weathering processes include the freezing of water in cracks accompanied by expansion of the ice and subsequent fracturing of the rock. Another weathering process is that of extreme temperature changes that will cause fracturing. The flow of water through rock can weaken some minerals and this will leave the remainder unsupported, so they are removed easily by wind or water erosion.

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 134 Large-scale weathering processes, such as river erosion or glaciation, not only will remove large masses of rock by abrasion but may also remove support from large masses and leave the balance of the rock in a state of stress that is conducive to fracture and further breakdown. The broken particles resulting from the weathering process are often transported by air, ice, water, or gravity and then redeposited. As the modes of deposition can be extremely variable, there may be considerable differences in the successive layers of the deposited or sedimentary material. Hence, sedimentary rocks are often characterized by the great variety in the material in the successive layers. Much time can elapse between depositional modes, and this provides opportunity for decomposition of the top surface of the layer. The result can be a very weak interface between the layers, and this will require washing and grouting to increase the strength and impermeability. If the sedimentary deposits are subjected to heat and pressure from overlying material or to cementation from dissolved minerals in water, the layers may greatly increase in strength. In addition to the weathering processes, tectonic activities such as earth movements may cause joints or faults to form in the sedimentary layers. These discontinuities may be tightly closed, open, or filled with other materials or minerals that have been transported into the openings. Many sedimentary rocks take their names from the size of the rock particles in the deposit. Thus, a rock made up of gravel or larger particles is called conglomerate (the contained particles are usually rounded or sub- rounded) or breccia (the contained particles are angular). If sand-size particles are compressed or cemented into a coherent mass, the result is a sandstone; similarly, silt-size particles constitute a siltstone and clay-size particles result in a claystone or shale. Calcareous mud or sand may become limestone. Deposits of highly carboniferous materials, such as vegetation, become coal. Any or all of these may be present in successive layers of a sedimentary rock formation. In addition to the mud or weak zones between successive layers, sedimentary rocks can have other problems. For example, if the cementing material in sandstone or conglomerate is water soluble, it will go into solution or become very weak when saturated by a reservoir, and the rock may revert to its original form, i.e., a sand or gravel. This action is considered the cause of the collapse of the St. Francis Dam in California. The sandy shaley conglomerate foundation disintegrated under the water action during the first filling of the reservoir. Some shales that are wet in situ tend to fall apart to a powder on drying out. These are called air-slaking shales. Limestones and dolomites can be dissolved by the weak natural acid formed by the combination of water and carbon dioxide in the water. Hence, over a long time period, water moving through these types of carbonate rocks can

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 135 dissolve out large portions of the rock and result in natural pipes or caverns (Figure 5-1). The overlying surface may then develop sinkholes and is called karst. Great care must be taken to grout and otherwise seal off cavernous limestone; otherwise a reservoir founded on this rock will not hold water. A similar action can occur in rock formations primarily composed of gypsum. Metamorphic Rocks These are formed from other rocks when they are subjected to great heat and pressure. In igneous rocks, like granite, the mineral grains will be reoriented to planar sheetlike forms called gneiss or schist. These may have a decided plane of weakness parallel to the mineral planes. In sandstone most of the minerals (except quartz) may disappear, and the quartz grains will fuse together into a glassy solid called quartzite. Under metamorphic processes, shale goes through transitional stages to ultimately form a slate. The latter will have decided cleavage planes, but these often are at angles to the original bedding of the shale. Limestone and dolomite when metamorphosed become marble. Figure 5-1 Caverns in dolomite foundation of a gravity concrete dam. (Later filled with concrete to ensure adequate bearing for the dam.)

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 136 Since the process of metamorphism is not uniform, metamorphic rocks have weak zones due to pressure, heat, or chemical action. These can result in sheeted areas, gouge seams, or even faults. Weakened areas, when found, must be removed and replaced with concrete so that the dam will have a uniformly strong foundation. Jointing in metamorphic rocks will be formed during the metamorphic processes and will often form irregular-shaped or wedge-shaped blocks. ROCK TYPES There are hundreds, probably thousands, of different varieties of rock that could be encountered in dam construction. Of course at any one site the likelihood is that there would be only a few to several rock types. However, because there are so many possibilities, it is not feasible to list here all the problems that might develop from every rock type that might be encountered at a dam. Therefore, only listed are those rock types that most commonly might be encountered or those that have been reported as causing problems at a dam. Rock Types And Their Performance Amphibolite: Metamorphic. Dark-colored with little or no quartz. May have poor weathering resistance. Tends to break along foliations. Anhydrite: Sedimentary. CaSO4. Usually white or slightly tinted. Unstable in the presence of water and tends to expand when wet, which causes rapid disintegration of the rock. Ash: Igneous. Usually noncoherent but may be somewhat cemented or welded. Light colored to gray. Deteriorates rapidly under water action and may be subject to considerable settlement because of its low density. High content of silica. (see Tuff) Basalt: Igneous. Dark colored, fine grained. Little or no quartz. Complex chemical formula including Na, Al, Si, O, Ca, Mg, Fe, and K in varying amounts. May contain small, highly visible pores called rugs, which may or may not be filled with clay-like material or be interconnected. However, some basalts do contain continuous tunnels or tubes as a result of gas flowing through the material during the time of its formation. Basalts tend to crack into well- defined chunks or blocks during the cooling process. High cohesion. If fractured, a plucking action can occur when high-velocity water flows over its surface. Claystone: Sedimentary. Clay constitutes greater than 25%. May be massive or stratified. Generally high in quartz. Some shales are classified as claystones and vice versa. There is no agreement as to the distinction except

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 137 that shale would be expected to have considerable fissility (easy cleavage or laminations). Coherent but subject to erosion under water and other forms of weathering action. May disintegrate into fine particles or clay under severe weathering conditions. Where stratified, may be a weakened zone that would be conducive to slides. Conglomerate: Sedimentary. Composed of easily visible, generally rounded fragments or pebbles in a matrix of granular material. Can be cemented with calcium carbonate, iron oxide, silica, or clay. Depending on the type of cement, it may be disintegrated easily by weathering agents. Diabase or Dolerite: Igneous. Dark colored, intrusive. Composed mainly of labradorite and pyroxene minerals. Little or no quartz. Fine to medium grained. Visible particles appear to be angular. Can be associated with cavities or numerous open fissures. Generally high strength. Sometimes called trap rock. Dolomite: Sedimentary. Composed chiefly of Ca, Mg, and CO3. White or tinted. May be crystalline or noncrystalline. Effervesces very slowly in HCl. Can be associated with a cavernous structure. Relatively high strength. Gneiss: Metamorphic. Foliated. Light to dark gray. Less than half of the minerals may show preferred parallel orientation. Commonly rich in quartz and feldspar. May contain considerable mica. Those high in mica may rapidly slake or have easy cleavage. Granite: Igneous. Primarily composed of quartz and feldspar. May contain some mica. Texture usually from medium to coarse grained. White to dark gray with occasional red or pink. May be block jointed or sheeted. The feldspar may disintegrate under weathering and leave a rather granular soft with some clay admix. Grains may be strongly or poorly interlocked. Frequently becomes a catch-all term for many types of feldspathic, quartzitic intrusive igneous rocks; thus, test values can have a considerable range. Gypsum: Sedimentary. CaSO4. 2H2O. Very soft. White or colorless but can be tinted. Easily disintegrated by normal weathering processes. Water action may cause solution channels. Limestone: Sedimentary. CaCO3. May have impurities of other minerals. White to tinted. Rock composed solely or almost entirely of CaCO3 includes chalk, coquina, and travertine. All effervesce freely with any common acid. The name always carries a warning that there may be minor or extensive cavern systems in the formation. Slowly soluble in water with a low pH. Can deteriorate under high temperatures. Marl: Sedimentary. Catch-all term describing soft, loose, earthy deposits that may be coherent or noncoherent. Chiefly clay and CaCO 3. Gray, but other colors are frequently present. Texture may be extremely fine or granular. Often easily eroded by water.

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 138 Micaceous Rock: Generally igneous or metamorphic. Any rock containing easily visible quantities of the sheetlike material called mica. Latter has a highly complex formula that contains Ca, Mg, Fe, Li, Al, Si, O, H, and F. Easily split into thin plates. Color ranges from colorless to black. In a rock mass under shear stresses, it can be expected to be a weak member owing to a relatively low coefficient of sliding friction. Pegmatite: Igneous. Very coarse grained with interlocking crystals. Composition similar to that of granite. High in quartz. Same color variations as granite. Larger constituents may weather loose from the binding matrix. More frequently tight interlocking is present and represents a highly durable rock. Peridotite: Igneous. Coarse grained. Chiefly olivine with other iron- magnesium minerals. Dark colored. Mentioned here primarily because it commonly alters to serpentine, often a highly undesirable foundation material. Phyllite: Metamorphic. Intermediate between slate and micaceous schist. Well-defined thin laminations. Usually black or dark brown. Can be split along bedding planes with some difficulty, and split surfaces may be slick. Resistant to weathering but tends to split or slab when original crustal stresses on it are relieved by excavation. Pumice: Igneous. Light colored. Highly porous or vesicular. Generally composed primarily of silica that has been produced by volcanic eruption. Very lightweight and abrasive. Stony or earthy texture. Very erodible and very low strength. Quartzite: Sedimentary or metamorphic. Resembles a very hard sandstone where the quartz grains are tightly cemented with silica. Also may be a metamorphic rock formed from the recrystallization of sandstone. Usually colorless. Breaks with irregular fractures across the grains. Very hard. Highly resistant to weathering forces but on occasion can disintegrate into granular material. Sandstone: Sedimentary. Medium grained, composed of rounded or angular fragments. Cementing material may be silt, clay, iron oxide, silica, or CaCO3. White, red, yellow, brown, or gray. May be friable, i.e., when rubbed by the fingers grains easily detach themselves. Can be well-defined bedding or very massive. Rate of disintegration depends on type of cementing material and weathering forces. May have a relatively high porosity and be considered a reservoir rock for water or oil. Schist: Metamorphic. Well-developed foliation generally in thin parallel plates that may show considerable distortion. Usually high in mica. Can be very competent as an engineering material but is frequently separated easily along the foliations or planes of schistosity. Can be subject to plucking action under high-velocity water. Because of the strong forces that develop

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 139 the inherent foliation, it is possible there will be shear planes between the laminations; if so, it will be a dangerous rock in slopes and tunnels. Serpentine: Igneous or metamorphic. Complex formula with Mg, Fe, Si, O, and H. Usually easily recognized because of its very greasy or soapy appearance or feel. Can be granular or fibrous. Often green or greenish yellow or greenish gray. May have a vein-like appearance. Generally a secondary mineral but can be found in thick beds. Always to be regarded with caution because of its tendency to disintegrate into very incompetent material under normal weathering action. Also, can have very low shear strength. Shale: Sedimentary. Extremely fine grained, composed mainly of clay-size particles but may occasionally contain silt or sand sizes. Characterized by its laminar or fissile structure. Easily cleavable. May be soft and easily scratched with a fingernail but can also be quite hard. Tends to slake rapidly when dried and then put into water. Can range from relatively light colors to black. Usually has a low shear strength and is to be regarded with caution when encountered in engineering works. Frequently regarded as a borderline material between soil and rock. Can disintegrate to a clayey mass. (See also the section Residual Soils.) Siltstone: Sedimentary. Generally hard and coherent. Tends to be massive rather than laminated. Gritty feel. At least two-thirds of the constituents will be silt size. Depending on the type of cement, may disintegrate rapidly into silty deposits. Not expected to have high durability when used as a rockfill. Slate: Metamorphic. No visible grains. Composed of clay-size particles. Will cleave into very hard, relatively thin plates. However, fissility occurs along planes that may not be parallel to the original bedding that is visible in the material. Very dark colored. Very durable but can break into large slabs in open slopes. Tuff: Igneous or sedimentary. Usually a product of volcanic eruption. May have relatively low density or, if the individual silica grains are welded together, a high density. Depending on the cement, may be easily eroded or very resistant to erosion. Gray to yellow in color. Usually low density. General Comments On Rock Types The physical properties of rock are extremely variable, even for one type of rock. This is illustrated in Table 5-1. For example, it can be seen in this table that the unconfined compressive strength of a ''granite'' can vary between 2,600 psi and 48,200 psi. One reason for these wide variations is the

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 140

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 141 inaccuracy of the description or "naming" of the rock, which points to the lack of an acceptable engineering classification system for rock. For example, if accurate petrographic descriptions are not obtained, the rock may be inaccurately categorized as basalt, trap rock, or granite. For this reason it is desirable to determine the mode of deposition of the rock; this may provide clues to the expected performance in engineering works. For example, if the rock is a member of a flow of molten rock on the surface or at shallow depths, attention should be directed at the possibility of gas caverns or extensive cooling fractures being present. Similarly, if the rock has been deposited in water, such as many sedimentary rocks, the possibility exists that such rocks will be susceptible to severe erosion under the weathering action of wind, temperature changes, or water. Another clue to predicting rock performance is to determine the geologic age of the material. This certainly is not a precise predictor but can assist when the age along with other information on the rock origin is known. For example, if the rock is relatively young, i.e., formed in the Cenozoic Era, it might be expected that the material would have a relatively poor coherence and tend to be highly erodible. Of course this is not always true, but it is one possible indicator. On the other hand, if the rock is extremely old, for example of the Precambrian Age, the rock might well be very durable and hard and a good- performing structural material. Obviously age by itself is not a good criterion because of the long period of time in which rocks have developed, e.g., between the Precambrian and the Cenozoic there is a period of over 500 million years. The properties noted in the above descriptions of various rock types are summarized in Tables 5-2 and 5-3. Table 5-2 correlates the easily visible surface defects or evidence of rock behavior to the possible cause for this behavior. Table 5-3 compares the rock type with the defects that may occur and that are easily visible by surface examination. Table 5-3 can be used in two different ways: (1) if the rock name or type is known, the expected defects can be identified from the table and (2) if certain defects are observed, it may be possible to identify the rock type associated with the de-feet. It must be recognized, however, that the defects noted are only those that more commonly occur, because almost any of the so-called surface defects may develop from any rock type. GEOLOGIC STRUCTURE Rock Foundation Defects Some of the potential defects in a rock foundation that will result from geologic structure are faults, joints, shear zones, and bedding and foliation

TABLE 5-2 Major Cause of Defect Defect Freeze-Thaw and Other Stress Relief Solubility Consolidation Water Pressure Water Lubrication Piping Temperature Changes Block loosening X X X X Cracking X X X Disintegration (granular) X X Seepage (clear) X X Seepage (muddy) X Settlement X X Slabbing X X X Slides X X X Softening X X GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 142

TABLE 5-3 Surface Defects Rock Type Block Cracking Disintegration Leakage Leakage Settlement Softening Slabbing Slides Foliation Loosening (granular) (clear) (muddy) Anhydrite X X Ash, volcanic X X X Basalt X X X Claystone X X X X Conglomerate X Diabase X X Dolomite X Gneiss X X Granite X X X X X Gypsum X X X X X X Limestone X X X Marl X X X X Micaccous X X X X X X rock Pegmatite X Peridotite X GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS Pumice X X X Quartzite X Sandstone X X X X Schist X X X X X Serpentine X X X Shale X X X X X X Siltstone X X X Slate X X X X Tuff X X X 143

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 144 planes. Faults result from a rupture in rock formations and are caused by high-magnitude tectonic forces. Joints can be formed as a result of tensile forces that develop from cooling of the liquid rock or from a weathering process, as described in the section General Geologic Considerations. The differentiation between faults and joints is that when portions of a formation move with respect to each other the discontinuity so produced is termed a fault. If there is a discontinuity but no movement has occurred, the break would be called a joint or fracture. Shear zones merely are another term for a faulted area in the crust. They generally have appreciable thickness or width and contain considerable ground-up rock derived from the parent rocks on both sides of the shear zone. Occasionally they also contain secondary material that has been transported into the zone by means of water. Foliation is a result of parallel arrangement of platy minerals and is common in metamorphic rocks such as schist and slate. Separations between bedding planes are a type of joint primarily associated with sedimentary rock. The term fracture can refer to a joint or a fault but always denotes a discontinuity in the rock mass. Water percolating through fractures can alter the mineral of the adjacent rock and in some cases actually dissolve portions of it. The result can be a weak, altered material. As previously noted, the fracture may be open, closed, or filled with some type of secondary material. Usually the secondary material is weaker than the surrounding rock, although occasionally there may be a high percentage of silica that actually welds the fractures together to form a rock mass that may have strength equivalent to the original unbroken mass. Joints generally tend to be more continuous and more open near the surface and to close with increasing depth. Regardless of whether the fractures are filled, it is necessary to reduce their permeability by washing and then grouting with a cementitious or resinous material. Continuous joints and fractures should always be of concern as they can result in instability in slopes adjacent to a dam or in the reservoir or downstream from the dam. Faulting, jointing, and shear zones in carbonate rocks may contribute to the development of karstic conditions. Clay material along such discontinuities may be washed away with increased head and/or the surging action common to a hydroelectric project. One such example is the Logan Martin Dam on the Coose River in Alabama. The rock foundation is dolomite with isolated beds of limestone and scattered masses of chert. (Chert is an amorphous or cryptocrystalline sedimentary rock comprised primarily of silica, with lesser amounts of quartz.) The rock is highly jointed and cavernous. Although the bedrock was extensively grouted during construction, under-seepage developed soon after reservoir impoundment. Upstream sinkholes

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 145 and downstream boils developed and persisted through periods of remedial grouting during the 18 years of operation. Seepage has been monitored by stream flow measurements downstream of the dam every spring and fall of this operating period. The reservoir was filled in 1964 and by the spring of 1977, stream flow measurements downstream of the dam had increased to 675 cfs. The seepage has been slightly reduced in recent years by multirow grouting of the dam foundation and by filling upstream sinkholes with cherry residuum. As a second level of defense, a trench drain and rock bolster have been constructed along a critical section downstream of the toe of the dam. Failures Due To Geologic Defects Waco Dam Structural defects have been responsible for numerous dam failures and accidents. For example, in 1961 a 1,500-foot slide occurred in the Waco Dam in Texas during construction. The earth embankment was as much as 13 feet below finished grade when the slide occurred. The dam was founded on three formations of clay shales that had varying strengths. This complexity in the foundation was due in part to faults. According to Beene (1967), the foundation failure resulted from a combination of depositional sequence and geologic structure disturbance. The weaker clay shale that failed was sandwiched between two stronger shales. Movement along closely spaced nonparallel faults caused shearing stresses in the weaker shale. The presence of a relatively pervious contact along a fault between the weak shale and the stronger shale permitted widespread distribution of uplift pressure. Figure 5-2 shows profile and embankment sections. Figure 5-3 shows pore-pressure contours in the weak shale after the slide. Beene concluded that influence of a joint system on the development and distribution of pore pressure in a clay shale cannot be predicted by laboratory tests; therefore, the embankment must be instrumented for movement and pore pressure. Baldwin Hills Dam Another failure resulting from foundation faults was that of Baldwin Hills Reservoir in California in 1963. This occurred some 12 years after the dam had gone into operation. The asphalt lining for the reservoir was founded on thinly bedded and poorly consolidated sands and silty sands. There were

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 146 Figure 5-2 Embankment sections of Waco Dam. Source: Beene (1967).

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 147 numerous known faults in the foundation. Because of this, special attention was paid to the design of the dam and the reservoir was highly instrumented. According to the State Engineering Board of Inquiry (Jansen et al. 1967), slow movements occurred on these faults with progressively increasing displacement. This ruptured the asphalt lining and allowed water under pressure to enter the faults and pipe out (remove) the filling material in the faults. This erosion proceeded very rapidly and undermined the dam with its subsequent complete failure. The long-term movement and development of these stresses appear to have been caused primarily by subsidence, and the latter had been observed for many years in this area (Leps 1972). Figure 5-4 shows a view of the dam after failure. Figure 5-3 Pore pressure contours at mid-pepper after slide. Source: Beene (1967).

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 148 Figure 5-4 Baldwin Hills Reservoir after failure. Teton Dam Open joints in the foundation need careful consideration in design. The failure at Teton Dam in 1976 in Idaho may in part have been caused by the existence of open joints in the foundation. According to the Independent Panel to Review the Cause of Teton Dam Failure (1976), the volcanic rock in the foundation was highly permeable and moderately to intensely jointed. The foundation was grouted during construction, but the grout curtain was not sufficiently effective, and there were open joints in the upstream and downstream faces of the right abutment key trench; these provided conduits for ingress and egress of water during reservoir filling. The independent panel considered the placement of highly erodible soil (the core of the dam) adjacent to the heavily jointed rock a major factor contributing to the failure. Malpasset Dam Often it may be the combination of deficiencies in the foundation that causes a failure or an incident. One such case was the Malpasset Dam failure in France in 1959. According to Bellier (1976), this was the first total failure in the history of arch dams. The failure was a result of foliation dipping downstream, arch stress parallel with the foliation, and a fault

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 149 plunging beneath the dam that created a watertight floor. Bellier stated that as the arch stress increased on the left abutment there was a reduction in permeability with a subsequent increase in uplift pressure that eventually caused rupture of the dam. Figure 5-5 shows the relationship between the geologic structure and the arch. This case history strengthens the requirement for using piezometers in rock abutments and foundations of arch dams as a positive monitoring device. Figure 5-5 Relations between the geological structure and the arch. The horizontal section shows that the arch is in line with the foliation on the left abutment. Note how the foliation is relative to the ground surface. Conditions on the right abutment were very different. The corresponding vertical sections clearly show the difference between the two abutments, in particular the zone compressed by the arch thrust in the left bank (Section BB). This zone could extend to great depth on account of the foliation. Malpasset Dam. Source: Bellier (1976).

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 150 Uniontown Cofferdam It also is important to understand the structural geology of foundations for temporary structures associated with dams, such as cofferdams, which are used during initial construction or in such subsequent modifications as adding a powerhouse to an existing dam. Cofferdam failures can cause loss of life to downstream inhabitants and construction personnel and/or appreciable property damage. A cofferdam failure during construction of the Uniontown locks and dam on the Ohio River in 1971 may in part have been the result of faults in the foundation (Thomas et al. 1975). The cofferdam was completed 10 days before the failure, and the dam foundation area had been dewatered. The failure was a translational slide along a coal and underclay stratum approximately 16 feet below the top of rock. The probable cause of failure was excessive pore pressure in the underclay and coal. A fault beneath one of the cofferdam's cells appears to have provided a likely avenue of communication to water outside of the cofferdam. Intense faulting in the area contributed to the reduction in strength of the sedimentary rock. SOILS The vast majority of dams are embankment dams composed of soils. Some concrete gravity dams may be in contact with soils at some portions of their foundations, particularly the abutments. A thorough understanding of the condition of an existing dam requires detailed knowledge of the types of soils in the embankment and foundation, their spatial distribution, and their physical characteristics (moisture content, strength, permeability, and presence of discontinuities affecting permeability and strength). For most existing dams, soils data from preconstruction site exploration and testing, design, or construction are not available; therefore, this extremely important information must be cautiously inferred by visual site inspection and limited sampling and testing. Such inferred conclusions must be made in conjunction with an understanding of the geologic setting along with experience with similar soils and structures. This section presents some very generalized information on soils. It also offers suggestions on how to obtain, from published information, more site- specific understanding of the nature and characteristics of the soils in, beneath, and around a particular dam. This approach can reduce the cost of drilling and testing and is essential for reliable interpretation of drilling and test data. It is emphasized that study of generalized information is no substitute for exploring and testing the soils in and under the specific dam.

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 151 Discussion of basic soil mechanics is not included in this section. For that information the reader is referred to texts, geotechnical engineering consultants, and sections of this book dealing with stability and seepage analyses. Soil Classification Classification of soils (dividing them into systems or groups having similar characteristics) may be done in many different ways depending on the particular characteristics of interest. For example, geologic classifications tend to be focused on origin and mappability. Pedologic classifications have been developed by soil scientists with primary emphasis on agricultural qualities of the soils, with mappability being an important factor here, too. Engineering classification systems place emphasis on physical characteristics most pertinent to the particular engineering utilization at hand. Engineers, pedologists, and geologists have become increasingly aware that soil data and maps developed by each of these disciplines can be very useful to others when properly interpreted. A common denominator is that most classification systems include textural descriptions based, at least in part, on the relative abundance of different grain sizes composing the soil. The following discussions outline the most commonly used soil classifications. Textural Classification A textural classification defines quantitatively the percentages of particles of various grain sizes in a soil sample. Although there are some differences among engineers, soil scientists, and geologists on the specific grain diameters defining boulders, cobbles, gravels, sand, silts, and (especially) clays, overall the differences are minor. By making appropriate sieve separation tests, a soil may be defined as, for example, 60% sand, 30% silt, and 10% clay; a more complete analysis of grain sizes can be usefully expressed on a graph (grain- size chart) showing the percentage of soils in each diameter. Engineering grain- size definitions along with a grain-size chart are shown in Figure 5-6; the figure includes some typical grain-size curves illustrating different degrees of uniformity of soil grain sizes. Descriptive Classification Descriptive classifications simply identify the main constituent, with less abundant or less important constituents' names being used as adjectives. For example, a soil containing mostly sand but including some silt and a little clay might be defined descriptively as a slightly clayey, silty sand.

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 152 Figure 5-6 Grain size chart and ASTM-ASCE grain size scale. Source: Sowers (1979). Generalized terms relating to density, consistency, moisture content, color, and origin are usually included in the description as the first modifying terms. Mixtures of sand, silt, and clay are often termed loam, particularly by soil scientists. Definitions of various loams are shown in Figure 5-7. Engineering Classification Several engineering soil classification systems are in use. The most common and appropriate one for dam engineering is the Unified Soil Classification System (USCS 1975). It is based on grain-size distribution and, for finer grained soils, on plasticity. Figure 5-8 shows the USCS soil classes and their definitions, and Figure 5-9 shows generalized engineering characteristics of each USCS soil class. This generalized information can be helpful in inferring information about an existing dam when used in conjunction with pedological and geologic maps and information, as discussed in the following sections. Pedological Classification The most detailed pedological classifications focus on the agricultural characteristics of surface and near surface soils. The basic unit of classification is termed a soil series, and series are classified into progressively larger and inclusive families, subgroups, great groups, suborders, and orders. Any

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 153 Figure 5-7 Soil triangle of the basic soil textural classes. Source: U.S. Bureau of Reclamation (1974).

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 154 particular soil series has specific visual, physical, and agricultural characteristics that can be recognized from place to place, usually within a localized geographic region. Over 10,000 soil series have been identified in the United States. Soil series are the basic mapping unit of agricultural soft scientists. All of the United States has been mapped by soil scientists at some scale, and within the last few decades many areas have been mapped in detail at scales of 1 inch = 2,000 feet or larger. In many areas the Unified Soil Classification of each soft series (and horizons within each series) has been determined, and many soil series have been subjected to other engineering tests to determine general engineering characteristics. Figure 5-8 Unified soil classification, including identification and description. Source: U.S. It is emphasized that agricultural soils maps, particularly the more modern ones where correlations have been made between soil series and engi

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 155 neering tests, can be very helpful in inferring generalized engineering characteristics at an existing dam. Furthermore, the text accompanying the soil map may include references to actual engineering performance of the soils in the vicinity of the dam. These may help in understanding a suspected or known deficiency in the dam, its foundation, or the reservoir rim. Geologic Classifications Soils may be classified as to their origin or mode of deposition. The broadest divisions are (a) residual soils, derived by in-place chemical and physical decomposition (weathering) of parent rock of soil materials and (b) transported soils, redistributed from their original or other site of deposition by Bureau of Reclamation (1974).

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 156

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 157

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 158 water, gravity, wind, or ice and deposited in water or on land. The regional distribution of these soils in the United States is shown in Figure 5-10. Discussed below are brief discussions of the major subdivisions of these two broad groups, along with examples of their implications for assessing and improving the safety of existing dams. Implications For Existing Dams Residual Soils The nature of residual soils is determined by a wide array of complex variables, including the parent materials' mineralogy, texture, and structure; climate; rate of surface erosion; topography; location of groundwater table; and types of vegetation. All of these factors change over time, and since most residual soils require tens of thousands to hundreds of thousands of years for their development in any significant thickness, they have been subjected to a wide range of each of these variables. Also, because of these many variables, residual soils may be erratic laterally and vertically on a specific site. A mature residual soil profile has the following generalized stratigraphy: A-Horizon. A layer at the surface containing organic litter. It is typically relatively sandy, the clays having been removed by rainwater seeping down through the layer. It is normally only a few inches thick and is gradational down into the: B-Horizon. A relatively clayey layer, the clays having evolved from chemical weathering of feldspars, micas, and other silicate minerals. Weathering has destroyed evidence of the parent materials' structure (such as layering and joints) and the soil has its own ''new'' texture and structure. This horizon may vary in thickness from a few inches to several feet and is gradational downward into the: C-Horizon. An intermediate zone between relatively unweathered parent materials and the highly weathered B-horizon. The structure of the parent material is present to some degree (increasingly so with depth), but the mineral grains of the parent material are partially weathered, breaking or loosening the intergranular cohesion of the parent materials. This horizon is gradational downward into unweathered materials and may be inches to tens of feet in thickness. The parent materials of residual soils may be rocks of igneous, metamorphic, or sedimentary origin; residual soil profiles may be developed on unconsolidated sediments (transported soils). Since most transported soils are

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 159 relatively young, residual soils developed on them usually are thin; exceptions do exist. Large regions of the United States are underlain by residual soils developed on metamorphic, igneous, and sedimentary rocks, and a vast number of existing dams are composed of and founded on residual soils. Some broad generalizations can be made about implications of residual soils for existing dams, recognizing that many exceptions exist. These generalizations are as follows: • The upper soils (B-horizon) are usually more clayey and less permeable than the deeper (C-horizon) soils. Even within the C- horizon, wide ranges in permeability may exist, depending on the variability of the parent materials. Borrow pits for embankment materials may be developed to use selectively the less permeable soils for a foundation cutoff trench and for dam core materials and the more permeable soils for building the outer zones of the embankment. However, in many existing dams the proper zonation was reversed during construction, placing less permeable soils in the outer part of the slope. This creates a downstream seepage barrier that raises the phreatic surface in the dam and increases uplift pressures. This may create an unstable embankment subject to structural or seepage failure. Another common problem is horizontal interlayering of less permeable and more permeable soils during construction. This problem is found most often where large or multiple borrow pits were developed for embankment materials and earth-moving equipment was extracting soils from different depths somewhat simultaneously. Such horizontal layering of more and less permeable soils within the embankment can produce unsafe seepage pressures in the embankment, leading to structural or seepage failure. • Relict joints, foliation or bedding planes, and faults in the C-horizon soils in a dam foundation usually control the quantities and preferential directions of seepage in the residual-soil foundation mass. Concentrated seepage along these relict structures, particularly near the outer toe of the dam, can develop into foundation piping. Furthermore, relict structures in the foundation residual soils may create zones of weakness subject to structural failure. When this is a problem, it usually develops during or soon after construction. Thus, this should be a consideration for analysis of foundation stability under earthquake loading of existing dams. • Residual soils developed from crystalline (igneous and metamorphic) rocks may contain platy minerals, such as micas, that have important effects on the engineering characteristics of the soil, both in-place and remolded (in the embankment). One of the more important and dramatic effects of platy minerals in the soft is their tendency to rotate during compaction into horizontal positions, giving the embankment soils a higher

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 160

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 161

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 162 horizontal permeability compared with their vertical permeability. In most applications this is an undesirable characteristic and can lead to serious defects in the dam from both structural and seepage standpoints. When residual soil is formed from granitic rocks, the result can be a very granular material resembling sand. Such deposits, sometimes referred to as "DG" (decayed granite) have attained thicknesses of 1,500 feet. They can be very unstable when excavations are made into them; also, they are usually very pervious. • Shales normally produce a clayey residuum, the nature of which varies widely depending on the mineralogy of the particular shale formation and many other variables. Though in the United States some shales have weathered to relatively stable soils suitable for dam construction and foundations when properly placed, some of the most treacherous soils in the nation are shale residuum and partially weathered shales, particularly those shales and soils containing significant amounts of sodium-montmorillonite clays. Among the most notable shales yielding problem soils are the widespread Pierre formation, Bearpaw formation, some other Cretaceous shales of the mid-continent and western regions, and some of the carboniferous shales in the mid- continent and Appalachian regions. Where these and similar problem shale soils exist, they are often suitable only for dam core construction due to their high-plasticity, swelling, and cracking characteristics and their sensitivity. However, they may have been incorporated in construction of existing homogeneous embankments and this can become a problem. Another problem with many C-horizon soils developed from shale is relict planes of weakness in the soils (bedding and joints). A related problem is the tendency of these soils to break into cobble-to-gravel-sized blocks during excavation; these materials can degrade (by swelling and slaking) in the embankment and under or around spillways and other appurtenant structures, causing serious defects. • Residual soils developed on relatively soluble rocks, such as carbonates, may contain cavities or natural "pipes" resulting from raveling of soil into cavities in the underlying rocks. These soil cavities may be open or filled with a wide variety of secondary residual and transported soils. This is obviously a serious foundation defect. A related characteristic is that the rock/residuum contact is often extremely irregular, thus causing large variations in soft thickness under the dam. Some of these soils are relatively compressible, and the resulting differential settlement of the dam can create cracks that may lead to structural or seepage instability. Some weathered limestone forms laterite, usually a reddish and very clayey soft. This soft often is erratic in its properties but is usually impervious and can be unstable in excavated slopes.

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 163 • Residuum derived from sandstones and conglomerates often has characteristics somewhat similar to those derived from some crystalline rocks. Permeability tends to increase with depth and may create foundation seepage deficiencies, especially where the original rocks contained few fine-grained minerals. Some sandstones contain significant amounts of carbonates, and their residuum and solubility may create problems similar to those outlined above for the carbonate soils. Transported Soils Any soil that has been moved from its site of origin and redeposited is in this broad group of soils. If the soil is deposited in a stable environment it eventually becomes indurated, forming a sedimentary rock. Some of the deeper sediments along the continental margins (Coastal Plains) and in some other areas are at least partially lithified or indurated, and there is no consensus as to whether these materials should be called soils or rocks. However, for the purposes of assessing and improving the safety of existing dams, most of the transported soils of interest are relatively unconsolidated or loose surficial deposits of somewhat recent origin. The following discussions outline the major categories and some of their possible implications for existing dams. Alluvial (Fluvial) Soils. Soils transported by streams and deposited in the streambed or adjacent floodplain are called alluvial soils or alluvium. They are almost universally present at dam sites. Alluvial soils may vary in thickness from a few inches to hundreds of feet and may vary in composition from large boulders to clays. Most commonly they are composed of gravels, sands, silts, and clays that have been sorted (in varying degrees) into discontinuous layers and lenses with wide ranges in horizontal permeability. They commonly are poorly consolidated (soft or loose), weak and compressible, often wet, and may contain layered or dispersed organic material. Removal of alluvial soils under at least the core of the embankment and often the entire embankment is usually required before dam construction to ensure structural and seepage stability. Many existing dams have defects resulting from improper use of alluvial soils in the embankment or inadequate treatment of an alluvial foundation. Problems have developed where contractors have minimized excavation of a cutoff trench or an embankment foundation or have used alluvium in the future reservoir area as borrow for embankment construction. As discussed in another section of this report, the Bearwallow Dam failure was at least partly caused by placing organic alluvium in the embankment. Also, this stripping of what may have been an impervious layer over the reservoir can

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 164 result in high seepage losses, such as occurred at Helena Valley Dam in Montana (Figure 5-11). Excessive seepage through permeable alluvial layers in the foundation is a common defect of existing dams. Although this can be precluded by proper cutoff trench construction, inadequate dewatering during construction can lead to a "messy" cutoff trench that does not adequately penetrate highly permeable layers. For example, a public water supply dam in North Carolina nearly failed from foundation piping due to an inadequate seepage cutoff in its alluvial foundation. An attempt had been made to construct a slurry wall cutoff before construction; the slurry trench was excavated by a dragline, and it failed to penetrate completely a gravel layer at the base of the alluvium. After the dam was completed and the reservoir was filled, seepage through the alluvial gravels caused severe boils and piping at the dam's toe. This required immediate draining of the reservoir to prevent failure of the dam. Problems with alluvial foundation soils are not limited to those dams that had no preconstruction subsurface investigations. Some investigators fail to fully appreciate the erratic geometry of the layers, pods, and lenses composing most alluvial deposits. Furthermore, even with the most diligent site investigation, highly permeable or weak and compressible layers may be undiscovered. Figure 5-11 Erosion caused by seepage through floor of Helena Valley Reservoir.

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 165 Some existing embankment dams are composed, at least in part, of alluvial soils that would be acceptable if they had been properly placed instead of the more and less permeable soils being segregated into horizontal layers in the embankment. Drilling and sampling normally would be required to discover this defect. Gravity-Transported Soils. There are two common types of gravity- transported soils: (1) colluvium and (2) talus, plus a special class (3) landslide materials. 1. Colluvisum. Soils (and rock fragments in a soil mass) that have been transported downslope on hillsides by mass wasting and gravitational creep are colluvial soils. In addition to gravity, their movement is aided by ice heave, tree roots, animal burrows (including worms), and water. They are evidenced by bent tree trunks, hummocky or irregular slopes, landslide scars, or heterogeneous soil mixtures (lack of a mature weathering profile). In some eases the transition from residual soils to colluvium is exhibited in natural or man-made excavations by relict structural planes bent downhill and grading upward into a jumbled mass. Natural rates of movement of these materials are probably on the order of a few inches per year maximum, more typically a few inches over several hundred years. Thickness varies from a few inches to over 100 feet, and the composition varies widely from region to region. Despite local and regional variability of colluvium, all colluvial soils have one thing in common: they are inherently unstable and are likely to develop into landslides; relatively rapid movement of large masses can be triggered by minor man-made changes in topography, loading, or drainage as well as by natural events, such as heavy rainfall, snow-melt, or earthquakes. Colluvium is particularly hazardous to existing dams where landsliding may impact the reservoir, spillways, or other appurtenant structures. Furthermore, it must be ensured that portions of the dam or associated structures are not founded on colluvium in a state of creep. 2. Talus. Also sometimes called scree or rubble, talus is an accumulation of rock debris at the base of a steep slope. It is classically developed at the foot of mountains in relatively arid or cold regions where mechanical weathering of exposed rock slopes outstrips chemical weathering and soil formation. Talus has many of the same characteristics as colluvium, and talus deposits often creep, forming a "rock glacier" in valleys. Particularly treacherous for unwitting dam builders are creeping talus deposits that are masked by a surficial deposit of more recent soils. Defects at sites of some existing dams may include talus foundations and damaging talus slides. 3. Landslide materials. Relatively large masses of otherwise intact soils beneath or around a dam and its reservoir may in fact have been displaced from their site of origin by landsliding. Old landslide blocks normally come

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 166 to rest at a state of marginal equilibrium and are subject to renewed movement when subjected to very minor changes in the environment. This may have very serious implications for existing dams, similar to the problems outlined above for colluvial and talus deposits. For example, special precautions had to be taken to protect the Oahe Dam on the Missouri River in South Dakota from landslides; the reservoir and associated highways and bridges are still being seriously affected by old landslides that were reactivated by the impoundment and by construction activities (Gardner and Allan 1979; see also Chapter 9 on reservoir problems). This instability results from high percentages of expansive clay (montmorillonite) both within the shale bedrock and between the shale layers. Aeolian Soils. The most common aeolian (wind deposited) soils are loess and dunes. Large areas in the central and western United States are covered with loess, which is a wind-deposited silt. Among the most notable areas are the Mississippi and Missouri river valleys and tributary areas where some loess deposits are tens of feet thick. Other areas covered by significant loess deposits include the High Plains, some of the Basin and Range valleys, the Snake River Plain, and the Columbia Plateau (palouse soil). From an engineering point of view, loess deposits are characterized by their erodibility and by their tendency to collapse or subside drastically when wetted and under a structural loading. The sensitivity of loess is due to its high vertical permeability, angular grains, and weak cementation between grains. Subsidence of loess in their foundation can produce serious differential settlement in existing dams and their appurtenant structures. Where loess has been used in constructing embankments, the sensitivity may have been eliminated by remolding and compaction, but the embankment soils still may be relatively weak. Though not nearly as widespread as loess in the United States, dunes are common in some of the western regions. Some older (Pleistocene) dunes may be covered by residual or other soils, and their presence may not be obvious. Loosely compacted, relatively clean sands in dunes are subject to liquefaction and seepage problems where they are present in the foundations of existing dams. Glacial Deposits. Glacial soils produced and deposited by Pleistocene continental ice sheets are prevalent in the northern United States. Much less common are soils associated with alpine (mountain) glaciers, but they are very, important at many dam sites in the Rocky Mountains, Sierra Nevada, and some of the other higher western mountains. "Glacial soils" is an extremely broad term and is used to include all forms of glacial drift, in

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 167 cluding soils moved and mixed by the large ice masses and then deposited by meltwaters (e.g., outwash plains, kames, eskers) as well as unstratified till (including moraines) deposited more directly by glaciers. Very important glacially related soils from an engineering standpoint are glaciolacustrine (lake) and glaciomarine deposits. In addition to wide variations in the origin and nature of glacial soils, the thickness of glacial deposits varies from a few inches to several hundreds of feet. Because of such wide variations, few generalizations on the implications of glacial soils for existing dams can be made. Particularly likely to create dam defects are loose, permeable unconsolidated drift and sensitive glaciomarine soils in the dam foundation or in the reservoir margin. With this simple word of caution the reader is referred to detailed published geologic and soils maps that may be available for the locality of a particular dam. For more general background on glacial soils and their engineering implications, excellent information can be found in Legget (1961) and Krynine and Judd (1957). Dispersive Clays Of particular importance for some existing dams are clay deposits that disperse (deflocculate, disaggregate) rapidly in water; they may be either residual or transported in origin and are discussed here as a special class. A few dam failures and many accidents or near-failures have been attributed to these soils. Problems with dispersive days include surface erosion, and particularly unusual gully erosion in natural slopes, cut slopes and embankments. Erosion along shrinkage cracks in embankments sometimes produces tunneling and jugging, common terms applied to piping and sinkhole development. In 1973, 20 dams in Mississippi were discovered to have unusually severe erosion problems from this cause, after a period of heavy rainfall following a dry period. Probably even more important is the susceptibility of dispersive clays to piping at the seepage exits of the embankment, particularly where cracks (even minute cracks) provide avenues for concentrated seepage and dispersed day removal. Piping failure of the dam can develop rapidly under these conditions. An excellent treatise on dispersive clays is that by Sherard and Decker (1977). It is a collection of 32 research papers. Some of the conclusions of the editors are paraphrased as follows: • Limited data indicate that dispersive soils are most commonly found as alluvium, lacustrine, slope wash, or weathered loessial deposits. How

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS Figure. 5-12 Seismic zone map of the United States. Source: Algermissen (1969) and Algermissen and Perkins (1976). 168

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 169 ever, they have been found in residual soils developed from igneous, metamorphic, and sedimentary deposits. According to Sherard and Decker: ''It must be anticipated that they could be found anywhere.'' • A simple field test is the Emerson Crumb test, conducted by dropping a small ball of soil into a container of water and observing the relative speed of dispersion. However, although field tests may serve as indicators, none is absolutely conclusive in identifying highly dispersive soils. • To determine the presence of dispersive clays, four tests should be performed: (1) the Pinhole test, (2) tests of dissolved salts in pore water (Na/total dissolved salts ratio), (3) SCS dispersion test, and (4) the Emerson Crumb test. No single one of these tests is always conclusive. • There must be some exit of concentrated leakage (such as cracks) for piping to develop in dispersive clays. • Piping can be prevented by installing a filter containing significant amounts of fine sand. Another method is to treat the upper first foot of the embankment with calcium (such as lime or gypsum). • Erosion of the upstream face can be prevented by lime treatment or a protective blanket of nondispersive soil. Sources of Geologic and Pedological Soils Maps Geologic maps covering the dam and surrounding areas may be obtained through the State Geological Surveys. These agencies have maps developed by their own staff, who are knowledgeable of other geologic maps that may be available from the U.S. Geological Survey, universities, and elsewhere. Agricultural soils survey maps may be obtained from the local Soil and Water Conservation District, the U.S. Department of Agriculture Soil Conservation Service, or the local Agricultural Extension Agent. EARTHQUAKE CONSIDERATIONS In any rock formation, high strains within the crust may induce stresses higher than the strength of the rock. This can result in a sudden release of stored energy that causes a fracture (fault) that may extend over a considerable area. For example, the 1906 San Francisco earthquake produced a break over 300 miles long. The energy from such a sudden rupture spreads out in all directions with decreasing ground motion at increasing distance from the break. Major earthquakes seem to occur where they have happened before. However, it is possible to have an earthquake almost anywhere. Hence, from studies of past earthquakes a given region can be identified as having a high or low probability of earthquake, such as is shown in Figures 5-12 and 5-13.

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 170 Figure 5-13 Seismic zones for Alaska and Hawaii. Source: Algermissen (1969) and Algermissen and Perkins (1976).

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 171 At any location on the earth's surface remote from the actual source or epicenter of an earthquake, energy is felt as a series of shocks in all directions. From seismometers installed at various locations, a record of these shocks can be obtained in terms of the acceleration, usually expressed as a fractional equivalent of the acceleration of gravity, e.g., 0.20 acceleration due to gravity. The shocks can be expected to vary in intensity with time in an irregular manner. The greatest accelerations usually are in a horizontal direction, and, because of the inertia effect, a horizontal shear force is set up at the base of the dam and then successively upward throughout the dam as it responds to the shear force at the base. However, it is acknowledged that in some major events the vertical acceleration has exceeded the horizontal (Kerr 1980). If the dam is moving toward the reservoir the force exerted on the surface of the dam by the water is momentarily increased because of the inertia of the water. Conversely, when the dam moves away from the reservoir this hydrodynamic force tends to decrease the water pressure. Thus the effect of the earthquake in the dam is twofold: the body force due to the inertia of the dam and the hydrodynamic force caused by the interaction of the dam and reservoir. Ground Motion Analyses To obtain data essential for estimating the performance of a dam during an earthquake, it is necessary to adopt a methodical approach, as is shown in Figure 5-14. Determination of ground motions at a dam site requires estimates to be made of the following: (1) magnitude and epicenter of earthquakes that are expected to affect the site and (2) the ground motion that can be produced by the estimated earthquakes. Obtaining such information will require a detailed study of seismological records for the region in combination with a study of the regional geology and the immediate site geology. The regional studies should be on the area within a 200-mile radius of the dam. A first step is to determine the seismic zone within which the dam is located (see Figures 5-12 and 5-13). A commonly used method of measuring and expressing ground motion involves the following four steps: 1. modified mercalli intensity (see Table 5-4), 2. individual ground motion parameters (maximum acceleration and velocity, predominant period of the wave, and duration of the strong shaking), 3. response spectra, and 4. accelerograms.

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 172 Figure 5-14 Procedure to determine ground motion at a site. Source: Boggs et al. (1972).

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 173 Intensity data can be developed by interviewing people that live within the 200-mile radius. From them it is necessary to obtain descriptions of any damage to structures, any earthquake movements they felt while living in the region, and displacement or damage that these movements may have caused to objects within residences or other buildings. Acceleration is one of the ground motion parameters and has an approximate relationship to magnitude and intensity (Table 5-4). Generally acceleration does not indicate the frequency or the duration of the shaking. Therefore, it is necessary to consider acceleration period and duration in order to describe acceleration. Numerous methods have been proposed for this purpose. The objective is, in terms of acceleration, to identify the maximum credible earthquake (MCE) and the operating basis earthquake (OBE). Both the MCE and the OBE are considered in the design and evaluation of dams. The latter is some fraction of the MCE, perhaps one-half. The OBE may be selected on a probabilistic basis from regional and local geology and seismology studies as being likely to occur during the life of the project. Generally, it is at least as large as earthquakes that have occurred in the seismotectonic province in which the site is located. Under the MCE the dam, if correctly designed, would suffer a small amount of damage, but there would be no release of reservoir water. Under the OBE there should be no permanent damage, and the dam should be able to resume operation with a minimum of delay after the earthquake. A first estimate of maximum acceleration can be obtained from the USGS Open File Report 76-416. A more precise determination of the maximum acceleration to be expected at a given locality requires the combined consideration of the following: 1. the earthquake record for the region, 2. the length and the depth of all major faults, 3. whether the foundation material is rock or soil, and 4. the distance of the dam site from the faults. After this information has been obtained, it is necessary to make three independent determinations to estimate the effect of an earthquake on a specific dam: 1. the amount of earthquake force, 2. the maximum stresses in the dam, and 3. the material strength required to resist these stresses. These determinations can be accomplished in two steps: (1) a Phase I analysis that uses a series of empirical parameters in a pseudo-static linear analysis (this gives a comparatively quick approximation of behavior expressed as maximum stresses or safety factors) and (2) a Phase II analysis

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 174

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 175 that will focus on questionable details and determine maximum stresses more accurately. For most dams the Phase I analysis should be sufficient. In borderline cases the Phase II analysis may be required. As previously noted, there are numerous methods for estimating ground motion values for magnitude and distance from a fault. A system that has been used frequently by the U.S. Bureau of Reclamation is based on work by Schnabel and Seed (1972). The relationship between acceleration and fault location is demonstrated by the curves in Figure 5-15. Because of the complex nature of earthquake-induced ground motion and its interaction with structures, the concept of response spectrum has been developed. This is a plot of the maximum values of acceleration, velocity, and displacement that will be experienced by a family of single-degree-of-freedom systems subjected to a time-history of ground motion. Maximum values of the parameters are expressed as a function of the natural period in damping of the system. The response spectrum for a given earthquake can be estimated directly if the magnitude and distance or the maximum ground acceleration, velocity, and displacement are known. It is Figure 5-15 Average values of maximum accelerations in rock. Source: Schnabel and Seed (1972) and Boggs et al. (1972).

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 176 also possible to estimate the response spectrum indirectly from sealed accelerograms. If only intensity data are available, an estimate must be made of the magnitude and distance of an earthquake that would produce the predicted site intensity. In these determinations the selected accelerogram records should be compatible with ground motions expected from the design earthquake. The accelerograms can be obtained from historical records, from artificially generated accelerograms, or from specialists in this type of analysis. In all cases the records should be evaluated by specialists in the field of earthquake engineering. The first step in determining ground motion is to locate the most recently available map. Estimates of the MCE and OBE from these maps are indicated in Table 5-5. Magnitude estimates can be made directly from instrument and intensity data and surface faulting. Indirect estimates can be developed from the site intensity and acceleration relationships (plus an attenuation factor) that determine the epicentral intensity. The design accelerograms are obtained by selecting real or synthetic accelerograms that can be adjusted to approximate the peak acceleration. Response spectra from design accelerograms must be similar to the design response spectra. Thus, several design accelerograms should be used for the analysis of any structure. Behavior of earth embankments and mechanical equipment depends not only on the magnitude of the seismic event but also on its duration. This means that the design and condition of the structures must be considered along with the response spectrum. A typical response spectrum is shown in Figure 5-16, which compares the natural period of the earthquake with the maximum acceleration. The earthquake magnitude can be correlated in a general way with the length of rupture along a fault. Generally, the greater the rupture length the larger will be the earthquake magnitude. Figure 5-17 depicts such relationships for a number of specific earthquakes and faults. (Each number on the figure refers to a specific fault and associated earthquake.) As can be seen on this figure, there is considerable scatter to the data; this indicates that expert knowledge is required to extrapolate this type of information TABLE 5-5 Earthquake Acceleration Region MCE OBE 0 0g 0g 1 0.1 g 0.05 g 2 0.2 g 0.1 g 3 0.4 g 0.2 g NOTE: g = acceleration due to gravity.

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 177 for any specific dam site. A detailed evaluation of the relationship between faults and earthquake magnitude is given in Slemmons (1977). Figure 5-16 Linear plot of response spectra. Source: Boggs et al. (1972). Analysis. The analysis of the behavior of a dam under the selected ground motions requires considerable experience and specialization. Generally speaking there are two methods used: (1) the quasi-static method wherein the behavior of the dam is studied under the action of the reservoir-water forces that are induced by an earthquake and (2) the dynamic analysis that considers both maximum horizontal and vertical accelerations and the frequency components. The second method requires a more complex and costly analysis that would be used only where the cost and type of structure would justify it. The objective of such analyses is to determine the dam response to different types of earthquake loadings. For example, in an arch dam, major consideration is given to the various movements of the rings and cantilevers that constitute an arch dam; for a gravity dam, attention also must be given to the shearing forces within the dam or along its base. For a concrete dam the forces are applied in upstream, downstream, and vertical directions in a mode that will produce stress everywhere in the structure and the maxima of these stresses are collected for study. For earth

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 178 dams the choice of method is more involved, and the reader is referred to the detailed discussions presented in Seed (1979) and Seed (1983) for a presentation of the latest state-of-the-art approaches. Figure 5-17 Relationship of earthquake magnitude to length of surface rupture along the main fault zone. Source: Slemmons (1977). The above information and procedures are relevant to the Phase I analysis. In the Phase II analysis a much more detailed study of dam behavior must be made. For example, the following information is needed:

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 179 1. Location of all active or capable faults in the vicinity. 2. The length and typical depth of each fault. 3. The product of the values in step 2 to find the energy released from the fault. 4. The attenuated energy received at the dam site (the perpendicular distance from the dam site to the fault is used). 5. The MCE, which will be the maximum of any of the energies determined in step 4. 6. The OBE, which can be estimated at one-half MCE. The duration of strong shaking can be estimated from empirical data, such as shown in Figure 5-18. The shaking duration is generally assumed to continue so long as faulting is occurring. Attenuation is the relationship between the amount of energy produced at the source (hypocenter) of the earthquake and the amount of energy available at some specific distance from this source. It is usually expressed as an attenuation factor, which is the acceleration at a site divided by the acceleration at the epicenter. Several approaches have been used, and the relatively wide range of these results is indicated in Figure 5-19. Generally, the empirical studies indicate that the range can be from a rapid attenuation in the first 20 miles to a more gradual attenuation at greater distances from the epicenter. At large epicentral distances the attenuation factors Figure 5-18 Duration of strong shaking. Source: Boggs et al. (1972).

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 180 may vary by several magnitudes. This means that engineering judgment must be used to consider the effects of focal depth, unusual geologic structure, unequal distribution of energy radiation with direction from the epicenter, etc. Figure 5-19 Attenuation factor versus distance. Source: Boggs et al. (1972). To determine the possible frequency of earthquake occurrence at a site it is necessary to use probability methods; the input data are obtained from existing earthquake records. There is some agreement (Boggs et al. 1972) that there is a definable relationship between earthquake magnitude and frequency of occurrence, such as:

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 181 where N equals the number of earthquakes, M equals magnitude, and a and b are constants established by observations. REFERENCES Algermissen, S. T. (1969) Seismic Risk Studies in the United States , 4th World Conference on Earthquake Engineering, Santiago, Chile, Vol. I. Algermissen, S. T., and Perkins, D. M. (1976) A Probabilistic Estimate of Maximum Acceleration in Rock in the Contiguous United States, U.S. Geological Survey Open File Report 76-416. Beene, R. R. W. (1967) ''Waco Dam Slide,'' Journal of the Soil Mechanics and Foundation Division, Proceedings of the ASCE, Vol. 92, No. SM4, July. Bellier, J. (1976) The Malpasset Dam, The Evaluation of Dam Safety, Engineering Foundation Conference Proceedings, November 28-December 3, ASCE. Blume, J. (1965) "Earthquake Ground Motion and Engineering Procedures for Important Installations Near Active Faults", Proceedings 3rd World Conference on Earthquake Engineering, New Zealand, Vol. III. Boggs, H. L., et al. (1972) Method for Estimating Design Earthquake Rock Motions, U.S. Bureau of Reclamation Engineering and Research Center, Denver, Colo. (revised). Cloud, W. K., and Perez, V. (1969) "Strong Motion Records and Acceleration," Proceedings, 4th World Conference on Earthquake Engineering, Chile, Vol. I. Gardner, Charles H., and Tice, T. Allan (1979) The Forrest City Creep Landslide on US-212, Oahe Reservoir, Missouri River, South Dakota , Proceedings, National Highway Geology Symposium. Gutenberg, B. and Richter, C. F. (1942) "Earthquake Magnitude, Intensity, Energy and Acceleration," Bulletin Seismological Society of America , Vol. 32, No. 3, pp. 163-191. Hunt, C. B. (1974) Natural Regions of the United States and Canada , W. H. Freeman, New York. Independent Panel to Review the Cause of Teton Dam Failure (1976) Failure of Teton Dam, Report to U.S. Department of Interior and State of Idaho. Jansen, R. B., Dukleth, G. W., Gordon, B. B., James, L. B., and Shields, C. E. (1967) "Earth Movement at Baldwin Hills Reservoir," Journal of the Soils Mechanics and Foundation Division, Proceedings of the ASCE, Vol. 93, No. SM4, July. Judd, W. R. (1969) Statistical Methods to Compile and Correlate Rock Properties and Preliminary Results, Purdue University Technical Report No. 2, Office of Chief of Engineers, Department of the Army, NTIS AD701086. Judd, W. R. (1971) Statistical Relationships for Certain Rock Properties , Purdue University Technical Report No. 6, Office of Chief of Engineers, Department of the Army, Washington, D.C. Kerr, R. A. (1980) "How Much Is Too Much When the Earth Quakes?" Science, Vol. 209, August. Krynine, D. P., and Judd, W. R. (1957) Principles of Engineering Geology and Geotechnics, McGraw-Hill, New York. Legget, R. F., ed. (1961) Soils in Canada, The Royal Society of Canada Special Publications, No. 3, University of Toronto Press. Leps, T. M. (1972) Analysis of Failure of Baldwin Hills Reservoir, Proceedings, Specialty Conference on Performance of Earth and Earth-Supported Structures, Purdue University and ASCE. Morrison, P., Maley, R., Brady, G., and Porcella, R. (1977) Earthquake Recordings On or Near Dams, U.S. Committee on Large Dams Committee on Earthquakes, California Institute of Technology.

GEOLOGIC AND SEISMOLOGICAL CONSIDERATIONS 182 Schnabel, P. B., and Seed, H. B. (1972) Accelerations in Rock for Earthquakes in the Western United States, Earthquake Engineering Research Center Report 72-2, University of California, Berkeley. Seed, H. B. (1979) "Considerations in the Earthquake-Resistant Design of Earth and Rockfill Dams," Geotechnique, Vol. 29, No. 3, pp. 215-263. Seed, H. B. (1983) "Earthquake-Resistant Design of Earth Dams" in Seismic Design of Embankments and Caverns, Proceedings of a Symposium Sponsored by the ASCE Geotechnical Engineering Division in conjunction with the ASCE National Convention, Philadelphia, Pa., May 16-20, 1983 (Terry. R. Howard, ed.). Sherard, J. L., and Decker, R. S., eds. (1977) Dispersive Clays, Related Piping, and Erosion in Geotechnical Projects, ASTM Special Technical Publication 623, American Society for Testing and Materials. Slemmons, D. B. (1977) Faults and Earthquake Magnitude, U.S. Army Engineer Waterways Experiment Station Miscellaneous Paper S-73-1, Vicksburg, Miss. Sowers, G. F. (1971) Introductory Soil Mechanics and Foundations, Macmillan Publishing Co., New York. Thomas, H. E., Miller, E. J., and Speaker, J. J. (1975) Difficult Dam Problems—Cofferdam Failure," Civil Engineering, August. Touloukian, Y. S., Judd, W. R., and Roy, R. F. (1981) Physical Properties of Rocks and Minerals, McGraw-Hill/Cindas Data Series on Material Properties, Vol. II-2, McGraw-Hill, New York. U.S. Bureau of Reclamation (1974) Earth Manual, 2d ed., Government Printing Office , Washington, D.C. U.S. Department of Agriculture, Soil Conservation Service (1975) Engineering Field Manual for Conservation Practices, NTIS Publ. PB 244668. Recommended Reading Buol, S. W., Hole, F. D., and McCracken, R. J. (1973) Soil Genesis and Classification, Iowa State University Press, Ames. Gary, M., McAffee, R., Jr., and Wolf, C. L., eds. (1972) Glossary of Geology, American Geological Institute, Washington, D.C. Hunt, C. B. (1977) Geology of Soils, W. H. Freeman, New York. Jaeger, C. (1972) Rock Mechanics and Engineering, Cambridge University Press. Legget, R. F. (1962) Geology and Engineering, McGraw-Hill, New York. National Academy of Sciences, Committee on Seismology (1976) Predicting Earthquakes, NAS, Washington, D.C. Sopher, C. D., and Baird, J. V. (1982) Soils and Soil Management, Reston Publishing Company, Reston, Va. Terzaghi, K., and Peck, R. B. (1967) Soil Mechanics in Engineering Practice, John Wiley & Sons, New York. Woods, K. B., Miles, R. D., and Lovell, C. W., Jr. (1962) "Origin, Formation, and Distribution of Soils in North America" in Foundation Engineering, G. A. Leonards, ed., McGraw-Hill, New York.

Next: 6 Concrete And Masonry Dams »
Safety of Existing Dams: Evaluation and Improvement Get This Book
×
Buy Paperback | $100.00
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

Written by civil engineers, dam safety officials, dam owners, geologists, hydraulic engineers, and risk analysts, this handbook is the first cooperative attempt to provide practical solutions to dam problems within the financial constraints faced by dam owners. It provides hands-on information for identifying and remedying common defects in concrete and masonry dams, embankment dams, reservoirs, and related structures. It also includes procedures for monitoring dams and collecting and analyzing data. Case histories demonstrate economical solutions to specific problems.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  6. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  7. ×

    View our suggested citation for this chapter.

    « Back Next »
  8. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!