Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 132
5 Geologic arid Seismological Considerations GENERAL GEOLOGIC CONSIDERATIONS Defective foundation and reservoir conditions have perhaps been responsi- ble for a majority of dam failures and accidents (see Chapter 2~. This chap- ter will discuss the major geologic features and rock types that may contrib- ute 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 platonic or intrusive rocks and are composed of masses of crystalline particles. Depending on the relative amounts and sizes of these constitu- 132
OCR for page 133
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 extru- sive 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 com- pletely into residual soil. Voicanic 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 permea- bility and credibility 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. Voicanic rock often tends to break down easily by weathering to leave a weak residue. Each lava flow lasts only a rela- tively 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 alterecl 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. Secti~nentary flock 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 wa- ter through rock can weaken some minerals and this will leave the remain- der unsupported, so they are removed easily by wind or water erosion.
OCR for page 134
134 SAFETY OF EXISTING DAMS 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 suc- cessive layers of the deposited or sedimentary material. Hence, sedimen- tary 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 overly- ing 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 ma- terials or minerals that have been transported into the openings. Many sedimentary rocks take their names from the size of the rock parti- cles 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 par- ticles are compressed or cemented into a coherent mass, the result is a sand- stone; similarly, silt-size particles constitute a siltstone and clay-size particles result in a claystone or shale. Calcareous mud or sand may be- come limestone. Deposits of highly carboniferous materials, such as vegeta- tion, 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, sedi- mentary rocks can have other problems. For example, if the cementing ma- terial 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 re- vert 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. Lime- stones 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
OCR for page 135
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 cav- ernous 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 reor- iented 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 proc- esses, 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 metamor- phosed 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.)
OCR for page 136
136 SAFETY OF EXISTING DAMS 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 encoun- tered at a dam. Therefore, only listed are those rock types that most com- monly 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. Un- stable 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 Turf) Basalt: Igneous. Dark colored, fine "rained. Little or no quartz. Com- plex chemical formula including Na, Al, Si, O. Ca, Mg, Fe, and K in vary- ing amounts. May contain small, highly visible pores called vugs, 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. CZaystone: 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
OCR for page 137
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 se- vere 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 "rained. Visible particles appear to be angular. Can be associated with cavities or numerous open fissures. Generally high strength. Some- times called trap rock. Dolomite: Sedimentary. Composed chiefly of-Ca, Mg, and CO3. White or tinted. May be crystalline or noncrystalline. Effervesces very slowly in HC1. Can be associated with a cavernous structure. Relatively high strength. Guess: 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 con- tain some mica. Texture usually from medium to coarse "rained. 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 soil with some clay admix. Grains may be strongly or poorly interlocked. Fre- quently 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 miner- als. White to tinted. Rock composed solely or almost entirely of CaCO3 includes chalk, coquina, and travertine. All effervesce freely with any com- mon 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 de- posits that may be coherent or noncoherent. Chiefly clay and CaCO3. Gray, but other colors are frequently present. Texture may be extremely fine or granular. Often easily eroded by water.
OCR for page 138
138 SAFETY OF EXISTING DAMS Micaceous Rock: Generally igneous or metamorphic. Any rock contain- ing 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 "rained with interlocking crystals. Composition similar to that of granite. High in quartz. Same color varia- tions 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 "rained. Chiefly olivine with other iron- magnesium minerals. Dark colored. Mentioned here primarily because it com- monly 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 sand- stone where the quartz grains are tightly cemented with silica. Also may be a metamorphic rock formed from the recrystallization of sandstone. Usu- ally 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 "rained, composed of rounded or an- gular 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 cement- ing 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 eas- ily along the foliations or planes of schistosity. Can be subject to plucking action under high-velocity water. Because of the strong forces that develop
OCR for page 139
Geologic and Se~smo~gical 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 sec- ondary mineral but can be found in thick beds. Always to be regarded with caution because of its tendency to disintegrate into very incompetent mate- rial under normal weathering action. Also, can have very low shear strength. Shale: Sedimentary. Extremely fine "rained, 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 rap- idly 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 mas- sive rather than laminated. Gritty feel. At least two-thirds of the constitu- ents 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 rock fill. 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. Luff: 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 be- tween 2,600 psi and 48,200 psi. One reason for these wide variations is the
OCR for page 140
140 ~q C) o o Cq bC L~ CD 1 U. 5 Ct V C) ._ 40 'C~ U) ._ o o o o o o - x - ._ C) ._ Ct - 4 - o - o U) C) .> V' L. C; ^ ._ ~ g Co; _ x _ C~ V7 o ~ Z ~ ._ x Ct ._ ._ q) b4 Ct V' o ~ Z ~ ._ x Ct ~s ._ .= ~; Ct s~ C~ ._ x Ct ._ bC Ct ~: ~; O ~ C~ C~ CD ~ ~ — CC C~ ~ ~ C~ C~ CC~ CC~ ~- C.Q C~ ~ ~ ~) 0 ~4 ~ 1 C~ a) CC) ~ C~ _I _ — C~ 1 C] oo oO a~ oo ~ O0 ~) ~ ~ _ 0 0 ~ 1 ~ ~ o ~ ~ ~ oo ° ~ C~ ~ °° o0 ~ 0 ~ ~ oo ~ CO 0 c~ . ~ . . . . . . . . . . . . . . . . . . . ~ ~ C~ C~ CC C~ ~ C~ C~ CO ~ CO C~ _ ~ _ ~ ~ c~ ~ ~ ~ _ oo ~ oo 1 ~ ~ ~ oo _ _ ~ oo ~ u: co ~ - ~ ~ c~ ~ '~ - ~ oo ~ cO c~ — — c~ — c~ — c~ — c~ c~ — o ~ ~ ~ ~ ~ ~ ~ 0 ~ oo ~ 1 ~ ~ o ~ ~ oo ~ D ~ ~ CD ~ C~ ~r ~ co ~ oo C~ oo ~ CO ~ oo oo ~ 1 ~ ~ ~ ~ ~ ~ _ ~ - . 1 oo ~ ~ 0 0 1 ~ ~4 '~ ~ co ~ - '~ - * IO C~ C5: C~ ~4 ~ )~ 1 0 0 1 0 ~ O O a) cc c~ c~ oo ~ ~ oO ~ 0 1 0 ~ 1 C~ ~ ~ U) U) . . . . . . . . . . . . . . . . CO ~ CO ~ o ~ C~ _ _ C~ _ C~ ~ ~ ~ C~ _ _ _ _ _ _ _ _ _ _ _ o C~ - Io ca oo U) O CO O O ~ 1 0 CO 1 0 C~ O — O 1 O0 00 00 ~ ~ ~ ~ CD O ~ 1 0 0 1 0 _ ~ O _ 1 ~ . . . . . . . . . . . . . . . . . _ _ CS CO C~ CO C~ o _ ~ _ o _ o o o - I C~ C~ t— CJ) ~ C~ CC) C~ cc O 00 1 1 c~ C~ )= CO ~ 1 ~14 c) ~ O oo C~ ~ t~ C~ O C5) ~4 1 1 _ o) c~ U) ~ 1 1O . . . . . . . . . . . . . . . . . cc ~r ~ _ oo C~ ~ ~ ~ ~ _ ~ 0 u: c~ ~ ~D _ _ ~ 1 ~ 1 ~ ~ ~ ~ o _ a: _ co 0 ~ ~ _ _ _ C] ~ 1 ~ 1 oo oo 0 ~ ~ oo ~ ~ . . . . . . . . . . O U) ~ _ C~ C~ 00 ~ ~ _ ~ 1O ~4 1O )~) ~4 ~4 C~ ~ =° 1 ~ 1 ~ o ~ ~ CD ~ ~ — . . . . . . . . . . c~ O ~ CO ~ ~ 0 ~ _ _ C~ I 00 1 O0 a~ O co 1 oO O 0 ~4 ~ ~ ~ O . . . . . . . . . . ~ ~ ~ ~ ~ ~ C~ o ~ _ C] _ C~ C~ _ _ C~ _ C~ _ U) U) CC ~ ~ CO CS _ C~ - ~ ~ ~ ~ oo ~ ~ . . . . . . . _ ~ ~ C~ U) o C~ ~ ~ ~ ~ ~ CO C~ C~ Cd ~ CO o _ ~ C~ o . . . . . . . * C~ o _ o o ~ C~ ~ _ CD - +1 _ CO oo . . . . . . . . C:Q Cp ~ ~ ~ C~ o _ C~ C~ D'- ~ a ~ ~ ~ .~ ~ ~ ~ ~ c ~ a; ~ C' ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ - 00 _ _ _ ~ Ct CD C) CD .= ~ _ _ O O ~ c) O ~ O * V)
OCR for page 141
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 exam- ple, 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 ex- tensive cooling fractures being present. Similarly, if the rock has been de- posited 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 dura- ble 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 ob- served, it may be possible to identify the rock type associated with the de- fect. It must be recognized, however, that the defects noted are only those that more commonly occur, because almost any of the so-called surface de- fects may develop from any rock type. GEOLOGIC STRUCTURE Rock Foundation Defects Some of the potential defects in a rock foundation that will result from geo- logic structure are faults, joints, shear zones, and bedding and foliation
OCR for page 142
142 to . . PA o . Ct :- ~ (,, — a; ;- — Ct ~ .` so PA o . Ct . - o V' o . - - o U) vat ALL ~ . ~ TV Cal he; c) a' o a) U) Cal V so o .= Cal In En En ~ ~ ~ so ~ Cam ~ C) a' a C: - be ~~ t0 air .o C., O be be _ . ~ ~ ~ ~ 0 5-' ·— C.) ~ 41) ~ _ O ~ ~ a ~ ~ ~ ~ ~ CQ
OCR for page 172
172 ,~, , WITH LOCATE SITE ON ALGERM I SSEN ZONE MAP . ~ ZONE LESS THAN 2 ? MONO - INVESTIGATE SUR FACE _ Dl SPLACEMENT ~REEVALl 6~ ~ 4~— ; ;~] ; | RELOCATE SITE | ~ YES SAFETY OF EXISTING DAMS NO )~ TONE 2 OR 3 GREATER THE 100 Ml LES ? YES ~ IS NO ~ DATA FOR RECONNAISSANCE STUDY . —~—E S RESEARCH HISTORY OF AREA IGNORE I . SKI SM IC EARTHQUAKE OFFS nets LOADING SELECT DESIGN EARTHQUAKE EVENTS USING END OF 1. MAGNITUDES ~ DISTANCES PROCEDURE 2. INTENSITIES \ 3. ENGINEERING JUDGMENT I EsTIMArE DESIGN SPECTRA C ACCELEROGRA,~S ~ YES 1 ~ SELECT PEAK ACCELERATION j SELECT ACCELEROGRAMS FROM 1. RECORDED HISTORY 2. SYNTHETIC RECORD SE L ECT ADD I T 1O N. AL ACCELEROG RAMS CHECK SPECTRA FROM SELECTED ACCELEROCRAMS WITH DES IGN SPECTRA ~ IS CHECK ~ YES WEND OFT (R~DUR£J / NO (END OF: < - OCEDUREJ FIGURE 5-14 Procedure to determine ground motion at a site. SOURCE: Boggs et al. (1972).
OCR for page 173
Geologic and Se~smolog~ca] Cor~lerat?ons 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 approxi- mate relationship to magnitude and intensity (Table 5-4~. Generally accel- eration 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 maxi- mum credible earthquake (MCE) and the operating basis earthquake (OBE). Both the MCE and the OBE are considered in the design and evalu- ation 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 Re- port 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 inde- pendent determinations to estimate the effect of an earthquake on a spe- cific 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 ex- pressed as maximum stresses or safety factors) and (2) a Phase II analysis
OCR for page 174
174 SAFETY OF EXISTING DAMS TABLE 5-4 Approximate Relationships: Earthquake Intensity, Acceleration, and Magnitude Modified Mercalli Intensity Scale I Not felt. Marginal and long-period effects of large earthquakes. II Felt by persons at rest, on upper floors, or favor- ably placed. III Felt indoors. Hanging objects swing. Vibration like passing of light trucks. Duration estimated. May not be recognized as an earthquake. IV Hanging objects swing. Vibration like passing of heavy trucks. Standing motor ears rock. Win- dows, dishes, doors rattle. Glasses clink. In upper range of IV, wooden walls and frame crack. V Felt outdoors; direction estimated. Sleepers wak- ened. Liquids disturbed. Doors swing. Shutters, pictures move. Pendulum clocks stop, start, change rate. VI Felt by all. Many frightened and run outdoors. Persons walk unsteadily. Windows, dishes, glass- ware broken. Books off shelves. Pictures off walls. Furniture moved or overturned. Weak plaster and adobe crack. Small bells ring. VII Difficult to stand. Noticed by drivers of motor ears. Fall of plaster, loose bricks, tiles, ete. Some cracks in masonry. Waves on ponds; water tur- bid. Small slides, caving of sand or gravel banks. Large bells ring. Concrete irrigation ditches dam- aged. VIII Steering of motor cars affected. Damage to ma- sonry; some partial collapse. Twisting, fall of chimneys, monuments, elevated tanks. Frame houses moved if not bolted down. Branches bro- ken. Changes in springs and wells. Cracks in wet ground and on steep slopes. IX General panic. Weak masonry destroyed, good masonry seriously damaged. Frame structures, if not bolted, shift off foundations. Frames racked. Serious damage to reservoirs. Underground pipes broken. Ground cracked, sand and mud ejected, earthquake fountains, sand craters. X Most masonry and frame structures destroyed with their foundations. Serious damage to em- bankments. Large landslides. Water thrown on banks of canals, rivers, lakes, etc. Rails bent slightly. XI Rails bent greatly. Underground pipelines com- pletely out of service. XII Damage nearly total. Large rock masses dis- plaeed. Lines of sight and level distorted. Objects thrown into the air. SOURCE: Boggs et al. (1972~. ao''- ° aos9- o - ~ 0.19 w w 2 3 4 w J In 5 w _ o _ l Z _ . ~ 6 7
OCR for page 175
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, ve- locity, 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 natu- ral 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 of 0.7 0.6 c 0 5 o ._ o - E ._ tic Cl 0.4 0.3 0.2 0.1 O If\ 1 2 4 6 10 20 40 60 100 Distance from Causative Fault-miles FIGURE 5-15 Average values of maximum accelerations in rock. SOURCE: Schnabel and Seed (1972) and Boggs et al. (1972~.
OCR for page 176
176 SAFETY OF EXISTING DAMS also possible to estimate the response spectrum indirectly from scaled accel- erograms. If only intensity data are available, an estimate must be made of the magnitude and distance of an earthquake that would produce the pre- dicted site intensity. In these determinations the selected accelerogram rec- ords 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 mo- tion 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 approx- imate the peak acceleration. Response spectra from design accelerograms must be similar to the design response spectra. Thus, several design accel- erograms should be used for the analysis of any structure. Behavior of earth embankments and mechanical equipment depends not only on the magni- tude of the seismic event but also on its duration. This means that the de- sign and condition of the structures must be considered along with the re- sponse 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 rela- tionships 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 ~ 0.Ig 0.05 g 2 0.2 g 0.Ig 3 0.4 g 0.2 g NOTE: g = acceleration due to gravity.
OCR for page 177
Geologic and Seismological Consi~lerat~ons 5.0 4.0 1 of O ~ 3.0 UJ J UJ Cow Cry ~ 2.0 X A: ~ 1.0 177 El Centro, California May 18, 1940 N-S ! p~ o'lol ,8~002~ p~='oOO~ 1 0 0.5 1.0 1.5 2.0 NATURAL PERIOD - SECONDS 2.5 3.0 ($ ~ damping values of 0, 2, 5., and 10% of critical) FIGURE 5-16 Linear clot of response spectra. SOURCE: Boggs et al. (1972~. for any specific dam site. A detailed evaluation of the relationship between faults and earthquake magnitude is given in Slemmons (1977~. Analysts. 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 fre- quency 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
OCR for page 178
178 SAFETY OF EXISTING DAMS ooo 500 A ~ 1 Go U" z ~ so o a: . Cal at: U" tr: In o I 10 z J 1 _ I I I T ~ LEGEND A ~ORMA~-SLIP e nEVERSE-SLIP C ~OR~Ac-Oe~iCUE-S~iP D QEVERSE-~)e~IQUE-S~iP E SYRt~E-ScIP WW ~VORLDW13E NA FORTH AMERICA eOr~A eC,NILLA AND eUC - ANAN' / NORTH AMERICA I eewv' GORILLA AND eUC-4ANAN ~vOQLDwlDE i , . ~ ' I ,. so .42E '~ 1 TOE. 17Ct 7~1~1 ·~^ IJ ~ .24 E 34 E - \C' /' ~r38^ BE- .4ea^E | S7E 25E- .47E .40A 27E. 1// /./L 1 i, | BBD ~ / 1 1 2 3 4 / / /ii / | ~ BBWW I/ / 1 ~117 1 ~/~1//~ 1 5 6 7 8 ~ EARTHQUAKE MAGNITUDE 500 100 In w - J 50 ~ z z o '.11 10 6 to o I 5 t: w FIGURE 5-17 Relationship of earthquake magnitude to length of surface rupture along the main fault zone. SOURCE: Slemmons (1977~. 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 pre- sentation of the latest state-of-the-art approaches. The above information and procedures are relevant to the Phase I analy- sis. In the Phase II analysis a much more detailed study of dam behavior must be made. For example, the following information is needed:
OCR for page 179
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 deter- mined 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 attenua- tion in the first 20 miles to a more gradual attenuation at greater distances from the epicenter. At large epicentral distances the attenuation factors . L~ ~~ t / ;~11' 1 2 3 4 _ 3 MAGN ITUDE 5 6 7 8 9 FIGURE 5-18 Duration of strong shaking. SOURCE: Boggs et al. (1972). 50 40 30 oh to is 20 ° LIJ 10 o
OCR for page 180
180 SAFETY OF EXISTING DAMS 1G:h 80 60 o 40 . ~CIoud, Eq. I `1969) 1 'l_~ 20 Gutenberg- A_ _ Blu me ~ ~ Richter ~ _ _ 0 ~ 1965) ~ —~ ~~~—~- 0 as so 75 100 EPICENTRAL DISTANCE, MILES FIGURE 5-19 Attenuation factor versus distance. SOURCE: Boggs et al. (1972~. may vary by several magnitudes. This means that engineering judgment must be used to consider the effects of focal depth, unusual geologic struc- ture, unequal distribution of energy radiation with direction from the epi- center, etc. 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: log N = a - bM
OCR for page 181
Geologic and Seismological Considerations 181 where N equals the number of earthquakes, M equals magnitude, and a and h 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. Algermmsen, 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 Founda- tion Conference Proceedings, November 28-December 3, ASCE. Blume, I. (1965) "Earthquake Ground Motion and Engineering Procedures for Important In- stallations Near Active Faults", Proceedings 3rd World Conference on Earthquake Engi- neering, New Zealand, Vol. III. Boggs, [I. 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 Ac- celeration," Bulletin Seismological Society of America, Vol. 32, No. 3, pp. 163-191. Hunt, C.13. (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 Di- vision, Proceedings of the ASCE, Vol. 93, No. SM4, July. Judd, W. R. (1969) Statistical Methods to Compile and Correlate Rock Properties and Prelimi- nary Results, Purdue University Technical Report No. 2, Office of Chief of Engineers, De- partment 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, Washing- ton, 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 Con- ference 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 Insti- tute of Technology.
OCR for page 182
82 SAFETY 0F EXISTING DAMS 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 Califor- nia, 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 Em- bankments and Caverns, Proceedings of a Symposium Sponsored by the ASCE Geotechni cat Engineering Division in conjunction with the ASCE National Convention, Philadel- phia, Pa., May 16-20, 1983 (Terry B. Howard, end.. Sherard, I. 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 Min- erals, McGraw-Hill/Cindas Data Series on Material Properties, Vol. II-2, McGraw-Hill, New York. U.S. Bureau of Reclamation (1974) Earth Manual, 2d ea., Government Printing Office, Washington, D.C. U.S. Department of Agriculture, Soil Conservation Service (1975) Engineering Field Manual for Conservation Practices, NTIS Publ. P13 244668. RECOMMENDED READING Buol, S. W., Hole, F. D., and McCracken, R. I. (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, I. V. (1982) Soils and Soil Management, Reston Publishing Com- pany, 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 Distribu- tion of Soils in North America,' in Foundation Engineering, G. A. Leonards, ea., McGraw- Hill, New York.
Representative terms from entire chapter: