Safety of Existing Dams Evaluation and Improvement (1983) / Chapter Skim
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5. Geologic and Seismological Considerations
5. Geologic and Seismological Considerations
Pages 132-182
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From page 132... ...
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.
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From page 133... ...
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.
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From page 134... ...
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.
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From page 135... ...
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.
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From page 136... ...
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.
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From page 137... ...
White or colorless but can be tinted. Easily disintegrated by normal weathering processes.
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From page 138... ...
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.
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From page 139... ...
Usually low density. General Comments on Rock Types The physical properties of rock are extremely variable, even for one type of rock.
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From page 141... ...
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.
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From page 142... ...
142 to .
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From page 143... ...
143 o ._ Ct ._ o Cut a)
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From page 144... ...
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.
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From page 145... ...
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.
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From page 146... ...
450 (c) FINAL 0 100 2C10 300 1 1 1 1 Scale, in teet FIGURE 5-2 Embankment sections of Waco Dam.
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From page 147... ...
. ~ future Berm _· ~ ~-420~ ~°,~ art+ Do I_ 400~ m9~\ South Fau It ~ \ FIGURE 5-3 Pore pressure contours at mid-pepper after slide.
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From page 148... ...
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)
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From page 149... ...
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.
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From page 150... ...
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.
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From page 151... ...
Geologic and Se~smo~gical 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.
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From page 152... ...
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.
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From page 153... ...
Geologic and Seismolog~cal Considerations ,00 90~ o ~A~, ~v\~L~\,~~vx~A ~~xa~ O ~ ~V\ - / V V~ \ ~ ^7 ~V \ r~,rS ~` ~ ^~ )
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From page 154... ...
has been determined, and many soil series have been subjected to other engineering tests to determine general engineering characteristics. It is emphasized that agricultural soils maps, particularly the more modem ones where correlations have been made between soil series and engi
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From page 155... ...
transported soils, redistributed from their original or other site of deposition by
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From page 156... ...
Typical Names of Soil Groups Group Symbols Permeabil ity When Compacted Shear Strength When Compacted and Satura ted Compressibil ity When Compacted and Saturated Well-graded gravels, gravelsand mixtures, little or no fines GW Pervious Excellent Negligible Poorly graded gravels, gravel-sand mixtures, little or no fines GP Very pervious Good Negligible Silty gravels, poorly graded gravel-sandsilt mixtures GM Semipervious to impervious Good Negl igible Clayey gravels, poorly graded gravel-sandclay mixtures GC I mpervious Good to fair Very low Well-graded sands, gravelly sands, little or no fines SW Pervio us Excel lent Negligible Poorly graded sands, gravelly sands, little or no fines SP Pervious Good Very low Silty sands, poorly graded sand-silt mixtures SM Semipervious to impervious Good Low Clayey sands, poorly graded sand-clay mixtures SC Impervious Good to fair Low Inorganic silts and very fine sands, rock flour silty or clayey fine sands with slight plasticity Inorganic clays of low to medium plasticity, gravelly clays, sandy clays silty clays, lean clays ML
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From page 157... ...
157 Relative Desirability for Various Uses (No. 1 is Considered the Best)
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From page 158... ...
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)
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From page 159... ...
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)
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From page 162... ...
· 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.
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From page 163... ...
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.
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From page 164... ...
Although this can be precluded by proper cutoff trench construction, inadequate dewatering cluring 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.
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From page 165... ...
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.
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From page 166... ...
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
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From page 167... ...
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.
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From page 168... ...
168 o o o o o ~ c5 i A to ¢ S ~ o ~ CD _ W~`j 1 ~ OIL.__ r .
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From page 169... ...
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.
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From page 170... ...
~' S E I S M I C Z O N E ~ a P H AWA I I ,; /3 ~9 - , PACIFIC OCEaN ~ ~ ~'~' SEISMIC ZONE MAP ALASKA 1 ~ / 2 ~oLoKal / r~ ~ ~ MAUI LANAiV} /K A H OOL aw E / /5~ ~ ......... \ HAWAII ~ V FIGURE 5-13 Seismic zones for Alaska and Hawaii.
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From page 171... ...
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)
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From page 172... ...
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.
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From page 173... ...
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.
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From page 174... ...
XII Damage nearly total. Large rock masses displaeed.
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From page 175... ...
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.
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From page 176... ...
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.
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From page 177... ...
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)
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From page 178... ...
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.
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From page 179... ...
4. The attenuated energy received at the dam site (the perpendicular distance from the dam site to the fault is used)
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From page 180... ...
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. 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.
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From page 181... ...
(1965) "Earthquake Ground Motion and Engineering Procedures for Important Installations Near Active Faults", Proceedings 3rd World Conference on Earthquake Engineering, New Zealand, Vol.
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From page 182... ...
(1977) Faults and Earthquake Magnitude, U.S.
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