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10 Proposed Earthquake Criteria BASIS FOR ANALYSIS As indicated in Chapter 6, at the current level of knowledge of earthquake occurrences in the United States, any estimate of annual probability or probable average return period for a major earthquake at a specific site is subject to considerable uncertainty. However, there are indications that, in most areas, earthquakes of the order of magnitude of the maximum credible earthquakes adopted for safety evaluations may have average return periods of, say, 500 to 1,000 years, or more, depending on the importance of the project. Earthquake experience along some major fault systems suggests shorter average return periods. It seems clear that the earthquakes adopted for safety evaluations do not represent as rare phenomena as do probable maximum precipitation estimates, which are generally considered to repre- sent rainfalls having average return periods of from 10,000 years to as long as 1,000,000 years. Of course, in some locations there is the possibility that there will never be further movement along a fault that has been judged potentially active. Other considerations apply to any comparison of criteria for safety of dams against floods and earthquakes. All dams are exposed to threat of extreme floods and require provision for such events, even though it may be extremely unlikely that a specific dam will ever experience a flood of the magnitude assumed for its safety evaluation. By contrast, only for dams located in active seismic zones are potential earthquake motions considered in safety evaluations and design, and, as indicated above, an individual dam 106

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Proposed Earthquake Criteria is more likely to experience the safety evaluation earthquake than it is to experience the safety evaluation flood. Also, failure of a dam as a result of an extreme flood would only add to the damages caused by the flood without the failure. But dam failure during an earthquake is apt to be a "sunny clay" event for which all damages are attributer] to the failure. In view of the above, the committee considers there is little basis for using less conservative criteria for any situation where failure of a dam under earthquake loadings could involve large damages and loss of life. Such com- 107 parisons of estimated frequencies as the above can raise questions regarding the major differences in probabilities of occurrence of design floods and design earthquakes. However, since both types of estimates represent proba- ble maximum events, these differences in probabilities do not represent inconsistent approaches. DESIGN CRITERIA The committee suggests that the federal agencies adopt the terminology "safety evaluation earthquake" (SEE). The SEE is defined as the earth- quake, expressed in terms of magnitude and closest distance from the dam site, or in terms of the characteristics of the time history of the free-field ground motions, for which the safety of the dam and critical structures associated with the dam should be evaluated. For high-hazard anti function- ally essential dams the SEE will be the same as the maximum credible earthquake (MCE) to which the dam will be exposed. For lower-hazard dams the SEE may be less severe than the MCE. Since the flooding caused by an earthquake-produced dam failure can rapidly affect a large area and population, and the public expects a high degree of protection from dams, the earthquake safety evaluation criteria should be stringent and conservative. The committee concludes that the safety evaluation earthquake criteria for high-hazard dams should be as follows. The dynamic response of the dam to a safety evaluation earthquake ground motion, attenuated from the source to the ~lam, should be such that it does not result in loss of the reservoir. Some damage to the dam and its appurtenant structures not critical to the stability of the dam may be allowable. For earthquake safety evaluations the assumed level of water in the reservoir usually should] be the normal full pool. The committee concludes that a safety evaluation earthquake ground motion developed by the deterministic-statistical method applied to known

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108 SAFETY OF DAMS causative faults, which is based, in part, on subjective evaluation, provides an appropriate level of conservatism for dam safety analyses. The committee suggests that federal agencies adopt this approach whenever possible. How- ever, where earthquake sources are not well defined, the seismotectonic province (semiprobabilistic) approach should be adopted. Other alterna- tives are not considered to be sufficiently well developed to be practicable at the present time and may not be so in the near future. The committee endorses the concept of an operational basis earthquake (OBE) ground motion, which is defined as the intensity of ground shaking that has a significant probability of occurring in a period of about 200 years. The dam should withstand this loading with no significant damage. The selection of an OBE is a matter of economics and public policy, and these might indicate the use of a period greater than 200 years. DESIGN APPROACH The committee endorses, in principle, the following approach to dam safety analysis and seismic design of dams currently being used by the federal agencies. Concrete Dams Adopt "defensive measures" in design and construction that will ensure foundation and abutment integrity, good geometrical configuration, and effective quality control. For gravity and buttress dams in areas of low seismicity, pseudostatic analysis methods may be used to check safety against sliding and overturn- ing; however, for high dams it may be appropriate to use more accurate methods. For high-hazard dams in areas of significant seismicity, dynamic analy- sis methods should be used for the analysis of structural response and induced stresses. For preliminary safety assessments, simplified methods may be used. Embankment Dams In design and construction, adopt such "defensive measures" as ample freeboard, wide transition zones, adequate compaction of materials in foun- dations and embankment, and a high level of quality control. For reasonably well-built dams on stable soil or rock foundations, the pseudostatic method of stability analysis may be used if estimated peak ground accelerations are less than 0.2 g.

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Proposed Earthquake Criteria 109 In areas where peak ground accelerations may exceed about 0.2 g, for dams constructed of or on soils that do not lose strength as a result of earth- quake shaking, a deformation should be estimated using dynamic deforma- tion analysis techniques. For dams involving embankment or foundation soils that may lose a significant fraction of their strengths under the effects of earthquake shak- ing, a dynamic analysis for liquefaction potential, or strength reduction potential, should be performed. COMBINED EARTHQUAKE AND FLOOD CRITERIA The committee does not consider it necessary to design dams to withstand the simultaneous occurrence of the safety evaluation earthquake and the safety evaluation flood. However, if an existing spillway is subject to fre- quent use, and an analysis with an OBE indicates that the spillway may be so damaged as to be unusable, the safety of the dam should be considered unsatisfactory. For especially high-hazard dams, more stringent combina- tions of earthquake and flood criteria may be justified.