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<br />I <br /> <br />I <br />Ie <br /> <br />I <br /> <br />I <br /> <br />I <br />I <br /> <br />I <br />I <br /> <br />Ie <br />I <br /> <br />I <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br />,- <br /> <br />I <br /> <br />. <br /> <br />. <br /> <br />A seismic risk map by Algermission and Perkins (1977) indicates that, in <br /> <br /> <br />the region of the Project area, there is a 90% probability that the <br /> <br /> <br />gravitational acceleration produced in hard rock by a seismic shock will not <br /> <br /> <br />exceed 0.04g in 50 years. The nearest epicentral location of reported or <br /> <br /> <br />recorded earthquake is mapped slightly over 3 miles south of the reservoir <br /> <br /> <br />site. This minor earthquake occurred in February, 1970, and had a magnitude of <br /> <br /> <br />4.0 and a focal depth of 33 kilometers (Kirkham and Rogers, 1981). <br /> <br />Well compacted clayey earth embankments with internal drains are known to <br /> <br /> <br />perform well under seismic loading. Stiff clays such as those of the existing <br /> <br /> <br />embankment and foundation are not subject to liquifaction or significant <br /> <br /> <br />deformation under cyclic seismic stresses. Also, compaction of clayey <br /> <br /> <br />embankments at moisture contents slightly higher than optimum make them more <br /> <br /> <br />plastic and resistant to cracking, which can result from both settlement and <br /> <br /> <br />seismic loading. Thick internal filter and drainage zones protect against <br /> <br /> <br />piping should cracking of the embankment occur from a seismic shock or <br /> <br /> <br />settlement. <br /> <br />It is recommended that for preliminary design purposes a peak horizontal <br /> <br /> <br />acceleration of 0.13g be used in pseudo-static: analysis of the enlarged <br /> <br /> <br />section. This value represents one-half of the estimated peak ground <br /> <br /> <br />acceleration resulting from the maximum credible earthquake. This <br /> <br /> <br />recommendation considers the low to moderate historically indicated seismic <br /> <br /> <br />activity of the area, the expected performance of the enlarged but relatively <br /> <br /> <br />low (44 feet high) clay embankment under seismic loads, and the apparent <br /> <br /> <br />moderate downstream risk. Final design considerc:tions may require different <br /> <br /> <br />seismic design criteria but such would not be expected to exceed the estimated <br /> <br /> <br />peak acceleration resulting from the maximum credible earthquake as reported. <br /> <br /> <br />Such values will not preclude design of an adequately stable embankment from <br /> <br /> <br />the locally available materials. <br /> <br />DAM AND RESERVOIR SITE GEOLOGY <br /> <br />Bedrock underlying the right abutment and most, if not all, of the dam <br /> <br /> <br />site is Cretaceous-age Dakota Formation. This bedrock consists of interbedded <br /> <br /> <br />sandstones and shales, and probably represents th" very upper-most part of the <br /> <br />-9- <br />