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<br />liquefac tion of the embankment or foundation ma terial. <br />Other causes of sliding instability are the reduction of <br />the shear strength of the material caused by the <br />infiltration of water through cracks in the enbankment or <br />foundation, the increased pore pressure due to <br />earthquake-induced ground motion, and the exceedence of <br />the shear strength of the material due to the increased <br />loading induced by the earthquake. Internal erosion, <br />or piping, can be caused by the impounded water <br />infiltrating cracks due to differential settlement; fault <br />movement; exceedence of the tensile strength of the <br />enbankment or foundation material and overtopping due to <br />the settlement or slope failure of the enbankment; seiches <br />in the reservoir; waves caused by slides or rockfalls into <br />the reservoir; or failure of the spillway or outlet works. <br /> <br />(2) Field Performance <br /> <br />Mo st enbankment dams which have been subjec ted to <br />earthquake s have performed well. General conc 1usions <br />which result from a close study of enbankment dam <br />performance during earthquakes are listed in section H. <br /> <br />b. Defensive Design Measures <br /> <br />Many problems which may develop into potential causes for <br />failure do not require extensive analytical treatment; rather, <br />simply the application of defensive measures will prevent <br />deleterious effects. Such defensive measures include the <br />fo llowing: <br /> <br />(1) Ample freeboard to allow for settlement, slumping, or <br />fault movements. <br /> <br />(2) Wide transition zones of material not vulnerable to <br />craCking. <br /> <br />(3) Chimney drains near the central portion of the enbankment. <br /> <br />t <br />\ <br /> <br />(4) Ample drainage zones to intercept possible flow of water <br />through cracks. <br /> <br />(5) wide core zones of plastic materials not vulnerable to <br />cracking. <br /> <br />(6) Well-graded filter zone upstream and downstream of the <br />core to choke c racks that may open. <br /> <br />(7) Crest details which will prevent erosion in the event of <br />overtopping. <br /> <br />(8) Flaring the embankment core at abutment contacts. <br /> <br />(9) Locating the core to minimize the degree of saturation of <br />materials. <br /> <br />-23- <br />