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<br />determine the tensile strength. For this te st, specimens <br />should be loaded at a rate which will simulate the <br />expec ted seismic loading rate. For existing dams where <br />testing of concrete cores is desired, the splitting <br />tensile test can also be used to determine the tensile <br />strength. <br /> <br />(2) Rock <br /> <br />When the foundation is included in the seismic analysis, <br />elastic moduli and Poisson's ratios for the foundation <br />materials are required for the analysis. If the <br />foundation is not considered as massless, the rock <br />densities and damping characteristics are also required. <br /> <br />Determining the elastic moduli for a rock foundation may <br />include several different methods or approaches, but the <br />effects of rock inhomogeneity (due partially to rock <br />discontinuities) on foundation behavior DlJst be taken into <br />account. The shear strength of weak discontinuities may <br />be the critical element in the analyses. Thus, the <br />determination of foundation compressibility should <br />consider both elastic and inelastic (plastic) <br />deformations. The resulting "modulus of deformation" is a <br />lower value than the elastic modulus of intact rock. <br /> <br />The rate of loading effect on the foundation moduli is <br />considered to be insignificant re1a tive to the other <br />uncertainties involved in determining rock foundation <br />properties and is not measured; to account for the <br />uncertainties, a lower and upper bound for the foundation <br />moduli should be u sed for the structural analysis. <br /> <br />(3) Damping Ratio <br /> <br />The damping ratio is usually taken to be 5 percent for <br />concrete gravity dams on competent rock in which cracking <br />of the concrete does not occur. If stresses indicate <br />c racking occurs, a value from 7 to 10 percent (based on <br />severity of cracking expected) should be used and the <br />analysis performed again. <br /> <br />I <br /> <br />3. Embankment Dams <br /> <br />a. Safety Concerns <br /> <br />(1) Types of Instability <br /> <br />Safety concerns for embankment dams subjected to <br />earthquakes involve two types of instability: sliding of <br />slopes and/or foundation and erosion of the dam or <br />foundation materials. The main cause of sliding <br />instabili ty due to earthquake-induced ground motion is <br /> <br />-22- <br />