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' San Luis Mine -Phase ^, Raise 1 Design Report <br />' a friction angle of 30° and cohesion of 350 psf. Lacking specific test data, previous design analyses <br />conservatively utilized a friction angle of 28° and no cohesion. Due to the possibility of a <br />continuous layer of slimes rich tailings from elevation 8552 to 8555 beneath the raise which may <br />' be saturated (as identified by the CPT probes) this layer was assumed to be saturated and to liquefy <br />under earthquake loading. Dynamic stability analyses presented in subsequent sections of the report <br />' conservatively assume that this layer is liquefied at the start of the earthquake. The post-liquefaction <br />residual or steady-state strength for this layer for use in stability analyses was selected on the basis <br />of procedures recommended by the U.S. Bureau of Reclamation (1989), and Seed and Hazder <br />' (1990). <br />Following these procedures, the average SPT N-value normalized to 1 tsf overburden pressure and <br />' (i0 percent of theoretical hammer energy, (N,}~, is determined for the layer in question. This (N,)~ <br />value is then corrected for fines content to determine the average equivalent clean sand (N,)~ value, <br />(N,)~„ which is used to estimate the post-liquefaction residual strength from ertlpirical data. For <br />the elevation 8552 to 8555 layer, the CPT data indicates an average (N,)~ value of 5.5 to 6. As <br />this layer is indicated to consist of tailings slimes with a high fines content, a fines correction of 4.5 <br />' to 5 was employed resulting in an (N,)~, value of 10 to 11. Utilizing the USER "best estimate" <br />recommendations for residual strengths, the resulting residual strength is approximately 275 to 325 <br />psf. For the dynamic stability analyses this layer has been assumed to liquefy and assigned a <br />residual strength of 300 psf. For static stability analyses this layer has been assigned a sheaz <br />strength consistent with the remainder of the tailings, i.e. a friction angle of 30° and cohesion of <br />350 psf. <br />' No phreatic surface was included in the stability analyses as the facility has been designed such that <br />the tailings adjacent to the embankment will drain rapidly. This was confirmed by the lack of <br />saturated tailings encountered in the C]?T probes and, although questionable, the piezometer data. <br />' The relative permeability of the embankment material to the tailings is very high thereby maintaining <br />unsaturated conditions in the embankment. <br />' 4.3 General Procedure <br />' For evaluating stability of the raise two separate cases have been considered; stability of the <br />upstream slope prior to deposition of tailings behind the raise and stability of the downstream slopes <br />' at the maximum height embankment section when tailings have been deposited to within 5 ft of the <br />crest of the raise. These two cases are considered the most critical as stability of the upstream face <br />will increase as the tailings level rises and stability of the downstream face will decrease as the <br />' July 13, 1993 6 SRK Pmjx[ No. 14719 <br /> <br />