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' San Luis Mine Phase 0, Raise 2 Design Report ' For comparison, the post-liquefaction residual strength of the saturated tailings was also estimated using the method of Ishihara, et. al. (1990). This study presents a data base of case histories for earthquake damaged embankments and tailings dams composed of silty sands or sandy silts for ' which cone penetration data was available. By comparing the back-calculated residua] strength for these case histories with the cone resistance data, an empirical correlation was developed. The use ' of the lower bound correlation presented in this study results in an overall average residual strength of 1068 psf for the saturated tailings. However, the correlation was developed primarily from case histories involving relatively low confining pressures and may be less accurate for deeper portions of the tailings deposit. Accordingly, the average residual strength for the upper 4 m (13.1 ft) of saturated tailings was also determined using the same procedure. The resulting lower bound residual ' strength for these tailings was 890 psf which is still far greater than that obtained using the method of Seed and Hazder, however, the later method was adopted for conservatism. Residual strength calculations are presented in Appendix E. ' A summary of the material properties incorporated into the stability analyses is presented on Figures ' 10 and 11 which summarize the resulu of these analyses. 4.3 Phreatic Conditions t No phreatic surface within the embankment was included in the stability analyses as the facility has been designed such that the embankment will drain rapidly relative to maximum potential inflows ' I from the tailings. The relative permeability of the embartlaetent material to the tailings is very high thereby maintaining unsaturated conditions in the embankment. This was confirmed by the ' piezometer data in the main embattlement and Raise 1 embattlmtent. ' Within the tailings themselves, the phreatic surface included in the analyses was above or consistent with the depths given in Table 1. For analysis of potential upstream failure modes, the depths presented in Table 1 were used to estimate an average phreatic surface. For analysis of potential ' downstream failure modes associated with future tailings levels, an increased phreatic level relative to the tailings level was included in the analyses. The phreatic surface was assumed to be coincident ' with the interface between the Raise 2 fill and the underlying tailings. This assumes a depth of only 15 ft to saturated tailings for this location relative to the embankment as opposed to approximately 32.5 ft as identified for current conditions. At a distance of 250 ft upstream of the crest of the ' raise, saturated tailings were assumed to occur at a depth of only 2 ft. As pr?esented in Table 1, saturated tailings presently occur at an average depth of 22.5 ft at a distance approximately 175 upstream from the embankment crest. Thus the phreatic surface assumed for the future ultimate projected tailings level should represent a conservative or "worst case" assumption for conditions likely to be encountered towards the end of operations. Upon closure of the facility, the large ' solution inflows associated with active tailings deposition will cease and the tailings will drain with u y 4-0 rorect o.