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In our analysis we used the mean shear strength of 17 degrees and cohesion of 706 psf <br />(4.9 psi) and selected a low bound strength o f 14 degrees and cohesion of 288 psf (2 psi). A unit <br />weight of 125 pcf was used, also considering the results of testing from these previous <br />investigations. <br />6.2.4 Clayey Gravel and Bedrock <br />In our opinion, the clayey gravel layer will have a drained shear strength value greater <br />than the clay. This layer was modeled as impenetrable to increase the efficiency of the software; <br />preliminary model calibration indicated this assumption was appropriate. <br />The intact bedrock is very low to moderate strength siltstone, claystone, and sandstone. <br />Deep- seated failures through the bedrock are not considered likely below the base of the fill; a <br />failure mode along the weaker CMS /CS very close to the base of the fill is the critical bedrock <br />condition. Therefore the bedrock was also modeled as impenetrable. <br />6.2.5 Groundwater Conditions <br />The field investigation indicated groundwater was at depths of 13 to 22 feet near the <br />valley centerline (Appendix A). Experience at the mine, including monitoring of the Streeter and <br />west Pit spoil fills, indicates build up of water within the spoil is limited. In the Clay Layer at <br />Base of Spoil model, we considered water levels relative to the base of the spoil at -15 feet, +20 <br />feet and +40 feet. In the CMS /CS at Base of Spoil Model we considered water levels of 0 and <br />+20 feet relative to the base of spoil. <br />6.2.6 Pseudo - Static Analysis <br />U.S. Geologic Survey data indicates that the peak ground acceleration with a 10 percent <br />probability of exceedance in 50 years is about 0.05g (USGS 2002). Our evaluation of the <br />anticipated seismic conditions was performed using a pseudo- static analysis with a horizontal <br />acceleration 0.05g. <br />7.0 RESULTS OF STABILITY ANALYSIS <br />Ll <br />The two sections shown in Figure 5 were analyzed using Spencer's limit- equilibrium method, <br />which satisfies force and moment equilibrium, using SLOPE/W software (GEO- SLOPE, 2007). <br />We analyzed circular slip surfaces and enabled SLOPE /W's optimization procedure. The <br />optimization procedure varies the location of individual points on the circular slip surface with <br />the lowest factor of safety until the slip surface with the minimum factor of safety (FS) is found. <br />23 -1- 01101 - 400 -R 1.docx /wp /1kn 23-1-01101-400 <br />10 <br />Exh. 23 -1 -17 Revision Date: 08/05/09 <br />Revision No.: PR -03 <br />