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TETRATECH RMC Mr. Mike Refer May 12, 2004 Page 3 - Buildings along west side of southwest cell - Power poles along east side of southwest cell It is our understanding that the Seltzer Well in the west cell will be relocated prior to slurry wall construction. It is also our understanding that all other structures located within the mine perimeter will be removed before the start of mining. STABILITYANALYSES We performed stability analyses in order to evaluate potential for damage to existing permanent structures due to mine highwall slope failures. Analyses were performed with the XSTABL version 5.202 computer program. XSTABL was used to generate 600 to 1,000 failure surfaces within the area of interest for each scenario and compute the minimum factor of safety using the Simplified Bishop's Method of Slices. We analyzed the appazent most critical soil profiles based on exploratory boring data and the mining plan. The soil profiles consist of a sandy-silty overburden layer overlying a sand and gravel layer, overlying a weathered claystone bedrock layer, overlying a less weathered claystone bedrock layer. In some cases the sand and gravel layer contains a mud lens. Thicknesses of the layers varied based on neazby drilling logs. Drilling records indicate the presence of a two to nine-foot thick mud lens, a layer of sandy clay within a gravel deposit, in the west and central parts of the site. It is our experience that these mud lenses are very difficult to dewater and represent a weaker part of a slope. Hence, we have included a mud lens in the appropriate scenarios below. To be conservative we used the greatest mud lens and overburden thicknesses for each area. These cross-sections represent conservative cases for stability and should not be used to estimate gravel volumes in the project. No laboratory strength tests were performed on soils at the site. The soil strength pazameters used were based on typical values for the anticipated soils and our experience at other sites in the area. A summary of the material properties used is found in Table 1. For the water table we assumed highest seasonal water levels and the effects of active ditches. TABLE 1-MATERIAL PROPERTIES USED IN SLOPE STABILITY CALCULATIONS Moist Saturated Effective Effective ID # In Material Unit Weight Unit Weight Cohesion c' Friction Angle ~' Xstabl (Pct (PeIJ (Psn (degrees) Overburden 114 126 150 28 1 Sand & Gravel 130 137 0 37 2 & 6 Weathered claystone Bedrock q24 134 0 14 3 (residual strength) claystone Bedrock l24 134 500 22 4 (peak strength) Mud Lens ll6 119 200 22 5 Slurry Wall 110 122 0 22 7