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TETRA TECH RMC Mr. Mike Refer <br />May 12, 2004 <br />Page 4 <br />We made the following assumptions: <br />- East cell would be mined after the installation of a slurry wall. <br />- West cell would be mined after the installation of a slurry wall <br />- Southwest cell would be mined either by dewatering or by dredging (both options were <br />modeled) <br />- Southwest cell would be mined fast <br />- Due to floodway restrictions, no significant berms of stockpiled materials or overburden <br />would be placed on the surface above the mine slopes <br />- The slurry wall would be constructed around the entire perimeter of the east and west cells, <br />crossing Tucson Road at the north and south ends, and running along the west side of <br />Tucson road to sepazate the cells into two lakes <br />- The slurry wall would be keyed three feet into claystone bedrock <br />- For constructability, the slurry wall must be 25 to 30 feet away from existing stmctures or <br />right-of--ways <br />The stability analyses were run in accordance with DMG requirements for temporary pit slopes as <br />indicated in a March 6, 2003 memo from Allen Sorenson: <br />Cell walls modeled as vertical slopes <br />Residual soil strength pazameters used for weathered bedrock <br />Factor of safety must be greater than 1.0 <br />The following scenarios were modeled for slope stability. The results aze presented in Table 2. <br />Details including graphical results and XSTABL output are found in the appendix. <br />Scenario 1: West Side of Tucson Road, including pavement and utilities, and power line <br />with a slurry wall. A 500 psf surcharge simulates live loads on the road and a mud lens is <br />present. If the west cell is mined first and the slurry wall is built only around the west cell, <br />the water level behind (east of) the slurry wall will stay at or neaz its natural level, azound <br />five feet below the surface. If the east cell is mined first or the slurry wall is built azound the <br />entire mine azea and the east side is dewatered, the water level behind the slurry wall will be <br />much lower. ht that case the setback can be reduced as shown in Table 2. <br />Scenario 2: East side of Tucson Road, including pavement and utilities, without a slurry <br />wall. A 500 psf surchazge simulates live loads on the road and a mud lens is present. <br />Because the entire east cell will be surrounded by a slurry wall, including the slurry wall <br />along the west side of the road, before mining, one can assume the azea (except for the mud <br />lens) to be dewatered. <br />Scenario 3: Maximum Highwall. In the east and central part of the east cell the bedrock is <br />10 to 15 feet deeper than in most of the rest of the site. Because the deepest part of the cell <br />is adjacent to a patch of land not included in the mine permit, the maximum highwall was <br />