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i All Quan Excavation Alternatives 1 2 and-31 Impact 4.2-2 Slope Stability The natural gentle (approximate one percent gradient) slope of the alluvial fan at the project site is stable and has a low potential for erosion. However, the project proposes creation of deep mining pits to elevation 85 feet NGVD and 120 feet bgs. During mining, temporary cut slopes would be excavated at a steepness of one third of a foot horizontal to one foot vertical (113:1). Fifteen-foot wide benches would be placed every 30 feet of cut slope height. The average slope of the benched excavation would be 314:1. The excavated slopes would be reclaimed by placing fill to create final fill reclaimed slopes. The reclaimed slopes would be graded to a steepness of 1.5:1 with 20-foot benches placed every 50 feet of slope height. The fill would consist of processing fines collected in ponds. The proposed reclamation plan estimates that approximately 90 percent of the processing s fines would be silt and clay. These fine-grained sediments would be compacted by earthmoving equipment, used to place the fill. Two slope stability analyses were performed for the pro posed project slopes to evaluate the potential for slope failure during mining and following reclamation. A preliminary slope stability analysis prepared by Foothill Engineering was submitted to the County with the proposed project application. This analysis evaluated the 1.5:1 reclamation slopes. The computed factor of safety for the slope was 2.1. The factor of safety generated by a slope stability analysis represents the ratio of factors which drive slope failure to the factors which resist slope failure. When the computed factor of safety is less than one, the slope is considered to be at or near failure. A second slope stability analysis was prepared for the proposed project (Kleinfelder, 1997).- This 997).This analysis evaluated the stability of the excavation and reclaimed slopes under static and pseudo-static (seismic shaking) conditions. A variety of types of slope failure were , evaluated including shallow failure of slope between benched and deep-seated failure of the composite (entire) slope. Due to the difficulty in obtaining undisturbed samples of the dense sand and gravel sediments, values of the angle of internal friction, cohesion and unit E J weight of the sediments at the project site (parameters used in the slope stability evaluation) were estimated on the basis of data collected from other mining projects within the vicinity of the project site. The seismic coefficient (expected seismic shaking) for the stability analysis was 0.15g. This value was chosen on the basis of professional judgement regarding the proximity of the project site to active faults and estimation of ' sustained seismic shaking. The CRSBBZ was not specifically considered as a seismic source by Kleinfelder. The seismic coefficient was determined to be approximately two- thirds of the expected peak acceleration (Heinzen, 1997). The results of the analysis indicated that the factor of safety for shallow slides on the excavation slopes ranged from 1.3 to 2.2 under static conditions and 1.1 to 1.8 under pseudo-static conditions. A deep-seated slide on the excavation slope had factor of safety values of 1.3 for static conditions and 1.0 to 1.1 for pseudo-static conditions. Draft Environmental Impact Report 4-16 ER-96-3