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<br />• sands obtained from test hole 3A. Based on our experience with similaz projects, we assumed that the sand was <br />the governing factor in the slope stability analysis, therefore, we used the strength values obtained on the sand <br />sample for the entire thickness of sands and gravels. <br />Based on the laboratory test results, we used a unit weight of 128.0 pcf, a cohesion value of 0 psf and a internal <br />angle of friction (phi) of 39 degrees for the sands and gravels. A unit weight of 100.0 pcf, a cohesion value of <br />I00 psf and a internal angle of friction (phi) of 0 degrees were used for the silts and clays. A unit weigh[ of <br />125.0 pcf, a cohesion value of 2000 psf and a internal angle of friction (phi) of 0 degrees were used for the shale <br />bedrock. <br />The subsurface profile used in the analyses was considered to be a "worn[ case" scenazio and was assumed to be <br />where the deepest cut will occur in the pit and with the greatest amount of overburden soils. The subsurface <br />profile used in the analysis consisted of approximately 8 feet of topsoil and silt materials overlying 33 feet of <br />sands and gravels and shale-claystone bedrock at the base of the excavation. We have also assumed that the <br />excavation will be dewatered in the analyses. <br />Based on our calculations and analysis, the minimum calculated theoretical factor of safety for cut slopes <br />excavated in the upper topsoil and silt layer excavated to a 3 (horizontal) to 1 (vertical) configuration will be 1.81 <br />• under static conditions. A minimum factor of safety of 1.56 was determined for a lazge failure occurring through <br />the topsoil and silt and the underlying sands and gravels. This analysis assumed a cu[ slope excavated in the sands <br />and gravels to a 1.25 (H) to 1 (V) configuration. A factor of safety of 1.04 was determined for a thin shallow <br />surface failure (sloughing) occurring in the sands or sands and gravels when excavated to a 1.25 (horizontal) to 1 <br />(vertical) configuration. <br />Generally, a minimum theoretical factor of safety ranging from 1.3 to 1.5 is desirable for permanent, unretained <br />cut slopes. A factor of safety of less than 1.3 but greater than 1.0 indicates some degree of risk of failure and a <br />theoretical factor of safety of less than 1.0 indicates the slope will probably fail. <br />Based on the above results, we recommend that the cut slopes constructed in the topsoil and silt layer be excavated <br />no steeper than to a 3(H) to 1(V) slope configuration and the cut slopes in the sands or sands and gravels be <br />excavated no steeper than to a 1.25(H) to 1(V) slope configuration. This recommendation is for temporary cu[ <br />slopes and is based on the assumption [hat the excavations will be dewatered and that the overburden spoils will <br />be placed back against the excavated slope shortly after the sands and gravels have been mined out. If the <br />excavated azeas aze not going to be backfilled (reclaimed) shortly after they aze constructed, then we recommend <br />that they be excavated to a 2(H) to 1(V) or flatter slope configuration. <br />. To prevent damage [o the existing gas line easement which crosses the southern portion of the site and the existing <br />overhead electric power line, which runs through the central portion of the pit, from the excavation operations and <br />!ob No. 003304 Northwes~ Colo'ado Consultants, Inc. Pagc 5 <br />