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■p File No. 20-1100-38 Ri KLEIN FELDER September 12, 1996 Summarized in Table I below are the soil properties for the three basic soil groups encountered in the test borings. As previously stated, the strength properties shown were based on laboratory tests on relatively undisturbed core samples and on back calculating what strength properties ` would be necessary for the existing nearby steep slopes to stand with a factor of safety of approximately 1.2. TABLE 1 SOIL PROPERTIES Angle of Moisture Unit Soil Internal Friction Cohesion Weight Type (degrees) (psf) (pef) _ Cemented Sand and Gravel 36 900 130 Silty Clay 24 500 120 Sandy Clay/Silty Sand 25 1350 1 120 The existing slopes at the adjacent Teichert Quarry confirm that the sand and gravel deposits are partially cemented. These lower materials have more inherent cohesion than would normally be assumed, considering grain size distribution only. In our opinion, the laboratory tests on samples r from our test borings justify relatively high shear strength parameters. The effect of ground shaking on the computed factors of safety was evaluated by applying a pseudo-static seismic load. The practice in this area is to utilize a mean peak ground acceleration corresponding to a maximum probable earthquake (10 percent chance of exceedance in 50 years) roughly approximated by a pseudo-static analysis. Based on the proximity of the nearest faults VP and estimates of maximum probable earthquakes, it is our opinion that a 0.15g design acceleration value is appropriate for this study. Circular arc failure surfaces were analyzed using the Bishop's Simplified method. This method utilizes the slope configuration, unit weight and shear strength properties of the exposed materials, and internal forces due to water pressures. After a potential failure surface has been assumed, the soil mass located above the failure surface is divided into a series of vertical slices. Forces acting on each slice include the slice weight, the pore pressure, the effective normal force on the base, the mobilized shear force (including both cohesion and friction), and the horizontal side forces due to earth pressures. The factor of safety is calculated by determining the ratio of the moment of resistance (cohesion and friction along the failure surface) to the moment of the driving forces about the center of the assumed circular failure surface arc. The computer program SLOPE/W was used to perform automatic searches of different potential failure surfaces and to compute a critical failure surface having the lowest factor of safety for a particular analysis condition. 10-1100-381MR960198 Page 4 of 6 01996, Kleinfelder,Ina