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C CORNERSTONE EARTH GROUP 4.6.2 Method of Analysis The lateral stability of a slope is influenced by the composition, inclination, and height of a slope. Stability is usually expressed as a ratio of resisting moments and forces divided by driving moments and forces termed the factor of safety (FS). Factors of safety are calculated for static and seismic conditions. The stability of the existing quarry slope was evaluated using the computer program GSTABL7 and circular modes of failure. Input parameters for the analysis include slope geometry, soil layers or zones, soil unit weights and strength parameters, and ground water conditions. In evaluating the stability of slopes under seismic conditions, GSTABL7 uses a "pseudo-static" method of analysis. The pseudo-static method models the effects of transient or pulsating earthquake loading on a potential slide mass by using an equivalent sustained horizontal force determined as the product of a seismic coefficient and the weight of the potential slide mass. The slope is first analyzed to establish the minimum factor of safety under static conditions. Once this minimum failure surface is located, an additional horizontal force acting in the direction of potential failure is imposed on the sliding mass. This additional force is equal to the soil mass multiplied by a seismic coefficient. The ground motion parameter used in apseudo-static stability analysis is referred to as the seismic coefficient "K". CDMG (1998) has published recommendations for the selection of the "K"-value in a publication titled "Guidelines for Evaluation and Mitigation of Seismic Hazards in California." The site is located near the active Calaveras and Hayward Faults and high ground shaking can be expected during a seismic event near the site. In accordance with the CDMG Guidelines, we have performed our pseudo-static analysis using a "K"-value of 0.2, which, in our opinion, is reasonable for analytical purposes. The minimum allowable factor of safety with respect to slope stability generally ranges from 1.5 to 2 for static conditions and 1.1 to 1.3 for seismic conditions. Apseudo-static factor of safety of one typically implies "movement" of the slope mass, but does not necessarily result in complete slope failure. In our opinion, acceptable factors of safety for static and seismic (pseudo-static) conditions may be considered to be 1.5 and 1.1, respectively. 4.6.3 Soil Properties Laboratory testing was performed during this investigation to further identify soil strength parameters in the upper 40 feet. Soil strength parameters were also estimated from the data generated from the previous geotechnical investigations at or near the site, strength data and correlations from our recently completed borings, and our experience and engineering judgment. For our analysis, the subsurface conditions were modeled as four layers, including an upper layer of sand and gravel, the upper silt and clay layer, underlying Livermore Gravels, and a lower layer of older alluvial soils consisting of interbedded clays and sands. The upper alluvial soils encountered were generally medium dense to dense or very stiff to hard. For the dense Livermore Gravels Formation, we assumed strength parameters that also represent an Ponderosa Active Adult Community Page 8 132-2-1