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!i The reclaimed slope stability analysis evaluated deep-seated failures. The results of the j t : analysis indicated that the reclaimed slopes would have factors of safety ranging from 1.5 to 1.6 under static conditions and 1.0 to 1.2 under seismic shaking. The conclusions of the second slope stability analysis (Kleinfelder, 1997) stated that ' I slumping or erosion of the project slopes should be expected. In addition,'the report suggested that unanticipated adverse conditions (including seepage from the adjacent water supply canals) could be encountered during excavation.. Inspection of the slopes during construction by a geotechnical engineer was recommended to identify such problems and recommend mitigations as needed. I� ' II Proximity of the project site to the. Greenville fault and Coast. Range-Sierran Block it Boundary Zone (CRSBBZ) suggests that expected earthquakes on faults within the zone could cause moderate to strong ground shaking at the project site. Seismic shaking can cause failure of unstable slopes during mining and reclamation activities. The slope F stability analysis prepared for the proposed project (Kleinfelder, 1997) indicates that the proposed reclamation slopes would have satisfactory stability under static conditions and under expected seismic shaking.. The modeling incorporated effects of seismic shaking, �. assuming a horizontal ground acceleration of 0.15 g. Stronger groundshaking may occur at the project site (0.3 to 0.5g). The higher groundshaking levels would be expected only during maximum credible earthquake on the CRSBBZ. The seismic potentia! for CRSBBZ ! is currently undergoing further evaluation to improve the understanding of the characteristics of potential earthquakes. Liquefaction is a condition caused by seismic shaking that results in the loss of strength FW of saturated, loose, unconsolidated granular sediments. Liquefied sediments can flow to a free face e.g., stream channel cutbank or submerged mining pit sloe resulting in lateral I I spreading or settlement of the ground surface. The natural sediments (Quaternary•: alluvium) at the project site that would be affected by mining are not saturated and would not, therefore, be subject to liquefaction. l In summary, the stability analysis submitted with the proponent's application to the County (Kleinfelder, 1997) indicates that the excavation slopes do not meet the County Development Title minimum factor of safety for static (1.5) and pseudo-static (1.0) conditions; and the proposed reclaimed fill slopes do meet the standard. As a result, the temporary cut slopes may be subject to failure prior to placement of the reclamation fill. E This is a significant impact. Alternative 4: No Project Under Alternative 4, the topography at the project site would remain relatively flat and no u slope instability would be expected. Draft Environmental Impact Report 4-17 ER-96-3 h i! I�