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4. ENVIRONMENTAL ANALYSIS - 4.7 GEOLOGY,SOILS,AND SE[SM[CrrY �- However,if either the temporary or permanent slopes become saturated,the predicted factors of _ safety would he lower than predicted and could lead to instability and failure. The seismic inertial forces within the slope may cause a slope to fail during an earthquake. Geomatrix simulated slope stability during a moderate magnitude earthquake by adding a horizontal predicted earthquake loading to the static slope stability analysis. Results from this simulation indicate that if the site undergoes an estimated 0.4 g to 0.5 g of peak horizontal ground acceleration,the factor of safety for a slope under earthquake conditions would be less than 1.0, and therefore unstable.11 The factor of safety of 1.0 or below suggests a momentary instability that could cause slope failure. Ground shaking duration typical of a moderate earthquake would cause many occurrences of momentary instability and result in an accumulation of slope displacement and failure(Geomatrix, 2002). Geomatrix then applied an assumed factor of safety of 1.0(indicating momentary instability of a slope) was then applied to obtain yield accelerations, which are earthquake accelerations that would cause momentary instability of a slope. The analysis determined minimum yield accelerations of 0.15g to 0.25g; well below earthquake accelerations that could be expected during the lifetime of the project. These findings suggest that in the event of moderate earthquake,the temporary and permanent slopes could experience deformation and failure. Geomatrix estimated that with peak ground accelerations for the — maximum magnitude earthquake on the Great Valley Fault Zone,slope displacement could be on the order of 2 feet(Geomatrix, 2002). SUMMARYAND CONCLUSION OFIMPACTANALYSIS The proposed permanent configuration of the quarry slopes are 2:1 at 100-feet in height without terraces. This configuration does not comply with Chapter 9-1415-3 of the San Joaquin County Development Title,Ordinance for Quarry Excavations that require slopes in excess of 50-feet be terraced with a maximum distance between terraces of 50-feet. Slope stability analysis indicates that under static conditions,temporary and permanent slopes would likely remain stable. Although San Joaquin County Quarry Excavation Standards do not — specify a required static factor of safety for temporary working slopes,the factor of safety of 1.6 for permanent slopes exceed the County Quarry Excavation Standards. Under seismic ground shaking conditions,slope analysis indicates that the temporary and permanent slopes would have _ a factor of safety below 1.0 and likely become unstable during a moderate earthquake with a peak ground accelerations exceeding 0.15 (temporary slopes)and 0.25 (permanent slopes). Failure of temporary slope in areas of active mining could cause injury and damage but due to the nature of open pit aggregate mining, if a failure did occur in the vicinity of the active mining, it would affect a small number of workers within the immediate work area. A future landslide failure of the proposed permanent final slopes would occur within a relatively remote,reclaimed agricultural area, and would not directly expose people or property to substantial adverse effects l l Geomatrix conducted an analysis to predict estimated ground motion at the proposed project site and determined a value of peals ground acceleration of 0.4 to 0.5 g. This value is consistent with that predicted using the Seismic -- Hazard Map,discussed previously in the Seismic Hazards section of this chapter. RMC PacJc Vernalis Quany Mining and Reclamation Project 4.7-20 ESA/203015 Adminikra ive Draft Eat March 2004