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4.Emironmemal Analysis _ ground acceleration produced during an earthquake can cause displacements in the soil. These displacements can decrease the soil strength and trigger down slope movement. Ground Motion and Slope Instability — Evaluation of slope stability in a region requires knowledge of conditions under which earthquake-induced landslides have occurred in the past. A magnitude 4.0 earthquake is relatively _ common in California,with light to moderate perceived shaking that causes either no or very light damage. As discussed in the setting above,the proposed project site could experience ground shaking up to a Modified Mercalli intensity of MM-VIII, which is an earthquake that everyone — would feel and one that could cause moderate to severe structural damage depending on location, age of structure,and construction. The estimated PGA could reach 0.4 g according to the regional probabilistic seismic hazard assessment completed by the California Geological Survey(CGS, 2003a). The Tracy-Vernalis area could be affected by earthquakes occurring in the Bay Area and the San Joaquin Valley and be subjected to earthquake magnitudes capable of causing failure in non-engineered, unsupported slopes or permanent reclamation slopes. Slope Stability Analysis for Proposed Project Site _ Geomatrix conducted a preliminary analysis to assess the stability of the temporary 3/4:1 (H:V) and permanent 2:1 (H:V)quarry slopes expected at the proposed project site. The preliminary stability analysis addressed slope stability under static and earthquake-induced conditions (Geomatrix, 2002), which was based on applicable,available subsurface information, laboratory analysis of a clay sample from the DSS Quarry site, and postulated seismic loadings.The 2005 _ Kleinfelder,Inc. study conducted a subsequent slope stability evaluation, which included a site- specific field investigation of the subsurface materials and static and pseudo-static(earthquake motion) slope stability computer modeling. _ Slope Configuration Requirements As presented in the Regulatory Framework section of this chapter, Chapter 9-1415.3 of the San Joaquin County Development Title,Ordinance for Quarry Excavations(Title) require that temporary cut slopes not exceed 1:1 (H:V) although temporary slopes at setback lines can be _ maintained at 1/2:1 (H:V) if geologic and engineering analysis determine that the slopes will have a minimum factor of safety of 1.5 against static deep-seated and near surface slope failure and a minimum factor of safety of 1.1 against seismic failure. Permanent slopes are required by the _ Title not to exceed 2:1 (H:V)but can be maintained at 1.5:1 (H:V) if slope stability analysis demonstrates that the slope would have the minimum required factor of safety(as above, 1.5 under static conditions and 1.1 under seismic conditions). Permanent 2:1 (H:V) slopes exceeding — 50 feet in height are required by the Title to be terraced with a maximum vertical distance between terraces of 50 feet. Terraces must be 12 feet wide and have appropriate drainage. Results of Slope Stability Analysis Based upon the findings of the 2005 slope stability evaluations and experience at other nearby — quarries with similar geology, Kleinfelder, Inc. concluded that the proposed 3/4:1 (H:V)temporary RMC Pacific Vernalis Quarry Mining and Reclamation Project 4.7-18 ESA 1203015 Draft Environmental Impact Report May 2006