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4.7 Geology,Soils,and Seismicity r Seismic-related ground failure,including liquefaction; and I„ — Landslides. • Result in substantial soil erosion or the loss of topsoil. INN This impact analysis focuses on potential project impacts related to seismic ground shaking, earthquake-induced landslides,and erosion. As discussed previously in this chapter, surface fault r rupture and ground failures due to liquefaction are not addressed because these hazards are not considered impacts related to the project. This EIR does not address expansive soils,unstable geologic units,and soils capable of supporting septic systems because these issues are not related r, to the proposed aggregate extraction project.The evaluation considered project plans,current conditions at the project site, and applicable regulations and guidelines. Approach to Analysis The following analysis of Impact 4.7.1 provides a summary of the factors that trigger slope l.. failure followed by a discussion of earthquake magnitudes that typically initiate slope failure. The analysis includes a summary of the findings of the 2002 Preliminary Geological and Geotechnical Study prepared by Geomatrix Consultants,Inc. (Geomatrix) and the 2005 Geotechnical Services Report, Slope Stability Evaluation, completed by Kleinfelder,Inc.The following impact analysis relies on the 2002 Geomatrix study and the 2005 Kleinfelder,Inc. study and draws conclusions from those documents and the other information provided during the preparation of this EIR. r. Slope Instability Factors Stability of a soil or alluvial soil slope can depend on a number of complex variables. The geologic material, its structure and attitude,and amount of groundwater can affect the potential for slope failure,as do external processes (i.e., slope geometry,and mining activity). The factors that contribute to slope movements include those that decrease the resistance in the slope materials and those that increase the stresses on the slope. Slope stability is usually expressed in terms of an index,referred to as the"factor of safety," which is typically defined as the forces that resist movement(the shearing strength available along a sliding surface) divided by the shearing stresses that tend to produce failure along a surface (Hunt, 1984). Slopes with a factor of safety of ` 1.0 or less are considered unstable and susceptible to failure. Slope failure occurs under static and dynamic forces. Static forces refer to forces such as gravity ` while dynamic forces are those produced by excessive ground motion of vibration or an earthquake. Slope failure occurs when those forces initiating failure overcome the forces resisting slope movement. For example, a soil slope may be considered stable until it becomes saturated INN with water(e.g.,during heavy rains)causing the water pressure in the individual soil pores to increase. Increased pore pressure breaks the bonds in soil and reduces its overall strength. Excavating into the slope and removing its lower base(known as the"toe of the slope")can n reduce or eliminate the slope support, which increases stresses on the slope and can lead to failure. While soil slopes may be stable under static conditions,earthquake ground motion can reduce the factor of safety to the point of failure,because the down slope component of the it IN. RMC Pacific vernalis Quarry Mining and Reclamation Project 4.7-17 ESA/203015 Draft Environmental Impact Report May 2006 V