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4 ENVIRONMENTAL ANALYSIS 4.7 GEOLOGY,SOILS,AND SEISMICITY r. and IV-A where steep 3/4:1 (H:V)temporary slopes border the proposed Plant site. However, surface failure would likely affect only the upper edge of the slope and would not extend outside the setback at the edge of slope. APPROACH TO ANALYSIS .. The analysis of Impact 4.7.1 provides a summary of the factors that trigger slope failure followed by a discussion of earthquake magnitudes that typically initiate slope failure. The analysis reviews findings of the 2002 Preliminary Geological and Geotechnical Study prepared by �- Geomatrix for the proposed project and presents a summary its findings and limitations. Geomatrix did not collect site-specific geotechnical and soils engineering data for the preliminary study and therefore,no soils engineering or geotechnical data can be directly applied to slope stability under the proposed slope conditions. The impact analysis relies on the preliminary 2002 Geomatrix study and draws conclusions from that and the other information provided during the s preparation of this EIR. SLOPE INSTABILITY FACTORS v 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 r 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 r. 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 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 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 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. Keefer(1984) studied data from 40 RMC PacRic Vemalis Quay Mining and Reclamation Project 4.7-17 ESA/203015 Administm w L a0 E1R Mamh 2004