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San Joaquin County Community Environmental Setting,Impacts,and Mitigation Measures Development Department Geology,Soils,and Seismicity Impact GEO-8: Potential Slope Failure (Less than Significant with Mitigation Incorporated) Proposed quarry slopes would be steep-0.75H:1 V—and would have a maximum height of about 175 feet. However,these slopes would be interrupted by 15-foot-wide benches every 30 vertical feet. Proposed slopes would be less steep-1.5H:IV—with benches at the same width and vertical spacing. When unsupported cuts are made in natural materials, as during excavation of the proposed mining pit, it is possible for substrates comprising slopes to become unstable and collapse in the form of small landslides(block or wedge failure)or rotational slip(or slump). In certain substrate conditions, such failures could be r progressive and could extend beyond the permitted limits of the pit and perhaps threaten nearby roadways, levees, and dwellings on properties adjacent to the pit. ` The potential for slope failures to occur in the proposed pit has been assessed through a slope stability analysis conducted for the project site. The analysis was performed by Kleinfelder using SLOPE/W,a computer program from Geo-Slope *" International. The soil properties used in the analysis are based on the results of the field and laboratory investigations performed for Kleinfelder's geotechnical report(Klcinfelder 2003a), as well as those performed previously in the project t area. Slope failures in cemented alluvium, such as those substrates encountered in the Vernalis area, tend to occur as block or wedge failures rather than deep,circular, or rotational failures (Kleinfelder 2003a). To account for this mode of failure, various block slip surfaces were modeled to estimate the most likely failure ` planes in the undisturbed material if adjacent areas were quarried at the project site. In addition,reclaimed slopes were analyzed for circular failures(slumps) because they would be constructed of disturbed alluvium that would no longer be cemented. Results are given in terms offactors ofsafety. A factor of safety of ` 1.0 indicates that gravitational forces(in the static case)or gravitational-plus- lateral seismic forces(in the pseudo-static case)are balanced by material strength. A factor of safety of 1.2,for example,indicates that material strength is y 20%higher than the failure forces. A factor of safety of less than 1.0 indicates that slope failure is probable. .. The results of these analyses are presented in the geotechnical report prepared by Kleinfelder(2003a)(Appendix E). They indicate that factors of safety for quarry excavation are 1.5 and 1.2 for static and pseudo-static conditions,respectively, and 1.8 and 1.3 for the non-saturated reclaimed slopes for static and pseudo-static conditions,respectively. These values meet or exceed the criteria normally used by be the geotechnical industry as established by the Corps,FEMA,Caltrans,and the DWR Division of Safety of Dams. Kleinfelder(2004)notes that for reclaimed slopes below the water table, the factor of safety would be about 1.0. For this reason,Kleinfelder recommends a bench of native(undisturbed)material be left at the groundwater elevation. This bench would support unsaturated y reclaimed material above it,which would have the higher factor of safety figures above. This impact is considered potentially significant. With provision of a DeSilva Gates Quarry Project Draft Environmental Impact Report 3.F-15 J85 05105.05 i