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i Environmental Impact Re Draft E ort Page IV.G-3 p p Forward Landfill Expansion �1 The seismic element of the San Joaquin County General Plan proposes to lessen these potential hazards through: Conducting an evaluation of potential hazards as a precondition to development; Prohibiting the location of critical use facilities within 1/8 mile of any known active fault; • Requiring facilities within ahigh-seismicity zone to be capable of withstanding the .,, maximum credible earthquake within that area;and • Curbing subsidence in the Delta area through regional and local efforts. � x Topography The regional topography is a relatively featureless plain of the Great Valley. Before development, the land surface was at an elevation of approximately 40 feet above mean sea level (amsl). The original topography has been modified by the landfill operations at the landfill by the creation of both depressions (excavations for borrow pits and sedimentation ponds, etc) and topographic highs created by the landfill cells. The excavation areas are approximately 15 feet amsl. As of April 2008 (the most recent available measurement), the highest point at the northern hill of the landfill (former Austin Road Landfill) was approximately 137 feet above Mean Sea Level (MSL),and the highest point at the southern hill of the landfill (original Forward Landfill) was approximately 194 feet. The current permit allows Forward to fill to a maximum of 210 feet above MSL. The proposed new expansion -•ti areas revised permit would permit a maximum of 183 feet above MSL in the expansion area. The expansion would be staged over 25 years, as landfill deposits are not evenly distributed over time. Some areas have developed more quickly than others based on proximity,landfill administration, refuse segregation plans, and location of borrow material. Soil Properties and Conditions Subsurface soil properties beneath the landfill and in the immediate vicinity of the landfill, including the expansion areas have been evaluated in numerous previous soil investigations as 1 5 of the joint Technical Document TD 2007 . While thee investigation documented m Table J (J ) g do not focus on the new expansion area the general soil properties can be extrapolated to the new expansion areas within the context of the natural heterogeneous nature of the soil M properties. The soils encountered during the geotechnical investigations generally consist of interbedded layers of sand, silt,and lean clay. The cohesive materials are generally of very stiff to hard consistency. The cohesionless materials can be characterized as dense to very dense. Because the sediments at the site were deposited as fluvial floodplain and channel deposits, the soil deposits at the site were found to be heterogeneous and laterally and vertically discontinuous. However,based on the 1992 CH2M HILL investigation, the subsurface soils properties were summarized into general layers. Beginning at the existing ground surface (approximately elevation 39 feet amsl), the six layers, from the surface on down, are as follows: • A 10- to 15-foot layer of light brown material ranging from lean clay to silt and silty clay. This layer is dry and very stiff to hard in consistency, with moisture contents typically at or below the plastic limit. 4