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• A 30-foot layer of material consisting of very stiff to hard silt and lean clay. <br /> This material is moist to wet, with moisture contents between the liquid and <br /> plastic limit. The material has an approximate OCR of 1.7 (CH2M HILL <br /> Boring BH-1, approximate depth of 78 feet). <br /> • Very dense poorly grade sand was observed near the bottom of CH2M HILL <br /> Boring BH-1, starting at an approximate depth of 100 feet. Sand was also <br /> observed in EMCON's Boring No. 17 to 178 feet. <br /> Based on the above subsurface profile, the subsurface soils at the site can be <br /> characterized as strong and dense and therefore not subject to significant <br /> stability or settlement issues. Moreover, since groundwater occurs below a <br /> depth of approximately 60 feet, the potential for liquefaction of site soils appears <br /> remote. <br /> Two soil borrow studies (Twining, 1991 and Lewis Engineering, 2000) were <br /> conducted in the existing borrow pit area south of the existing Austin Road <br /> Landfill. The results of these studies indicate that soils at elevations of <br /> approximately 15 to 25 feet amsl are classified as low plasticity clays (CL) based <br /> on the Unified Soil Classification System (USCS) and have laboratory <br /> permeabilities ranging from 1 x 10' to 1 x 10' cm/sec when compacted to <br /> above 90 percent relative compaction based on American Society for Testing <br /> Materials Method D1557 (ASTM D1557). <br /> The other soil borrow study (Twining 1996) was conducted in the borrow area <br /> north of the existing Austin Road Landfill. The results of this study indicate that <br /> the soils encountered at a depth of approximately 4 to 11 feet below ground <br /> surface are classified as a sandy clay and have a laboratory permeability of <br /> approximately 3 x 108 cm/sec when compacted to above 90 percent relative <br /> compaction based on ASTM D1557. <br /> 3.6 SURFACE WATER HYDROLOGY <br /> Anticipated rainfall intensities for a 1,000-year storm event near Stockton, <br /> California, are presented in Table 6. The intensity estimates were developed from <br /> short-duration events recorded at Modesto, California, and adjusted for Stockton, <br /> California. <br /> Figure 8 presents the regional 100-year floodplain map. As shown on this map, the <br /> majority of the Forward Landfill site is within flood zone designation C (Federal <br /> Forward Landfill)TD 3-9 <br /> L:\Allied\2000.193\Reports\td:Sec•3.0:05/21/02 BRYAN A.STIRRAT 6 ASSOCIATES <br />