ARCHIVED REPORTS_2001 - Laserfiche WebLink

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Group B. Top: 28 to 40 feet deep, to bottom: approximately 55 to 75 feet deep (25 to 35 feet thick)- Interbedded sand, silty sand, silt, and a little clay. Sands are fine grained. Overall a little sandier than group A. Sediments are distinctly less oxidized than Group A showing a gray brown color. Group C. Top: 55 to 75 feet deep, bottom: approximately 110 feet deep - Sediments dominated by fine-to medium-grained sand; some coarse- grained or gravelly layers. A few layers of silt and clay material occur. Color is generally light brown to yellow brown. Grains are large enough to assess that the sediments appear to be granitic in origin. Several monitoring wells at.the existing Austin Road Landfill were completed in a coarse-grained zone approximately 160 feet below ground. Based on these wells, an approximately 40-footthickuait-sof.day appears to exist between 120.,and 1,60 feet below ground, although the extent of this unit is uncertain. 3.4 FAULTING AND SEISMICITY The Forward Landfill is locate&in•an area-of,rAatia��ly4ow-seismic activity. Itis flot located in an area of rapid geologic change. The nearest known fault to the site, the Tracy-Stockton Fault Zone; passes through the City of Stockton, 6.5 miles northwest of the site. This fault zone is the closest known to the landfill and does not show evidence of Holocene displacement. No special setback requirements are therefore needed for the=landfill to mitigate-rupture,,ofa Holocene Jt. The seismic exposure of the proposed Forward Landfill was evaluated by Geosyntec Consultants ([Geosyntec], 2001). The evaluation consisted of assessment of seismic hazards near the site and development of corresponding design ground motions. In order to assess the seismic loads that may occur-tat the site following regional earthquakes, Geosyntec reviewed available information regarding significant active and potentially active faults and seismic source zones within 100 miles (160 km) of the project site (Table 4). Also listed in Table 4 are the strong ground shaking duration (DJ and Peak Horizontal Ground Acceleration (PHGA) estimates for both the maximum probable earthquake (MPE) and the maximum credible earthquake (MCE) events. Both estimates are for a hypothetical bedrock outcrop at the geometric center of the Forward landfill site. As shown on Table 4, the Great Valley blind thrust fault system, approximately 21 miles (34 kilometers) southwest of the site, has the greatest potential to Forward Landfill jTD 3-5 L:\AI lied\2000.193\Reports\fii naljtd:Sec-3.0:08/20/01 BRYAN A.STIRRAT&ASSOCIATES