Laserfiche WebLink
4.0 Geology/Hydrogeology <br /> 4.1 Regional Geology <br /> The Pacific Bell facility is located in the Great Valley physiographic province The Great <br /> Valley is a geologic sequence, consisting primarily of alluvial, flood, and delta plains formed <br /> by the valley's two major rivers (the Sacramento River and the San Joaquin River) and their <br /> tributaries The resulting topography of the area is flat with an average elevation at the <br /> facility of 12 feet above mean sea level The site is immediately underlain by sands, silts, <br /> clays, and gravels of nonmarine origin, which are of Quaternary age (Norris and Webb, <br /> 1990) These sediments are generally loosely consolidated <br /> 4.2 Site Geology <br /> During drilling activities conducted beneath the Lindsay Street sidewalk, three distinct soil <br /> layers were encountered. The upper layer consisted of imported fill material, emplaced upon <br /> removal of three and replacement of one 25,000-gallon underground storage tanks at the site <br /> This fill was composed of sands, silts, and gravels and was noted to extend to approximately <br /> 18 feet below grade Native soils were encountered underlying this fill layer, which consisted <br /> of unconsolidated fluvial sediments A layer of silty sand was noted between 18 and 25 feet <br /> below grade A layer of silt with varying concentrations of sand was noted from 25 to 30 <br /> feet below grade, the total depth explored Review of boring logs for wells MW-1, MW-2, <br /> and MW-3, installed surrounding the site in December 1991, indicates that a sand layer <br /> underlies the silt to 60 feet below grade, the total depth explored A cross-section and <br /> location map are provided as Figures 4 and 5 <br /> A particle size analysis (using the hydrometer method) was conducted on a sample collected <br /> at 28 5 feet below grade from well VW-A Results indicated the sample was composed of <br /> 66.1% fines and 33 9% coarse grained material, which can be described as brown silt slightly <br /> cemented using the Unified Soil Classification System (USCS) <br /> 4.3 Site Hydrogeology <br /> Survey and monitoring data indicated that groundwater flows to the east-northeast with an <br /> approximate gradient of 0 01 ft/ft The water table was encountered at approximately 42 feet <br /> below grade Table 1 contains historical site groundwater elevation data <br /> MZ/04-06-95/UST/95-0089 4 <br />