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SECTION 2.0 GEOLOGY AND HYDROGEOLOGY <br /> i <br /> 2.1 REGIONAL AND LOCAL GEOLOGY <br /> The site is located near the center of the Great Valley of California, a geomorphic province, that <br /> is a nearly flat alluvial plain bounded on the east by the Sierra Nevada and on the west by the <br /> Coast Ranges The Great Valley is comprised of a thick sequence of sedimentary rocks of <br /> Jurassic to Recent age The Great Valley is an elongated, northwest trending, asymmetric trough <br /> with a long stable eastern shelf and a short western flank (Bailey, 1996) The Stockton Arch, an <br /> area of uplift, extends from the Sierran slope across to the western flank of the trough The uplift <br /> is thought to have occurred during Eocene time and may have continued into the Miocene as a <br /> result of upward movement on the south side of the Stockton fault <br /> The United States Department of Agriculture (USDA) classified soil in the vicinity of the site as <br /> fine, montmonllonitic, thermic typic pelloxerert The soils are vertisols, which indicates they are <br /> clayey soils that have cracks and high bulk densities The soil is formed from transported material <br /> of mixed origin and occupies nearly level or very gently sloping plains Erosion is not a problem, <br /> but because of the basin position, some areas are subject to flooding Internal dramage is poor as a <br /> result of the heavy-textured soil which becomes plastic and swells when wet, and movement of <br /> moisture is further retarded because the substratum is comparatively impervious The soil forms <br /> cracks when it is dry When the soil becomes wet, the cracks close and the soil is then slowly <br /> pervious to moisture (USDA, 195 1) <br /> Based on boring logs from previous investigations, the stratigraphy consists primarily of alluvial <br /> deposits of clays, silts, and sands Figure 2-1 depicts a plan view of the location of geologic cross <br /> sections at the site Figures 2-2 through 2-4 depict generalized geologic cross sections of the site <br /> and the neighboring Caltrans site (Shop 10 Maintenance Station) across South B Street to the <br /> west Fill material is found near the surface and is underlain by a silty and sandy clay layer which <br /> has been encountered to a depth of 3 to 12 feet bgs Below the clay layer are silts, silty sands, and <br /> clayey sands with zones of poorly graded sand and silty clay to a depth of approximately 55 feet <br /> bgs A zone of silty sands and poorly graded sands is found to the total depth of borings <br />' (approximately SO feet bgs) advanced at Stockton Yard However, a gravel zone (at MW-1) and a <br /> silty clay layer (at MW-6 and MW-11) have been encountered at approximately 70 feet bgs as <br />' shown on the geological cross sections <br /> 2.2 REGIONAL AND LOCAL HYDROGEOLOGY <br /> rThe regional hydrogeology of the Central Valley is dominated by a large basin-fill aquifer <br /> system comprised of a structural trough that is about 400 miles long and from 20 to 70 miles <br /> wide and extends over more than 20,000 square miles The trough is filled to great depths by <br /> marine and continental sediments which are the result of millions of years of inundation by the <br /> ocean and erosion of the rocks that form the surrounding mountains Sand and gravel beds in this <br /> S 1PROJ4 1040100041SYYARDIMAINTYARDSXDOC 2-1 URS Greiner Woodward Clyde <br /> 1 <br />