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8.B. - Page 45 <br /> 4.6 GEOLOGY <br /> The following discussion is based on a Soil Engineering Study prepared by Earth Systems Pacific in <br /> December, 2010 and an additional memo report completed by Earth Systems Pacific in September, <br /> 2011. The report and letter are attached as Appendices B and C,respectively. <br /> 4.6.1 Existing Setting <br /> 4.6.1.1 Geology and Soils <br /> The project site is located on the eastern side of the Santa Cruz Mountains. The site is adjacent to <br /> Redwood Creek and is underlain by Pleistocene aged alluvial deposits�� which are typically dense <br /> gravely and clayey sand, clayey gravel and sandy clay. The alluvial deposits are overlain by <br /> Holocene aged stream channel deposits associated with Redwood Creek, consisting of poorly to well <br /> sorted sands, silts, and gravel with minor cobbles. <br /> A subsurface exploration was completed at the project site in November 2010. Three exploratory <br /> borings were drilled at the approximate locations shown in Figure 7. The borings were drilled to <br /> depths of 10, 15, and 50 ft below ground surface (bgs). In general,the soil materials at the project <br /> site consist of five to nine feet of dark gray expansive clays overlying brown silty to sandy clays and <br /> clayey sands and silts to the ma�mum depth explored of 50 feet. The site also has occasional sand <br /> layers,including granular silt layers located 35 to 40 ft bgs. There are up to five feet of creek bank <br /> deposits along the creek margins, overlying the expansive clay soils. <br /> The subsurface soils at the project site are mostly clays and have a high expansion potential. The <br /> project site would not be exposed to slope instability,long-term erosion, or landslide related hazards <br /> due to the flat topography of the site,the concrete-lining along the adjacent creek channel, and the <br /> project design which does not place loads on the channel lining, as discussed below.�� <br /> 4.6.1.2 Groundwater <br /> Groundwater was encountered at the project site during subsurface explorations at approximately 20 <br /> ft bgs. Surface water was present in the creek channel west of the site,however,the channel is <br /> concrete lined and thus the water is isolated from the surrounding soil. Fluctuations in the level of <br /> subsurface water can occur due to variations in rainfall,temperature, and other factors and <br /> groundwater should not be considered constant. The historic groundwater level in the area <br /> surrounding the project site is ten to 20 ft bgs. Based upon soils samples collected from the site, it is <br /> likely that the groundwater at the site has historically been as high as at least 18 ft bgs. <br /> 4.6.1.3 Seismicity and Seismic Hazards <br /> The San Francisco Bay Area is one of the most seismically active regions in the United States. An <br /> earthquake of moderate to high magnitude generated within the San Francisco Bay region could <br /> cause considerable ground shaking at the project site. The degree of shaking is dependent on the <br /> magnitude of the event,the distance to its zone of rupture and local geologic conditions. The project <br /> site is not within a defined Alquist-Priolo Earthquake Fault Zone and the risk of fault rupture at the <br /> site is low.13 <br /> 11 Alluvial soils are fine-grained fertile soil deposited by water flowing over flood plains or in river beds. <br /> 12 Earth Systems Pacific. Responses to City Plan Review Comments. September 2011. <br /> 13 California Depariment of Conservation. California Geological Survey-Alquist-Priolo Earthquake Fault Zones. <br /> 2010. <http://www.conservation.ca.�ov/c�s/r�hrn/ap/Pa�es/affected.aspx> Accessed July 13,20ll. <br /> 1410 Valota Road Residential Project 33 Initial Study <br /> Redwood City January 2012 <br />