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Section 5 <br />5.6 GEOLOGY/SOILS <br />The following analysis is based on a geotechnical report prepared for the Mid <br />Basin Centennial Park Injection Well Project by Leighton Associates in <br />September in 2015. The geotechnical report is presented in its entirety in <br />Appendix E. <br />5.6.1 Affected Environment <br />Geologic Setting <br />The project site is located within the floodpWn of the Santa Ana River. The site is <br />underlain by over 1,000 feet of alluvium deposited by the ancestral Santa Ana <br />River. The alluvial deposits underlying the site are estimated to be Late <br />Pleistocene to Holocene in age and typically consist of sand, silt and clay in the <br />upper 25 feet below grade. <br />Subsurface Conditions <br />Boring samplIes were conducted on the project site work areas to identify <br />subsurface conditions. The pavement sections of the project site consisted of 3 <br />inches to 7 inches of asphalt concrete overlaying 3 to 12 inches of aggregate <br />base. Subsurface soils that underlie the pavement sections generally consisted <br />of light olive brown to brown, medium dense sand with varying amount of silt to a <br />depth of approximately 10 feet, The sand is dry to slightly moist and considered <br />susceptible to caving due to the relatively low moisture content and lack of <br />cementation. Below the sand to the maximum depth explored, the soils consisted <br />primary of layers of loose to medium dense silt and medium stiff clay. <br />Groundwater <br />Groundwater was encountered at depths between 13 feet and 23 feet. The <br />historic high groundwater table within the site was reported to be at 5 feet below <br />the ground surface. <br />Sail Corrosivity <br />The subsurface soils at the project site showed negligible potential for sulfate <br />attack on concrete and are considered to have low corrosion potential to buried <br />ferrous metal in direct contact with the soils. <br />Faulting and Seismicity <br />There are no known active faults traversing the site and the site is not located <br />within a State of California designated Aliquist-Priolo Earthquake, fault Zone- The <br />project area is located within a seismic active region and would be susceptible to <br />ground shaking from known active and potentially active faults in the region, <br />including the San Joaquin Hills Blind Thrust, Newport Inglewood, Puente Hills <br />Blind Thrust and the Elsinore Faults. <br />Mid Basin Centennial Park 9J§7 <br />c9n1A;Project Final EIR 5-82 <br />