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SAN LEANDRO SHORELINE DEVELOPMENT DRAFT EIR <br />CITY OF SAN LEANDRO <br />GEOLOGY, SOILS, AND SEISMICITY <br />other geotechnical engineering measures specified in project specific geotechnical reports shall be <br />implemented during design and construction. These measures include use of deep foundations <br />engineering and removal or improvement of potentially liquefiable soils. Documentation of the <br />methods used shall be provided in the required design -level geotechnical report(s). <br />Significance After Mitigation: Less than significant. <br />Lateral Spreading <br />Lateral spreading is a phenomenon in which relatively flat land areas undergo sudden lateral movement <br />generally toward a slope or channel margin during an earthquake. Lateral spreading occurs most <br />frequently where there is laterally continuous liquefiable layer or layers present extending to or near a <br />slope. Within the Project site lateral spreading could be a risk along the channel margins created by the <br />dredged channels both inside and outside of the marina and adjacent fill dikes. This is considered to be a <br />significant impact. <br />Impact GEO-36: The Project could result in a significant impact related to development on unstable <br />geologic units and soils or result in lateral spreading. <br />Mitigation Measure GEO-313: The potential for lateral spreading shall be evaluated as a part of the <br />required geotechnical reports. Where necessary, corrective measures shall be included in the required <br />design -level geotechnical report(s) and implemented during construction. These measures could <br />include retaining structures to stabilize channel margins, use of deep foundations, removal or <br />improvement of liquefiable soils, and/or the use of relatively rigid foundations. <br />Significance After Mitigation: Less than significant. <br />Settlement (Subsidence, Collapse) <br />Settlement, also referred to as subsidence and/or collapse, is a process in which compressible soils <br />undergo a reduction in volume in response to an increase in pressure. This pressure can be the result of <br />the addition of soil material or structures. Soils most susceptible to settlement are soft, saturated clays <br />and silts such as the Bay Mud dredged fill materials that underlies the area west of the original shoreline. <br />The existing fills were placed more than 50 years ago by placing soil and rock material over the underlying <br />Bay Mud. Since that time the Bay Mud and fill has consolidated under the load exerted by the existing fills <br />and it is likely that the potential for additional settlement under existing conditions is low. However, the <br />Project would include construction of new structures, which would place a load on their foundations and <br />the underlying materials as well as potentially the placement of some new fill. For buildings constructed <br />with shallow foundations such as slabs or spread footings, the new load would be applied directly to the <br />existing fill materials. Larger buildings would likely be constructed using deep foundations such as driven <br />piles, which apply the load to the alluvial materials beneath the Bay Mud, which are firmer and less prone <br />to settlement. If not properly engineered, buildings could undergo excessive settlement. Parking areas, <br />underground utilities and/or other non -building improvements could also be impacted by new fill <br />placement. This is considered to be a significant impact. <br />Impact GEO-3C: The Project could result in a significant impact related to development on unstable <br />geologic units and soils or result in subsidence or collapse. <br />PLACEWORKS 4.5-13 <br />