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` MOUNTAIN HOUSE PROPOSED MACHADO TENTATIVE MAP 4. ENVIRONMENTAL CHECKLIST 4.6 GEOLOGY AND SOILS L regionally active strike-slip faults such as the San Andreas fault,the Hayward fault, the L Calaveras fault,the Concord-Green Valley fault, and, nearer to the project site, the Greenville-Marsh Creek fault. Small earthquakes occur frequently on these active faults, and periodic major earthquakes can cause significant surface displacement and regionally L significant seismic ground shaking. Site Soils LSoils at the project site consist of Capay clay and Stomar clay loam. Both soil types are very deep and moderately well drained. The Capay clay is present in interfan basins,while the Stomar soils are on alluvial fans. Both soil types exhibit slow permeability, high shrink-swell potential, and low strength(Anderson,2005). L Soil expansion is a phenomenon in which clayey soils expand in volume because of an increase in moisture content and shrink in volume upon drying. This is generally related to increased moisture in the winter and spring months and decreased moisture during the L summer and fall. Changes in soil volume as a result of changes in moisture content can cause L stress and result in cracking of foundations,pavement, sidewalks and damage to other site improvements. i L Liquefaction hazards may be present in loose to medium dense, saturated soils such as sands or silty sands in which the space between individual particles is completely filled with water. The potential for an earthquake generating ground-shaking characteristics capable of causing liquefaction at the project site is a possibility. However, since the subsurface soils have high clay content and/or relatively dense,the potential for liquefaction is considered remote. LGroundwater Three bore-holes were drilled at the project site in October 2005 as part of the Limited Phase L II Environmental Site Assessment. Groundwater was encountered in three holes at 17.5 feet, 26 feet,and 29 feet below ground surface (bgs). It is likely that groundwater flows from southeast to northwest following the pattern of regional topography(Anderson, 2005). The Lbore-hole with groundwater at 17.5 feet bgs was drilled close to the east side of the settling pond, so the higher groundwater level may have been influenced by pond water. SIGNIFICANT IMPACTS IDENTIFIED IN 2005 SPIIIEIR Six potential impacts were identified in the Initial Study for the 2005 SPIHEIR. There were no impacts associated with surface rupture along an earthquake fault and use of septic tanks. L' Impacts associated with expansive soils were less than significant. Impacts associated with liquefaction,landslides, and soil erosion were less than significant with the incorporation of mitigation measures presented in the MEIR along with the regulations and implementation measures of the MHMP that control construction of improvements at the Mountain House Community. The impacts associated with strong seismic ground shaking were found to be potentially significant and unavoidable. L L 4-27 L