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MOUNTAIN HOUSE PROPOSED ALTAMONT HFIGH'€'S TENTATIVE MAP 4. ENVIRONMENTAL CHECKLIST 4.6 GEOLOGY AND SOILS c) Be located on a geologic unit or soil that is unstable, or that would become unstable as a result of the project, and potentially result in on- or off-site landslide, lateral spreading, subsidence, liquefaction or collapse? The proposed project is not located in an unstable geologic unit. As stated in the discussion of Item a)above, the potential for landslides and liquefaction at the site are considered remote, and construction of the proposed project would not change these conditions. . Following construction of the proposed project, the site would slope down gradually from east to west at less than four percent, with the exception of Court B. Court B would slope down to the north at 4.6 percent so the adjoining lots would be at lower elevations than the rest of the site. Retaining walls, varying in height from four to seven feet, would be installed along the north and west boundaries of the site. A retaining wall four feet high or less would be installed along a portion of the southern boundary of the project site, and a retaining wall three feet high or less would be installed along the southern boundary of flag Lot 50. There would be little chance of lateral spreading or subsidence provided the engineered fills are compacted to the appropriate relative compaction. The geological and soils impacts of constructing the proposed project would be less than significant. Mitigation Measures No mitigation measures are required. d) Be located on expansive soil, as defined in Table 18-1-B of the Uniform Building Code (1994), creating substantial risks to life or property? The soils at the project site are expansive. The term generally applies to any soil that has a potential for swelling with seasonal or man-made increases in moisture content. These soils also have a potential to shrink or settle due to decreases in moisture content or drying of the soil (Kleinfelder, 2006). Unless properly mitigated, soil expansion has the potential to cause cracks in concrete floor slabs and foundations and exterior walls of masonry buildings as well as wavy surfaces along sidewalks and streets. Given the extensive cut and fill and the construction of retaining walls which are proposed for the project,proper design and construction to accommodate the presence of expansive soils will be important. Methods of controlling expansive soils include constructing post-tensioned slab foundations, moisture conditioning of soils prior to construction, or replacement of.expansive clay soils with non-expansive fill. Positive site drainage should be provided away from building pads and pavement areas toward drop inlets or other drainage devices(Kleinfelder, 2006). A geotechnical engineer and a structural engineer(not a civil engineer) should evaluate each area of the project site during design and construction to determine the appropriate option for expansive soil control. If the recommendations of the engineers are followed, the impacts of expansive soils would be less than significant. Mitigation Measures No mitigation measures are required. 4-32