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MOUNTAIN HOUSE NEIGHBORHOOD"F"PROJECT INITIAL STUDY Condition of Approval GEO-1: The following recommendations were made in the project's Geotechnical Report(as distinguished from the Geotechnical report for the Bethany Elementary School)and these recommendations shall be followed as written in the original report and summarized below: • Foundation support shall be strengthened and stiffened shallow foundations, slabs, or mats, with specific design recommendations presented in the Geotechnical Report~ • Critical infrastructure improvements shall be designed with the potential ground deformation in mind Flexible connections shall be provided where critical underground utilities enter buildings to accommodate the anticipated differential movements due to liquefaction. • Given the presence of potentially-liquefiable deposits underlying much of the site and the potential for seismically-induced ground deformation, it was recommended that a Seismic Zone Factor(Z) of 0.4 be used for the design of emergency and critical facilities These standards are recommended even though the site is located in Uniform Building Code(UBC)Seismic Zone 3. b) Result in substantial soil erosion or the loss of topsoil? Substantial soil erosion is not expected to occur at the site because it has an average slope of less than 1 percent toward Old River(over one mile to the northeast of the site). Ground elevations range from about 90 feet near the southwest corner of Neighborhood"I"'to about 50 feet near the northeast corner. Project grading is not expected to require cuts and fills in excess of 10 feet, with typical site grading for building areas involving cuts and fills of less than 5 feet. Topsoil would be lost on up to 406 acres for the proposed project. To support slabs, pavements, foundations, and new engineered fills, the project's Geotechnical Report recommends stripping such areas of all vegetation, debris, organic topsoil, or any existing fill or other unsuitable material or soil. Stripping is recommended to extend at least 4 feet beyond the limits of the proposed improvements. Organic topsoil is not present over much of the site;however, organic matter and roots are present in the upper 6 to 12 inches due to cultivation of the area. It is recommended that site strippings be stockpiled for later use in landscape areas. Inorganic, on-site soil that is free from rocks greater than 4 inches in least dimension, debris, or other unsuitable material, may be used as engineered fill provided that it is placed as described in the Geotechnical Report. 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? Due to the site's level terrain, landslides would not be of concern. Lateral spreading and liquefaction are addressed under(a)above. 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 Geotechnical Report concluded that the project would be located in an area of moderately to highly expansive soils. Because of this, it is important that grading and site work be performed in accordance with the requirements of the Uniform Building Code(UBC)and with the recommendations of the Geotechnical Engineer of Record during construction. The following is a summary of mitigation measures that are described in greater detail in the Geotechnical Report in addition to those identified under(a)above: C IJOBSWCUNmAL SMY MALINMALSMMY-PARMDOC(7/20=) 89 V.I. GEOLOGY AND SOILS