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City of Pleasanton—Hidden Canyon Residences and Preserve Draft EIR Geology, Soils, and Seismicity FirstCarbon Solutions 3.6-31 https://adecinnovations.sharepoint.com/sites/PublicationsSite/Shared Documents/Publications/Client (PN-JN)/2148/21480016/EIR/3 - Draft EIR/21480016 Sec03-06 Geology.docx unstable with the development of the proposed project. Impacts related to being located on an unstable geologic unit or soil would be less than significant with mitigation incorporated. Level of Significance Before Mitigation Potentially significant impact. Mitigation Measures Implement MM GEO-1. Level of Significance After Mitigation Less than significant impact. Expansive Soil Impact GEO-4: The proposed project would not be located on expansive soil, as defined in Table 18-1-B of the Uniform Building Code (1994), creating substantial direct or indirect risks to life or property. Impact Analysis As previously discussed, expansive soils are characterized as soils with a clayey texture that have shrink and swell potential when undergoing fluctuations in moisture content that shrink when dried and expand when wet. The main development areas associated with the central and eastern portions of the project site away from Devany Creek consist of the Diablo clay series which have expansive capabilities that undergo changes in moisture content due to California’s Mediterranean climate seasonally fluctuating from hot dry summers, to cool moist winters. In addition, colluvial and residual soils comprised of stiff clays range from an approximate depth between 2-11 feet below the surface, but many vary throughout the project site. Previous plasticity index tests performed on surficial clays on the project site at one to three feet in depth indicated moderate to high plasticity suggesting that soils are capable of undergoing deformation without fracturing, being a measure of having moderate to high expansive potential. Since the soil series on the project site consists of Diablo clays that have expansive capabilities, the region is associated with a Mediterranean climate, and previous plasticity index tests determined clays to have moderate to high plasticity, the proposed project may have a potentially significant impact related to creating substantial direct or indirect risk to life or property due to being located on expansive soils. The Updated Geotechnical and Geological Report provides recommendations to reduce the expansive capabilities of the colluvial soil on the project site. The implementation of MM GEO-1 would require a design-level geotechnical investigation and a review of the proposed project’s structural, civil, and landscaping plans by a qualified Geotechnical Engineer prior to the issuance of grading or building permits. The qualified Geotechnical Engineer would perform a variety of tests to indicate the expansive capabilities of the colluvium on the project site including, but not limited to, conducting an Atterberg Limits Test to investigate the fluctuations in moisture content of the colluvium soils such as calculating the plasticity index, the shrinkage limit, or the activity number. The qualified Geotechnical Engineer would provide appropriate recommendations for structure foundations and pavements that would be incorporated into the proposed project grading and