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City of Pleasanton—Hidden Canyon Residences and Preserve Geology, Soils, and Seismicity Draft EIR 3.6-30 FirstCarbon Solutions https://adecinnovations.sharepoint.com/sites/PublicationsSite/Shared Documents/Publications/Client (PN-JN)/2148/21480016/EIR/3 - Draft EIR/21480016 Sec03-06 Geology.docx potential to induce lateral spreading from development of the proposed project. However, the younger alluvium soils composed of sands and gravels on the southeastern portion of the project site are deposited toward a free face being the Devany Creek channel which may be susceptible to lateral spreading. However, no development or grading is proposed within the southeastern portion of the project site and impacts would be less than significant. As previously mentioned, soils highly susceptible to subsidence include clays and silts that have expansive properties and fluctuate in moisture content seasonally. The project site is situated on the Diablo clay series and Millsholm silt loam series where both soil series consist of a clayey and silty texture and are known to fluctuate in moisture content seasonally, especially the Diablo clay series which contains cracks during the dry season that close in the wet season. In addition, the Updated Geotechnical and Geologic Report identified colluvium and residual soils comprised of clays to be situated within the main development area. Although the proposed project is situated on soils that could be susceptible to subsidence, the proposed project would connect to the existing water main underneath Dublin Canyon Road and would not extract groundwater as a source of potable water or irrigational uses. In addition, the proposed project is not located within an area of existing land subsidence. As such, impacts related to soils becoming unstable from development resulting in subsidence would be less than significant. The project site is underlain by colluvium and residual soil comprised of a clayey texture. As previously discussed, naturally occurring collapsible soil consists of sand and silt sized particles held together by clayey material that is naturally present in alluvial and colluvial deposits or residual soils in semi-arid environments that can be shallow or deep. As such, soils on the project site contain characteristics of collapsible soil where additional wetting could occur naturally from the infiltration of rainwater, or from the development of the proposed project including a leakage from installed water lines and BMPs from construction. Furthermore, additional loading from developed structures can cause the rearrangement of particles and a reduction in void ratio, causing compression leading to collapse. Since the main development area of the project site consists of colluvium and residual soil composed of clays that are indicators of collapsible soils, additional loading from the development of residential units, or additional wetting from construction activities or natural processes could result in collapse. Overall, surface and subsurface conditions on the project site indicate potentially significant impacts related to landslides, liquefaction, and collapse if not engineered properly. These conditions likely vary in different areas on the project site, as specified above. However, 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 any grading or building permits. The qualified Geotechnical Engineer would perform a variety of on- site and laboratory tests on the soil properties situated on the project site including, but not limited to boring, trenching, and in situ standard-penetration tests. The qualified Geotechnical Engineer would provide recommendations in site plans and design features that would be incorporated into the project grading and building plans to reduce potential impacts associated with unstable soil conditions, in accordance with the CBC. The City Engineer would review and approve the plans and verify incorporation of recommendations prior to issuance of a grading or building permit. With MM GEO-1, the proposed project would not be located on a geologic unit or soil that would become