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City of Pleasanton—Hidden Canyon Residences and Preserve Geology, Soils, and Seismicity Draft EIR 3.6-26 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 the CGS.23 Additionally, younger alluvium consisting of sands and gravels with perched groundwater are positioned along Devany Creek along the southeastern portion of the project site. Accordingly, the Updated Geotechnical and Geologic Report indicates the southeastern and northeastern boundaries of the project site to have a moderate to high liquefaction potential. Therefore, development within these areas of the project site could result in potentially significant impacts related to direct or indirect loss, injury, or death involving liquefaction generated from strong seismic shaking may be potentially significant. As provided above, the proposed project would be required to demonstrate compliance with General Plan policies and programs, including but not limited to Program 5.1 and 5.2 which require site-specific studies or geotechnical engineering studies. Further, the Updated Geotechnical and Geologic Report and recommends future developments constructed in these areas to implement design features to resist liquefaction- induced settlement. Therefore, prior to obtaining any grading or building permits, the proposed project would implement Mitigation Measure (MM) GEO-1 by hiring a qualified Geotechnical Engineer to prepare a design- level geotechnical investigation and review existing structural, civil, and landscaping plans. The qualified Geotechnical Engineer would present additional findings on any seismic-related hazards, including liquefaction, on the project site and would provide recommendations that would be required to be incorporated into the design of the proposed structures prior to the issuance of any grading or building permits. With implementation of MM GEO-1, the proposed project would not be susceptible to the risk of injury, loss, or death involving seismic-induced ground failure such as liquefaction. iv) Landslides The project site is situated on complex topography with moderately steep to steep hillsides ranging from 774 feet in elevation AMSL on the western portion of the project site that slopes downward to the east, decreasing in elevation to approximately 438 feet AMSL along Devany Creek. The Updated Geotechnical and Geologic Report identified three small-scale drainage swales with eroded side slopes filled with colluvium on the southeastern aspect of the project site that drains into Devany Creek. The colluvium-filled swales were discovered to be the result of seasonal pervasive downhill creep of the colluvial soils. The northern portion of the project site was previously mapped having a large landslide that extends north beyond Dublin Canyon Road, but further analysis indicated that the topography associated with the northern portion of the project site is related to erosional processes rather than a large landslide.24 The Updated Geotechnical and Geologic Report concludes that while a majority of natural slopes located on the project site are relatively stable, the proposed cuts proposed during future grading could affect the stability of finished slopes. This could result in a potentially significant impact related 23 California Department of Conservation. 2025. California Geologic Survey Seismic Hazards Program: Liquefaction Zones Map. Website: https://gis.data.cnra.ca.gov/datasets/cadoc::cgs-seismic-hazards-program-liquefaction- zones/explore?location=37.696002%2C-121.952585%2C15.87. Accessed May 1, 2025. 24 Cornerstone Earth Group. 2020. Updated Preliminary Geotechnical and Geologic Report. March 31.