_DEIR - Laserfiche WebLink

City of Pleasanton—Hidden Canyon Residences and Preserve Draft EIR Geology, Soils, and Seismicity FirstCarbon Solutions 3.6-11 https://adecinnovations.sharepoint.com/sites/PublicationsSite/Shared Documents/Publications/Client (PN-JN)/2148/21480016/EIR/3 - Draft EIR/21480016 Sec03-06 Geology.docx As such, the Updated Geotechnical and Geologic Report identified low potential of liquefaction for the main development area and a moderate to high liquefaction potential for development on the northeast corner of the project site. Slope Disturbance Slope disturbance from long-term geologic cycle of uplift, mass wasting, intense precipitation or wind, and gravity can result in slope failure in the form of mudslides and rock fall. The project area is seismically active with known faults; however, the project area does not contain active faults that would cause geologic uplifting. Mass wasting refers to a variety of erosional processes from gradual downhill soil creep to mudslides, debris flows, landslides, and rock fall—processes that are commonly triggered by intense precipitation or wind, which varies according to climactic shifts. Often, various forms of mass wasting are grouped together as landslides, which are generally used to describe the downhill movement of rock and soil. Soil creep is a long-term, gradual downhill migration of soil under the influence of gravity and is generally on the order of a fraction of an inch per year. These soils can creep away downslope sides of foundations and reduce lateral support. The City of Pleasanton is located within the Tri-Valley region in Alameda County, expanding from the City of Dublin in the north to the City of Livermore in the east, with elevations ranging from approximately 320 to 600 feet AMSL that separates Mt. Diablo to the north from the Diablo Ranges to the south, while the East Bay Hills borders the Tri-Valley to the west. Most of the urbanized areas in Pleasanton are situated in the Tri-Valley on flat terrain and are not located in a CGS Zone of Required Investigation for seismic landslide hazards. Areas that are more susceptible to landslides within the municipality that are delineated in Zones of Required Investigation by the CGS are areas of higher elevation located south and southeast of Bernal Avenue, and on the eastern aspect of the East Bay Hills west of the Calaveras Fault line and Foothill Road.20 The project site is located adjacent to the western limits of the City and on the eastern side of the East Bay Hills. The CGS identifies areas of the northern and southeastern portions of the project site to be within a State-designated Landslide Zone of required investigation; see Exhibit 3.6-2. The project site is transected by a ridgeline striking in a westward direction with the highest point in elevation approximately 743 feet AMSL on the far northwestern side of the project site. The ridgeline produces two slopes with northeast and southeast aspects that are 200 feet in height with a moderately steep incline. The northeast aspect of the slope faces toward Dublin Canyon Road and I- 580 consisting of scattered vegetation at the higher elevations and dense trees toward the lower elevations along Dubin Canyon Road. The southeast aspect contains steeper slopes covered with annual grasses and consists of gullies on the eastern side of the project site that drains into Devany Creek where vegetation becomes denser. The Geotechnical Review prepared by Ninyo & Moore on February 12, 2025, indicates that the far western end of the northeast aspect of the ridgeline has a relatively high susceptibility to landsliding but concluded no landslide to be present; the southeast aspect of the ridgeline contains four shallow landslides within colluvium-filled gullies above Devany Canyon generated from seasonal downhill creep of surficial colluvial soils indicating potential slope instability.21 20 California Department of Conservation. 2025. Earthquake Zones of Required Investigation. 2025. Website: https://maps.conservation.ca.gov/cgs/informationwarehouse/eqzapp/. Accessed February 25, 2025. 21 Ninyo & Moore. 2025. Update Geotechnical Review Hidden Canyon Residences and Preserve, Dublin Canyon Road, Pleasanton, California. February 12, 2025.