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City of Pleasanton—Hidden Canyon Residences and Preserve Geology, Soils, and Seismicity Draft EIR 3.6-2 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 faults that are overlain by colluvial and residual soils. Other formations in the project area include an unnamed Miocene siltstone and clay shale (Tmc), the Miocene Clairmont Shale member of the Monterey Formation (Tm), and an unnamed Miocene sandstone (Tms). Locally overlaying these geological formations within low-lying areas and the valley bottom near Devany Creek are Quaternary alluvium (Qa). However, there have been some historic variations on the classification of the Monterey Formation either being defined as a formation or group. In this analysis, the Monterey Formation will be classified as a formation that contains the Claremont Shale member. Subsurface Conditions Subsurface conditions are discussed in this section including existing soils on the project site, regional groundwater levels, and potential subsurface hazards including subsidence, expansive soils, and collapsable soils. Existing Soils The project site is comprised mainly of Diablo clay series on the southeastern and northeastern slopes, and a portion of Millsholm silt loam on the southwestern side. The Diablo clay is characterized as well drained with slow permeability consisting of a heavy clay loam, silty clay or clay texture that is typically associated with rolling to steep slopes between 5 to 50 percent in grade. Diablo clay typically contains cracks in dry soil ½ to 2 inches wide from the surface to a depth of 20 to 40 inches during the dry season which close during the wet season. Moist Diablo clay consists of a medium to rapid surface runoff due to its slow permeability, expansive capabilities, and topographic setting.1 The Millsholm silt loam series is well drained with a moderate permeability situated with slopes between 5 to 75 percent in grade with a clay, silty clay, gravely clay, gravely sandy clay loam, or loam texture and may have 35 percent rock fragments, and fluctuates in moisture content seasonally from California’s Mediterranean climate. The Millsholm silt loam associated with the southwestern side of the project site has a moderate K Factor, which measures the soil’s susceptibility to sheet and rill erosion from water under natural conditions. However, the Millsholm silt loam is classified as having a severe susceptibility to erosion if exposed to disturbance activities, most likely due to the project site’s steep terrain along with having a moderate K factor. Furthermore, soils with a clayey texture tend to have larger and more durable aggregates that do not contribute to fugitive dust emissions and have a high resistance to wind erosion under natural conditions unless aggregates are broken down by surface disturbance.2 In addition, the Updated Preliminary Geotechnical and Geologic Report from Cornerstone Earth Group analyzed subsurface conditions and determined that soils on the project site vary but are mostly comprised of residual soil and colluvium consisting of stiff to stiff lean to fat clays with a shallow depth between 0.5-foot to 2 feet at the highest elevations of the project site and increase in depth within the troughs and drainage swales between the higher ridges with an approximate thickness between 5 to 11 feet. Soils differ on the northern portion of the project site with colluvium and residual soil approximately 6 feet thick that is underlain by an additional layer of very stiff sandy 1 United States Department of Agriculture (USDA). Diablo Series. 2017. Website: https://soilseries.sc.egov.usda.gov/OSD_Docs/D/Diablo.html. Accessed February 13, 2025. 2 United States Department of Agriculture (USDA). Natural Resources Conservation Service. Web Soil Survey. n.d. Website: https://websoilsurvey.nrcs.usda.gov/app/WebSoilSurvey.aspx. Accessed February 13, 2025.