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City of Pleasanton—Hidden Canyon Residences and Preserve Geology, Soils, and Seismicity Draft EIR 3.6-4 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 Expansive Soils Expansive soils are clays or silty clays that experience significant volume changes with the removal or addition of moisture and can be considered a geological hazard as the shrinking and swelling potential in volume can generate damage to foundations, underground public utilities, and structures including walls and ceilings. Most of the project site is situated on Diablo clay which have expansive capabilities as the seasonal fluctuations in moisture content can cause soils to shrink during the dry season exposing cracks two inches wide with a depth of 20 to 40 inches that close through expansion during the wet season. Previous studies on the project site referenced in the Updated Preliminary Geotechnical and Geologic Report measured the plasticity index of the colluvial soil at depths ranging from one to three feet on the northern edge of the project site. Soil plasticity is the ability of a soil to undergo deformation without cracking which is caused from the adsorption of additional water between clay particles that allows particles to move over one another without fracturing. The plasticity index, which is the range in moisture content where soils experience plasticity, indicated moderate to high plasticity and expansion potential for the colluvial soil.8 Collapsible Soil Collapsible soil generally consists of sand and silt sized particles that are held together by a bonding material such as clay. Soil particles can decrease in volume or become rearranged when undergoing additional wetting or loading with the occurrence depending on a variety of factors including soil texture, structure, initial moisture content, and void ratio.9 Collapsible soils naturally occur in alluvial and colluvial deposits or residual soils in arid and semi-arid environments that are either shallow or deep, or can occur on poorly engineered artificial fills which can lead to structural damage. As previously discussed, soils on the project site consists of stiff clays from residual soil and colluvium with expansive capabilities that fluctuate in moisture content seasonally, and therefore, contain some characteristics of collapsible soil. Seismicity The term seismicity describes the effects of seismic waves that are radiated from an earthquake fault in motion that generates strong seismic shaking. While most of the energy released during an earthquake results in the permanent displacement of the ground, as much as 10 percent of the energy may dissipate immediately in the form of seismic waves. Ground movement during an earthquake can vary depending on the overall magnitude, distance to the fault, focus of earthquake energy, type of geologic material, and structure resilience. The composition of underlying soils, even those relatively distant from faults, can intensify ground shaking. Strong ground shaking from an earthquake can result in damage, with buildings shifted off their foundations and underground pipes broken . Seismicity from strong seismic shaking can generate a variety of secondary seismic-related hazards such as surface fault rupture, liquefaction, landslides, and lateral spreading. Seismic ground shaking and secondary seismic-related hazards in the surrounding region are discussed in detail further below. 8 Cornerstone Earth Group. 2020. Updated Preliminary Geotechnical and Geologic Report. March 31. 9 California Department of Transportation (Caltrans). 2024. Geotechnical Manual: Collapsible Soil. February.