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CORNERSTONE C. EARTH GROUP 4.2 ESTIMATED GROUND SHAKING Moderate to severe (design-level) earthquakes can cause strong ground shaking, which is the case for most sites within the Bay Area. The Probabilistic Seismic Hazard Analysis performed by the California Geologic Survey indicated a peak horizontal ground acceleration for the site with a 10 percent chance of exceedance in 50 years of approximately 0.6g for alluvial soil conditions. This value is intended for reference only and is not intended for use in design or analysis. 4.3 LIQUEFACTION POTENTIAL During strong seismic shaking, cyclically-induced stresses can cause increased pore pressures within a soil matrix that can result in liquefaction triggering, soil softening due to shear stress loss, potentially significant ground deformation due to settlement within sandy liquefiable layers as pore pressures dissipate, and/or flow failures in sloping ground or where open faces are present (lateral spreading) (NCEER 1998). Soils most susceptible to liquefaction are loose, saturated, uniformly graded, fine-grained sands and soft, saturated low plasticity silts and clays. Limited field and laboratory data is available regarding ground deformation due to settlement; however, settlement on the order of 2 to 3 percent of the liquefied layer thickness can occur in saturated clean sand layers. C Although localized layers of loose to medium dense sand were encountered in our borings within the upper 15 feet, they are generally located above anticipated future ground water levels of approximately 40 feet below existing site grades. Below this depth, granular soils are generally dense to very dense. For these reasons, the potential for liquefaction during seismic shaking is considered low. 4.4 SEISMIC SETTLEMENT/UNSATURATED SAND SHAKING Strong seismic shaking can cause unsaturated cohesionless soils to consolidate. Soils most susceptible to seismic settlement are loose to medium dense sands and gravels with relatively low fines contents. Our borings encountered loose to medium dense sand and silty sand at depths ranging from approximately 5 to 14 feet. The thickness of these layers ranged from 1 to 6 feet thick. Sands encountered at these depths had fines contents ranging varying from approximately 7 to 40 percent. For our seismic settlement analysis, we assumed a low ground water depth of at least 50 feet. We evaluated the potential for seismically-induced settlement using the method proposed by Pradel (1998). This analysis method estimates seismically-induced settlement based on the Standard Penetration Test (SPT) blow count or N-value and the design earthquake characteristics. Due to the inherent variability of the SPT and sensitivity of the SPT to test procedures, it is necessary to correct SPT N-values to normalized values. Ponderosa Active Adult Community Page 6 132-2-1