Reso13 PC 13-09 2859 Initial Study Mitigated Negative Declaration Kensington

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Proposed Senior Assisted Living Facility PSI Project No. 575-308-2 August 3, 2011 Page 7 The site Coefficients Fa and F� presented in the above table were obtained from CBC Tables 1613.5.3(1) and 1613.5.3(2) as a function of the site classification and mapped spectral response acceleration at the short (SS) and 1 second (S�) periods. Considering the use of the proposed structures, the values of Sps and Sp� calculated from the above, and the design spectral acceleration, we expect the structure to fall within Seismic Design Category D, as defined by Tables 1613.5.6 (1) and 1613.5.6 (2) of the code. Design of structures should comply with the requirements of the governing justification's building codes and standard practices of the Structural Engineering Association of California. 4.3 Hazard Assessment Seismicallv-Induced Settlement of Soils — In the areas explored, the site is surfaced with between approximately 2 and 3 inches of asphalt pavement over 3 to 4 inches of aggregate base. Below this pavement section, the material encountered consisted primarily of inedium to dark brown firm to stiff silt and sandy silts to a depth of approximately 5 to 7 feet underlain by. inter-bedded layers of sand, gravelly sand, silty sand, clay, silty clay, and sandy silts to the total depth explored of 51-'/2 feet bgs. Based on the anticipated earthquake effect and the stratigraphy of the site, seismically-induced dry settlement is estimated to be less than '/4 inch. Liquefaction - Liquefaction typically occurs in loose granular soils and low-cohesive silts and clays (PI<12) with relatively shallow groundwater. During an earthquake, ground shaking causes a rapid increase in the porewater pressure within the soil mass and a corresponding decrease in the soil's effective stress, which can result in a sudden loss of soil bearing strength. Highly plastic fine-grained soils are not generally susceptible to liquefaction. Liquefaction potential has been found to be the greatest where the groundwater level is within a depth of 50 feet and loose fine sands or fine soils with low plasticity occur within that depth. The liquefaction potential decreases with increasing grain size and PI and gravel content, but increases as the ground acceleration and duration of shaking increases. Based on our borings, potentially liquefiable soils (saturated sand) were identified in boring B-1 from about 23 to 27 feet and from about 32 to 36 feet below existing grade. The Geotechnical Hazards Synthesis Map for San Mateo County (Leighton, 1976), indicates that the site is in a zone of moderate potential for seismically-induced liquefaction. During the time of drilling, groundwater was encountered in boring B-1 at approximately 20 feet bgs, however, based on groundwater monitoring data for the Shell Service Station at 3201 EI Camino Real, Menlo Park, California, located approximately one-third mile southeast of the Subject Property; historic high groundwater is estimated to be approximately 12 feet bgs. Much of the soil encountered in the borings consist of inedium to high plasticity clay and silty clay, which due to their plasticity, are not considered to be susceptible to soil liquefaction. However, the gravely sand layer in Boring B-1 at a depth ranging from about 23 to 27 feet and the silty sand at a depth of about 32 to 36 feet below grade were considered to be potentially susceptible to soil liquefaction.