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 4 3.0 SUBSURFACE EXPLORATION 3.1 Site Geoloav The subject site is located within a large region known as the Coast Ranges geomorphic province. This province is characterized by extensively folded, faulted, and fractured earth materials. These structural features trend in a northwesterly direction subparallel to the San Andreas Fault, and make up the prominent system of northwest-trending mountain ranges separated by sediment-filled valleys (CGS, 2002). The subject site is situated on an alluvial flood plain approximately 3-'/2 miles southwest of San Francisco Bay. Our observations and review of readily available, pertinent geologic literature indicate that the subject site is underlain by medium to coarse-grained, younger alluvial fan deposits (Leighton, 1976). These soils are described as unconsolidated, moderately sorted, permeable sand and silt. 3.2 Pre-Field Activities Prior to initiation of field drilling activities, PSI outlined the site in white paint, marked the boring locations and contacted Underground Service Alert (USA) a minimum of 48 hours prior to beginning work to locate any potential buried utilities. The USA inquiry identification number (or "Ticket Number") for the utility locate request was #0210416. A drilling permit was obtained from the San Mateo County Environmental Health Department (SMCEH). A copy of this permit is included in Appendix A. 3.3 Subsurface Conditions In order to evaluate soil conditions at the site, six (6) soil borings were advanced using a truck- mounted, hollow-stemmed auger drill rig. Five borings (B-1 through B-6 with the exception of B- 5) were drilled in the footprint of the proposed structure to a depth between approximately 16-'/2 and 51-'/2 feet below the existing ground surface (bgs). The other three borings (B-5, B-7, and B-8) were drilled in proposed pavement areas to a depth between approximately 2-'/2 and 6-'/2 feet bgs. Boring B-5 was drilled with the drill rig while borings B-7 and B-8 were extended using a hand-auger. Locations of the soil borings, as well as the existing and proposed structure, are shown on Figure 2. Logs of the soil borings are presented in Appendix A. In the areas explored with the drill rig, the site is surfaced with between 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 bgs. Below this, inter-bedded layers of sand, gravelly sand, silty sand, clay, silty clay, and sandy silts were encountered to the total depth explored of 51-1/2 feet bgs. Bedrock was not encountered in our borings. No existing fill was encountered in the borings but some fill may be present beneath or immediately adjacent to the existing buildings. The above subsurface description is of a generalized nature to highlight the major subsurface stratification features and material characteristics. The boring logs, included in Appendix A, should be reviewed for specific information at individual boring locations. These records include