3A Public Hearing 2006 1204 - Laserfiche WebLink

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GEOTECNIA Project Number: 060805 2888 Darius Way, San Leandro October 27,2006 Page 5 were encountered in the three borings drilled for this study. The potentials for landsliding, fault rupture, and seismic shaking are discussed below. Landsliding As part of our study, we reviewed the soil and geologic investigation report for the subdivision (Terrasearch, Inc., 1989) and published landslide maps of the site vicinity (Nilsen, 1975a and 1 975b ). These references do not show the presence of mapped landslides or colluvial soils at the site. During our site reconnaissance, w9~id~ot observe evidence of deep-seated, active slo~ instability at the site. Based on the above. discussion, it is our opinion that the potential for deep- seated landsliding at the site is low provided the recommendations in this report are implemented. Fault Rupture The subject property does not lie within the Alquist-Priolo Earthquake Fault Zone associated with the Type-A Hayward Fault. No active faults are shown crossing the site on reviewed published maps, nor did we observe evidence of fault rupture during our study. Therefore, we conclude that the potential risk for damage to the planned improvements at the site due to surface rupture from faults is low. Seismic Shaking Earthquake shaking results from the sudden release of seismic energy during displacement along a fault. During an earthquake, the intensity of ground shaking at a particular location depends on a number of factors including the earthquake magnitude, the distance to the zone of energy release, and local geologic conditions. We expect that the site may be exposed to strong earthquake shaking during the life of the improvements since the site is only about y,. mile from a Type-A fault. Lateral loads should be estimated in accordance with the currently enforced building code based on the seismic parameters recommended below. RECOMMENDATIONS General As discussed above, we recommend that drilled-pier-and-grade-beam foundations be used for the majority of the planned structures at the site (main house, carriage house, pool house, and retaining walls), except that shallow footings may be used in excavated areas in the bedrock at least 10 feet from slopes. Detailed recommendations are provided below for drilled piers and shallow footings. In addition, the recommendations contained in the latest adopted edition of the UBC should be followed for reducing potential damage to the structures from earthquake shaking.