3A Public Hearing 2006 1204 - Laserfiche WebLink

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GEOTECNIA Project Number: 060805 2888 Darius Way, San Leandro October 27,2006 Page 3 from about 3,500 to over 4,500 pounds per square foot (psf), and Standard Penetration Test (SPT) blow counts (N-values) ranging from about 11 to 37 blows per foot (bpf). Gabbro bedrock was encountered below the clay layers in Borings B~ 1, B-3, B-4, B~6, and B-8 at depths ranging from 2'1z to 12lf2 feet, and from the surface to the maximum depths explored in Borings B-2, B-5, and B-7. The bedrock was soft to finn, plastic to friable, included occasional clay lenses, and was highly weathered (soil-like). The N-values in the bedrock ranged from about 11 bpfat a depth of 10 feet in Boring Bw5 (where clay lenses were present) to 50 blows of an SPT sampler for only 2 inches of penetration at the bottom of Boring B-7 (at a depth of about 4 feet). Groundwater Free groundwater was not encountered in any of the eight borings at the time of drilling. The groundwater level is anticipated to fluctuate with changes in seasonal and annual precipitation, well pumping, irrigation, and other factors. CONCLUSIONS General On the basis of our geologic reconnaissance, results of our field exploration and laboratory testing programs, and review of available data, we conclude that the proposed development at the site may be constructed as planned provided the recommendations in this report are implemented. The main geotechnical conditions and geologic hazards at the site include (1) the presence of expansive soils and associated slope creep, and (2) seismic shaking during earthquakes. These items, as well as the potentials for landsliding, fault rupture, and seismic shaking, are addressed below. It is our opinion that the new foundations to support the proposed structures at the site should consist of drilled-pier-and-grade-beam foundations where the piers need to be at least 16 feet deep or extend at least 10 feet into the bedrock, whichever is deeper. The intent of the piers is to resist the anticipated differential expansive soil movements and provide adequate resistance to lateral loads due to the presence of steep slopes and creeping soils. Geotechnical Conditions Presence of Expansive Soils The results of our field exploration and laboratory testing program and review of available data indicate that the site is underlain by expansive clays. The potential for expansion is tabulated below qualitatively as a function of the PI. As shown on the table, the in-situ clays (with PIs of