SR 06:060 - Laserfiche WebLink

1readweI1&RoI1o Mr. Jonathan Scharfman Silver Stone Conununities 10 June 2005 Page 5 Historically, ground surface displacements closely follow the trace of geologically young faults. The site is not within an Earthquake Fault Zone, as defined by the Alquist-Priolo Earthquake Fault Zoning Act, and no known active or potentially active faults exist on the site. The nearest known fault is an inactive extension ofthe Verona Fault which is about one kilometer south of the site. In a seismically active area, the remote possibility exists for future faulting in areas where no faults previously existed; however, we conclude the risJc of surface faulting and consequent secondary ground failure is very low. We therefore conclude the risk offault offset at the site from a known active fault is low. If the buildings are designed in accordance with the 1997 Uniform Building Code (UBC), the requirements for Zone 4 should be used as a minimwn. The site'is about 4.1 kilometers from the Calaveras Fault (a B-type fault). The anticipated soil conditions at the site indicate the soil profile type is an SD, with near-source factors of I .09 and 1.32 for N. and Nv, respectively. Foundations We anticipate that the buildings can be supported on shallow footings bearing on native soil. The primary concern from a foundation standpoint is whether or not weak soil is present at the site, its depth and extent. This can only be determined with on-site exploration. Weare presenting herein our preliminary foundation recommendations for the condition where the soft silt is absent, and where it is present. If the soft surface layer of silt is absent, we conclude the proposed buildings can be supported on conventional continuous and isolated footings that bear in undisturbed native soil or engineered fill. We anticipate that allowable bearing pressures in the range of 2,000 to 3,000 pounds per square foot (pst) will be appropriate. Footings would have minimwn widths of about 18 to 24 inches and minimwn depths of 18 inches. If site exploration discovers soft soil similar to what was found on the adjacent sites, several approaches can be taken to mitigate its effects, which would primarily be excessive settlement. The weak soil could be removed and replaced as engineered fill if its depth is not too great. If removal is impractical, some form of in-situ improvement could be undertaken. The use of rammed aggregate piers (Geo Piers) has been used effectively in the Pleasanton area. Deep soil compaction may also be viable. Alternatively, a stiffened foundation system may be possible where soft soil is too thick to remove and recompact, provided anticipated total and differential settlements can be tolerated. Stiffened, shallow foundation systems appropriate for the site include grids (interconnected, well-reinforced continuous footings) and post-tensioned slabs bearing on native soil or engineered fill.