SR 06:060 - Laserfiche WebLink

Mr. Jonathan Scharfman Silver Stone Communities 10 June 2005 Page 4 1read\\eI1&RoI1o DISCUSSIONS AND CONCLUSIONS From our review of the available subsurface data, we conclude the site is suitable for the proposed project from a geotechnical standpoint. The primary geotechnical concerns for this site are the potentially soft silt and the steep slope at the northern property boundary. These issues are discussed in the remaining sections of this report. Seismic Hazards During a major earthquake on one of the nearby faults, strong to very strong shaking is expected to occur at the project site. Strong shaking during an earthquake can result in ground failure such as thatassociated with soilliquefaction2, lateral spreading3, and cyclic densification4. We used the results of the borings to evaluate the potential of these phenomena occurring at the project site. The results of our evaluation are presented below. The seismicity of the site is governed by the activity of the Calaveras and Hayward Faults. However, ground shaking from future earthquakes on any of the nearby faults could be felt at the site. The intensity of earthquake ground motion at the site will depend upon the characteristics of the generating fault, distance to the earthquake epicenter, magnitude and duration of the earthquake, and local subsurface conditions. We judge ground shaking at the site during a large earthquake on one of the nearby faults will be strong to very strong. When soi1liquefies, it experiences a temporary loss of shear strength due to a transient rise in excess pore pressure generated by strong ground motion. Soil most susceptible to liquefaction is loose, clean, saturated, uniformly graded, fine-grained sand and silt oflow plasticity that is relatively free of clay. Flow failure, lateral spreading, differential settlement, loss of bearing strength, ground fissures and sand boils are' evidence of excess pore pressure generation and liquefaction. Because the granular soil encountered in the vicinity are dense, we conclude the risk of liquefaction and associated lateral spreading is low. Seismically induced compaction or cyclic densification of non-saturated sand (sand above the groundwater table) due to earthquake vibrations may cause differential settlement. Based on available subsurface data, we conclude the risk of seismically induced densification is low. 2 Liquefaction is a transformation of soil from a solid to a liquefied state during which saturated soil temporarily loses strength resulting from the buildup of excess pore water pressure, especially during earthquake-induced cylic loading. Lateral spreading is a phenomenon in which surficial soil displaces along a shear zone that has formed within an underlying liquefied layer. Upon reaching mobilization, the surficial blocks are transported downslope or in the direction of a free face by earthquake and gravitational forces. Cyclic densification is a phenomenon in which non-saturated, cohesionless soil is densified by earthquake vibrations, causing ground-surface settlement. 3 4