9 First American Way - Soils Report

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Lutzky Associates Development. LP - Geolechnical Investigation MACTEC Project 4953-03-263 1 September 18,2003 According to the City of Santa Ana General Plan (1982), the County of Orange Safety Element (1995), and the California Division of Mines and Geology (1998), 'the site is within an area identified as having a potential' for liquefactÂ·ib-n. To evaluate the site-specific liquefaction potential, the magnitude-7.5-adjusted peak ground acceleration (PGA) corresponding to the Design Basis Earthquake (DBE), which is defined as an event with a 10% probability of exceedance in 50 years, was computed. The magnitude-7.5- adjusted PGA of the DBE, hereinafter referred to as the liquefaction PGA, was computed probabilistically using EZFRISK, Version 5.71 with the ground motion attenuation relations discussed in Abrahamson and Silva (1997), Boore et al. (1997), and Sadigh, et al. (1997), for a "soil" site type with a shear wave velocity in the upper 30 meters equal to 234 m/s. The liquefaction PGA for the subject site was calculated as 0.30 g. Our liquefaction analysis was based on the depth to ground water beneath the site (using historic high ground-water levels of 10 feet below the existing grade), SPT and CPT results obtained during our site investigation, and the liquefaction peak ground acceleration from our probabilistic ground motion studies. The results of the liquefaction analysis indicate that some of the medium dense sand layers between Elevation 17 and Elevation 2 have the potential to Jiquefy during a DBE event. The liquefaction-induced settlement is expected to be on the order of'/2 to 3% inch. Seismic-induced settlement is often caused by loose to medium-dense granular soils densified during ground shaking. Uniform settlement beneath a given structure would cause minimal damage; however, because of variations in distribution, density, and confining conditions of the soils, seismic-induced settlement is generally non-uniform and can cause serious structural damage. Dry and partially saturated soils as well as saturated granular soils are subject to seismic- induced settlement. Generally, differential settlements induced by ground failures such as liquefaction, flow slides, and surface ruptures would be much more severe than those caused by densification alone. Based on project description, soils beneath the site and ground water level at the site, the seismic induced settlement at the site is considered to very low. 13