19E - SANTA ANA GRADE SEPARATION PROJECT

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Santa Ana Boulevard Grade Separation September 13, 2011 Page 2 General Subsurface Conditions According to the attached As -Built LOTB sheets for the nearby structures, a total of twelve auger borings were drilled between March and May, 1989. The deepest boring was advanced to about elevation +50 feet. Existing grade at the borehole locations at the time of the investigation ranged between elevations +130 feet and +168 feet. The existing grade at the subject site is about elevation +145 feet. At the nearby bridge site, the surficial material is embankment till consisting predominantly of medium dense to dense silty sand with few interbedded very stiff to hard sandy silt. Thickness of this embankment till varies from 10 to 25 feet. The embankment till is underlain by about 40 feet of native deposits composed predominantly of loose to very dense silty sand and medium stiff to hard sandy silt with occasional silty clay and clayey silt interbeds. Below about elevation +105 feet, the stratigraphy included very dense silty sand, sand with gravel and cobbles, sandy gravel, and clayey gravel with some interbedded loose to medium dense silty sand and sand layers down to about elevation +80 feet. Very dense silty sand, sand with gravel and cobbles, sandy gravel, and clayey gravel layers were encountered below about elevation +80 feet down to the depth explored. Groundwater was not encountered in any of the borings that were drilled down to the deepest elevation of about +50 feet. Preliminary Seismic Recommendations It is our understanding that the Santa Ana Boulevard GS will be constructed using the American Railway Engineering and Maintenance -of -Way Association ( AREMA) guidelines ( AREMA, 2009). We performed a site - specific probabilistic ground motion analysis using FRISKSP (Blake, 2004 & 2000) computer program for the bridge site. The probabilistic analysis was performed using four attenuation equations published by Abrahamson and Silva (1997), Boore, et al. (1997), Campbell (1997), and Sadigh et al. (1997). These peak horizontal ground acceleration (PGA) values were obtained by averaging the results of the above- referenced attenuation equations and are provided in Table 1. Table 1. Seismic Design Parameters Ground Motion Level (Frequency) Return Period PGA Site (g) Coefficient, S 1 (occasional) 100 Years 0.214 2 (rare) 500 Years 0.351 1.5 3 (very rare) 2,400 Years 0.524 The structural engineer can use the data provided in Table 1 to develop the site - specific response spectrum following the procedure outlined in Section 1.4 of Chapter 9 of AREMA (2009). =Y . oics Inc. -- - G Mee U4ae119,1aanf1g