Item 18 - Appeal Application Nos. 2023-02 and 2023-03 for Cabrillo Town Center project

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Our evaluation utilized the information collected from the excavations and laboratory test results, along with utilizing the more recent studies as indicated in SP 117A by Bray and Sancio, 2006 as a screening tool to determine if the encountered fine grained soils (clays) are susceptible to liquefaction and analyzed as such. Our evaluation included performing grain size distribution, Atterberg limit, and moisture content testing on representative fine-grained layers (i.e. clayey/silty Sands) encountered within the geotechnical borings excavated on -site. The laboratory test results indicated that the encountered fine-grained layers have a plasticity index of 5, 7, 10 and 12, and moisture contents less than 85 percent of the liquid limit and is considered as being susceptible to liquefaction. The liquefaction analysis was performed using the LiquefyPro program with a user provided factor of safety of 1.0. The liquefaction analysis was performed considering a minimum 5- foot remedial removal and recompaction and the existing condition below with the highest historic groundwater elevation at a depth of 40 feet below the ground surface. The liquefaction analysis was performed using the following input data: • Groundwater at a depth of 40 feet below the ground surface during seismic event, and boring groundwater at elevations ranging 51.5 feet. • A Peak Horizontal Ground Acceleration (PGAM) of 0.593g for a Design Earthquake Magnitude of 7.71. • Fines content as determined from laboratory testing during this investigation. • The hammer used for determining blow -counts for both the ring and SPT sampling was an auto -trip hammer with a 140 lb weight and a 30-inch drop. Therefore, based on the type of hammer used, an energy correction factor (CE) of 1.3 is considered acceptable for use in the analysis. Based on our site evaluation, liquefaction analysis, and our professional opinion, there is a potential for specific layers to liquefy between a depth of 44 to 51.5 feet below the existing site grades. During a strong seismic event, seismically induced settlement can occur within loose to moderately dense, dry or saturated granular soil. Settlement caused by ground shaking is often non -uniformly distributed, which can result in differential settlement. Based on blow counts and in -situ densities, the native alluvial soils between a depth of 5 to 40 feet below the existing ground surface are susceptible to seismically induced dry sand settlements and were evaluated in our analysis. Based on the results of the liquefaction/seismically induced settlement analysis, we estimated the amount of total liquefaction -induced and dry sand settlement possible for the design conditions is approximately 3-inches or less, and a differential settlement of approximately 1.5-inches. We estimated these settlements based on the procedures proposed by Tokimatsu and Seed (1987). The graphical output of our liquefaction analysis which also shows the graphical output of seismically induced saturated and dry sand settlement is included in Appendix D. Project i y ouncil 18 — 823 1 3 , 2021