3400 RIVERFRONT BLVD Geotech Report 2022-10-25

S • • friction for strength, is susceptible to liquefaction until the excess pore pressures can dissipate. 5 Sand boils and flows observed at the ground surface after an earthquake are the result of excess pore pressures dissipating upwards, carrying soil particles with the draining water. In general, loose, saturated sand soil with low silt and clay content is the most susceptible to liquefaction. S Low plasticity, silty sand and silt may be moderately susceptible to liquefaction under relatively higher levels of ground shaking. 40 Two of the required inputs for a liquefaction analysis are the earthquake PGA and magnitude values that represent the overall seismic hazard at the site. We used the PGA value of 0.47 g • provided by USGS seismic design maps (USGS, 2014) for the site. We used an earthquake • magnitude of 7.07, which the USGS interactive deaggregation tool (USGS, 2013) indicates is the i mean magnitude from all seismic sources that contribute to the earthquake hazard at this site for an earthquake with a probability of exceedance of 2 percent in 50 years. • • The other main parameter used in liquefaction evaluations is the groundwater level at the site. 5 We modeled the groundwater as being present at an elevation of 8 feet, which corresponds to the average depth at which water was observed in the borings and test pits at the site. We performed a liquefaction evaluation utilizing the simplified procedures recommended by • Idriss and Boulanger(2008) and updated in Boulanger and Idriss (2014). We evaluated • liquefaction using SPT data from the site. Our analyses indicate that there is a 20-to 30-foot- thick i layer of sand and low plasticity silt across most of the site that will likely liquefy during a design-level earthquake. Less liquefaction will occur along the western edge of the site due to the presence of medium plasticity silt. • • 5.3.2 Liquefaction-Induced Settlement We evaluated the potential for liquefaction-induced settlement to occur at the site as the soil • consolidates due to the seismic shaking. We used the procedures recommended by Ishihara and • Yoshimine (1992) and Tokimatsu and Seed (1987)to estimate the amount of liquefaction- • induced settlement. The results of our analysis indicate that 5 to 8 inches of liquefaction- induced settlement will occur across most of the site. The western edge of the site will likely • experience less than 2 inches of liquefaction-induced settlement due to the presence of medium • plasticity silt. IP As part of our analysis, we estimated the total settlement at numerous boring locations and then evaluated the magnitude of differential settlement between adjacent boring locations. Based on the distance between the borings, we anticipate that the average differential settlement over the • maximum length of a typical townhome (assumed to be 100 feet)will be less than 4 inches. The • structural engineer should determine if the magnitude of seismic settlement is within acceptable • limits. • 5.3.3 Non-Liquefaction Settlement • We also evaluated the potential for seismic settlement of the non-saturated sand above the • liquefied zone using the procedures recommended by Tokimatsu and Seed (1987). This • • G EODESIGN& 10 Polygon-128-01:091615 • •