04 ATTACHMENT 6 - Laserfiche WebLink

Project No. E9164-04-0 2 - 4 - April 15, 2020 which saturated cohesionless soils are subject to a temporary loss of shear strength due to pore pressure buildup under the cyclic shear stresses associated with intense earthquakes. Primary factors that trigger liquefaction are: moderate to strong ground shaking (seismic source), relatively clean, loose granular soils (primarily poorly graded sands and silty sands), and saturated soil conditions (shallow groundwater). Due to the increasing overburden pressure with depth, liquefaction of granular soils is generally limited to the upper 50 feet of a soil profile. We evaluated the potential for liquefaction and resultant settlements at the site using the soil boring data and the methodology of Youd et. al. (2001) and Idriss and Boulanger (2006 and 2008). Our evaluation incorporated an earthquake moment magnitude (Mw) of 6.9 and a design groundwater depth of 15 feet. The groundwater depth used in our analysis was assigned based on the soils conditions encountered in soil borings at the site and groundwater conditions reported in previous borings by others. Our recent soil borings did not encountered groundwater. It is our opinion the groundwater previously encountered in borings by others is likely seasonal or intermittent. As such, our evaluation of liquefaction potential is likely conservative. Based on USGS seismic design criteria for 2019 CBC, a ground motion/Peak Ground Acceleration (PGA) of 0.908 g was used in our analysis Our liquefaction analysis identified potentially liquefiable sandy layers below the design ground water depth of 15 feet. The liquefiable layers are located between approximately between depths of about 15 and 20 feet below the existing grade and generally exist between overlying clayey alluvium and Tertiary age formational materials below. Consequences of liquefaction can include ground surface settlement, ground loss (sand boils) and lateral slope displacements (lateral spreading). Dublin Creek is located north of the proposed development. Based on our investigation, the liquefiable layers apparently discontinuous in nature towards the creek, therefore, in our opinion, the potential for lateral spreading is considered low. For liquefaction-induced sand boils or fissures to occur, pore water pressure induced within liquefied strata must exert enough force to break through overlying, non-liquefiable layers. Based on methodology recommended by Youd and Garris (1995), which modified and advanced original research by Ishihara (1985), a capping layer of non-liquefiable soil can prevent the occurrence of sand boils and fissures. In our opinion, due to the depth to the liquefiable layer, the potential for ground loss due to sand boils or fissures in a seismic event is considered low. The likely consequence of potential liquefaction at the site is ground surface settlement. Our analysis indicates that, if liquefaction were to occur, total foundation settlements less than 1 inch may result. SP117A indicates that localized differential settlements of up to ⅔ of the total estimated settlements should be assumed for design. We recommend foundations should be designed to accommodate approximately ¾ inch of differential seismic settlement across a horizontal distance of 50 feet. 4.5 Landslides The site is not mapped in a State of California Seismic Hazard Zone for seismically induced landslides. We did not observe overt evidence of global instability in slopes at the site and the slopes within the Dublin Creek drainage are heavily vegetated with mature trees. We consider the potential for landslides impacting proposed structures at the site to be generally low. 4.6 Tsunamis and Seiches The site is not located within a coastal area. Therefore, tsunamis (seismic sea waves) are not considered a significant hazard at the site.