1018 14TH ST 2023-02-09 - Laserfiche WebLink

SHANNON&WILSON.INC. groundwater table (assumed to be at elevation+11 feet) and elevation -12 feet would liquefy(see Section 4.1.1). ■ 200-year Design Ground Motion: We understand that the Port has requested that the new bulkhead be designed for a ground motion where soil liquefaction would be localized and considered "minimal." Based on our liquefaction analyses (see Section 4.1.1), this condition would correspond to a ground motion with a return period of about 200 years. This ground motion is similar to the seismic design criteria that was used in 2002 for the North Marina Bulkhead Project (14th Street bulkhead), connected to the north end of the project. The 2002 U.S. Geological Survey (USGS)National Seismic Hazard Data (consistent with the 2009 IBC)were used to develop the 200-year design site response spectrum. The USGS Earthquake Ground Motion Tool (USGS, 201 1) was used to retrieve the short period spectral acceleration, Ss, and spectral acceleration at the 1-second period, S1 for the design ground motions. The site soil response factors are shown in Figure 4. The site soil response factors are based on determination of the site classification type. Based on the subsurface explorations at the site, it is our opinion that the site is best classified as Site Class D, considering the minimal liquefaction, as discussed below. Additionally,the fundamental period of the structure is assumed to be less than 0.5 second. Therefore,the site classification can be determined with a non liquefied soil profile, in accordance with American Society of Civil Engineers (ASCE) 7-5 (ASCE, 2005). Figure 4 summarizes the factors for determining the code-based ground response spectrum for the 200-year design ground motion; the Site Class D IBC design ground motion has been included for reference. 4.1.1 Liquefaction One of the hazards associated with the occurrence of a seismic event at a site where cohesionless, granular soils are present in the subsurface is the potential for liquefaction and related lateral spreading. The liquefaction potential of a soil primarily depends on its gradation and density, and the intensity and duration of ground shaking. Based on the soil conditions encountered at the site,we identified the potentially liquefiable soils as including fill and alluvium below the groundwater table. Using the SPT N-values measured in the borings and the estimated percent fines content of the samples, we performed analyses to estimate the factor of safety (FS)against liquefaction for various earthquake ground motion levels. The analyses were performed using three accepted empirical methods for evaluating liquefaction potential: Youd and others (2001), Idriss and Boulanger(2006), and Cetin and others (2004). Each method uses laboratory testing, computer modeling, probabilistic techniques, and liquefaction data from 21-1-21761-003-R 1 fdocx/wp/cIp 21-1-21761-00 3 8