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353 Main Street Apartments Redwood City, California May 1, 2018 lOriginol Report Dated November L8,20L6) Liquefaction Soil liquefaction is a phenomenon where saturated granular soils undergo a substantial loss of strength due to increased pore water pressure resulting from cyclic stress applications induced by earthquakes or other vibrations. ln this process, the soil acquires mobility sufficient to permit both vertical and horizontal movements, which may result in significant deformations. Soils most susceptible to liquefaction are loose, uniformly graded, fine-grained sands. ln addition, recent literature indicates that fine grained soils may also be susceptible to liquefaction or cyclic strain softening. Examples of highly susceptible fine-grained soil include "nonplastic silts and clayey silts of low plasticity (Pl<12) at high water content to liquid limit ratios (w./LL>0.85)." Examples of soils moderately susceptible to liquefaction include "clayey silts and silty clays of moderate plasticity (12<Pl<18) at wc/LL>0.80" (Bray and Sancio,2006l. lt is generally acknowledged that liquefaction will not affect surface improvements if these deposits are located at a depth greater than 50 feet below the ground surface. ln the deeper deposits, the greater overburden pressure is sufficient to prevent liquefaction effects from occurring. SPT Based Liquefaction Methodolosv The liquefaction analysis at the subject site was evaluated using the data collected from Boring 84 and using the methodology suggested in the 1996 and L998 National Center for Earthquake Engineering Research workshops (NCEER) on evaluation of liquefaction resistance of soils (Youd & ldriss, editors), and the 1.999 Southern California Earthquake Center (SCEC) "Recommended Procedures for lmplementation of DMG Special Publication 117". A brief outline about the variables involved in the liquefaction evaluation process is described below. One of the steps of liquefaction potential evaluation consists of normalizing the SPT blow count to the effective overburden stress of 100 kilopascal (or L.Q44 tons per square foot). This is denoted as Nr(oo) and is found through the following formula: Nr(oo) = N. CN¡C¡CeCnCs where: Nm = measured standard penetration resistance Cru = factor to normalize Nmto a common reference effective over-burden stress Ce = hammer energy ratio (ER) correction factor Ce = borehole diameter correction factor Cn = rod length correction factor Cs =correction factor for samplers with or without liners 7301842-001 1610-038.SER.REV2 7.B. - Page 65