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Phelan Development Holly Master Plan - Buildings H & 1 15188.000.006 Geotechnical Exploration recommendations does not constitute any kind of guarantee that significant structural damage would not occur in the event of a maximum magnitude earthquake; however, it is reasonable to expect that a well-designed and well-constructed structure will not collapse or cause loss of life in a major earthquake (SEAOC, 1996). 3.3.3 Liquefaction Soil liquefaction results from loss of strength during cyclic loading, such as imposed by earthquakes. Soil most susceptible to liquefaction is clean, loose, saturated, uniformly graded, fine-grained sand. Empirical evidence indicates that loose to medium-dense gravel, silty sand, and low- to moderate-plasticity silt and clay may be susceptible to liquefaction. In addition, sensitive high-plasticity soil may be susceptible to significant strength loss (cyclic softening) as a result of significant cyclic loading. The results of our liquefaction analysis are presented in Appendix C. We summarize the results of our analysis below. 3.3.3.1 Liquefaction-Induced Settlement We evaluated the liquefaction potential of the site soil with CPT data using methods published by Robertson (2009). The Cyclic Stress Ratio (CSR)was estimated for a Peak Ground Acceleration (PGAM)value of 0.53g, which is the mapped Maximum Considered Earthquake (MCE) Geometric Mean Peak Ground Acceleration based on the 2016 ASCE 7 Standard for a Site Class D. We also used a moment magnitude (MW) of 6.9 in our analysis, which corresponds to the maximum magnitude for the Great Valley 7 fault based on the United States Geological Survey (USGS) national seismic hazard maps. We assumed a groundwater depth of 7 feet in our analysis based on the groundwater depths discussed in Section 2.6. The results of our liquefaction analyses indicate interbedded sand layers and low-plasticity silt and clay layers below the groundwater depth of approximately 7 feet are potentially liquefiable and susceptible to cyclic softening. Cyclic softening potential was determined using the Bray and Sancio (2006) criteria, which categorizes fine-grained soil into susceptibility categories based on the relationship between plasticity index and the ratio of moisture content and liquid limit. A sample of silty clay with sand was retrieved from test pits in the vicinity of Building H at a depth of 73/4 feet below existing grade and was tested for these parameters. Based on the laboratory results, the soil was categorized as "susceptible" to cyclic softening; as such, settlement due to cyclic softening of these layers was included in the analysis. This material was encountered in the vicinity of Building H. In addition, we reviewed bearing capacity of this layer for footings greater than 7 feet in width and acceptable factors of safety are estimated. Layers of liquefiable soil range in thickness from '/2 to 3'/2 feet in thickness. Consequences of liquefaction include surface disruption, settlement, and downdrag on deep foundations. Given the relative thickness of non-liquefiable surface soil and potentially liquefiable soil, it is our opinion that sufficient confinement is available to resist surface disruption. Based on our analysis, we estimate up to approximately 1 inch of potential total settlement from liquefaction during a design-level seismic event. We provide recommended differential settlements to be used for design in Section 6. 3.3.4 Densification Due to Earthquake Shaking Densification of loose granular soil above and below the groundwater level can cause settlement due to earthquake-induced vibrations. Based on the encountered clayey soil within the upper 5 feet, we believe the potential for dry sand densification is low to negligible at the subject site. —NGE0 Page 19 December 13, 2021 Expect Exceltence