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January 3, 2025 <br />HWA Project No. 2021-159-21 <br />Geotechnical Engineering Report 18 HWA GEOSCIENCES INC. <br />Port Gardner Storage Facility <br />strength.” This reduction in shear strength can result in liquefaction-induced slope instability. <br />Liquefaction-induced slope failures can occur as either a lateral spreading event or as a flow <br />failure. <br />Liquefaction-induced lateral spreading occurs as the shear strength of liquefiable soils decrease <br />during seismic shaking but does not decrease to the point that a complete flow failure would <br />occur. Lateral spreading occurs cyclically when the horizontal ground accelerations combine <br />with gravity to create driving forces that temporarily exceed the available strength of the soil <br />mass. This is a type of failure known as cyclic mobility. The result of a lateral spreading failure <br />is horizontal movement of the partially liquefied soils and any overlying crust of non-liquefied <br />soils. Displacements associated with lateral spreading are only generally quantifiable and on the <br />order of several feet. The actual magnitude of displacement depends on the site geometry, soil <br />characteristics, and earthquake loading. <br />In contrast, liquefaction-induced flow failures result when the residual strength of the liquefied <br />mass is not sufficient to withstand the static stresses that existed before the earthquake. Upon <br />initiation of liquefaction-induced flow failure, the liquefied soil behaves like a debris flow, <br />characterized by very large displacements. Flow failures involve horizontal and vertical <br />movements of the liquefied soils and any overlying crust of non-liquefied soils. The chaotic <br />nature of flow failures is such that estimation of the magnitude of displacement is not reasonable. <br />Limit Equilibrium Analyses (LEA), as described in Section 4.3, were conducted to evaluate <br />post-liquefaction instability due to the reduction in shear strength of the liquefiable soils at the <br />project site. <br />4.3 LIMIT EQUILIBRIUM GLOBAL SLOPE STABILITY EVALUATION <br />The stability of the project site, including the existing structures and the proposed improvements <br />under static, pseudo-static, and post-liquefaction conditions were evaluated using limit <br />equilibrium methods utilizing the computer program SLIDE 2018 8.032 (Rocscience, 2020). <br />Limit equilibrium methods consider force (or moment) equilibrium along potential failure <br />surfaces. Results are provided in terms of a factor of safety, which is computed as the ratio of the <br />summation of the resisting forces to the summation of the driving forces. Where the factor of <br />safety is less than 1.0, instability is predicted. With limit equilibrium, the shear strength available <br />is assumed to mobilize at the same rate at all points along the failure surface. As a result, the <br />factor of safety is constant over the entire failure surface. <br />The models used in the slope stability analyses reflect conservative interpretations of Geologic <br />Profiles A-Aʹ and B-Bʹ (Figures 3A and 3B).