3800 RIVERFRONT BLVD Geotech Report 2023-05-04

March 26, 2019 HWA Project No. 2015-061-21 Task 600-7 HWA has no evidence of such conditions within the refuse and HWA does not believe such conditions to be likely. 4.2.3 Liquefaction Induced Settlement Loose sand deposits tend to densify when they are subject to earthquake shaking. For saturated sand deposits, excess pore water pressure builds up during the earthquake excitation, leading to loss of strength or liquefaction. After the shaking stops, excess pore water pressures dissipate toward a zone where water pressure is relatively lower, usually the ground surface. The dissipation is accompanied by a reconsolidation of the loose sand (Ishihara and Yoshimine, 1992). The reconsolidation is manifested at the ground surface as vertical settlement, usually termed as liquefaction-induced settlement or seismic settlement. Some reconsolidation of the pockets of potentially liquefiable soils is expected to occur. However, given the relatively thin layers of liquefiable soils and their depth below the ground surface, we would expect that this reconsolidation would attenuate significantly by the time the settlement reaches the ground surface, resulting in very little noticeable settlement along the roadway. Given the magnitude of long-term consolidation settlement expected along the roadway, we do not expect that liquefaction induced ground settlement will be a design consideration for the roadway. 4.2.4 Liquefaction Induced Slope Failures Liquefaction induced slope failures can either occur as a lateral spreading or as a flow failure. Liquefaction induced lateral spreading occurs as the shear strength of liquefiable soils decrease during seismic shaking but do not decrease to the point that a complete flow failure would occur. Lateral spreading occurs cyclically when the horizontal ground accelerations combine with gravity to create driving forces which temporarily exceed the available strength of the soil mass. This is a type of failure known as cyclic mobility. The result of a lateral spreading failure is horizontal movement of the liquefied soils and any overlying crust of non-liquefied soils. Displacements associated with lateral spreading are generally quantifiable and on the order of a few meters. In contrast, liquefaction induced flow failures result when the residual strength of the liquefied mass is not sufficient to withstand the static stresses that existed before the earthquake. Upon initiation of liquefaction induced flow failure, the liquefied soil behaves like a debris flow, characterized by very large displacements. Flow failures involve horizontal and vertical movements of the liquefied soils and any overlying crust of non-liquefied soils. The chaotic nature of flow failures is such that estimation of the magnitude of displacement is not reasonable. To evaluate the potential for lateral spreading to affect the proposed roadway, HWA has completed the lateral spreading analysis at the location of cross sections C-C' and D-D'. We believe that these two cross sections represent the most critical geometry with respect to lateral spreading, as they are closest to the river. These analyses suggest that flow sliding and lateral spreading are expected to occur within the soils directly adjacent to the river bank. These seismically induced failures are expected to result in damage to the riverbank and trail system to Geotechnical Report Revision 3 26 2019.doc 10 HWA GEOSCIENCES INC.