3535 RIVERFRONT BLVD Geotech Report 2023-02-22

September 24, 2018 HWA Project No. 2015-061-21 4.3 GLOBAL SLOPE/EMBANKMENT EVALUATIONS We assumed soil strength parameters and ground water conditions based on our field exploration observations and laboratory test results. We checked the validity of these parameters by back calculating bank stability during the seismic loading imposed by the Nisqually Earthquake. Since no bank failures appear to have occurred as a consequence of that event, this represents a baseline seismic case for a condition wherein the factor of safety was at least one. Ground accelerations measured in Monroe were used for input, and we used 50% of this (i.e. 0.08g) for our pseudo-seismic analyses, as presented in applicable design guidelines including the International Building Code (IBC, 2015). The results for this moderate level earthquake relative to subsurface and slope conditions for cross-sections A-A', C-C' and D-D' (oriented as indicated on Figure 2C) are provided on Figures 6 through 8, respectively. As evident, the factors of safety for each bank condition represented exceed one and confirm that the actual effective soil parameters were and likely currently are at least the levels assumed, and could in fact be somewhat greater. We evaluated global slope stability conditions for the river bank area bordering the 3-Acre Park site using limit equilibrium slope stability methods. We analyzed three bank loading scenarios: static loading, pseudo-static earthquake loading, and post-liquefaction static loading. In the pseudo-static earthquake loading analysis, for the slope stability assessment of the river bank slopes, we applied a constant horizontal acceleration as appropriate for the IBC design level earthquake (0.35g), which is roughly equivalent to a PGA for an event with a 500-year return period. In the post-liquefaction static loading scenario, we considered the slopes under static conditions (no horizontal acceleration), but with a reduced shear strength for the liquefied sand layers, modeled as sand with a reduced friction angle (i.e. residual shear strength condition), to simulate conditions immediately after the earthquake shaking has stopped. Our limit equilibrium analyses were performed using the computer program SLIDE 5.0. Global factors of safety with respect to potential deep-seated failure surfaces were determined under the three load cases. The factor of safety computed is the ratio of the summation of the driving forces to the summation of the resisting forces. Where the factor of safety is less than 1.0, instability is predicted. For global slope stability design, minimum acceptable factors of safety under static loading conditions are commonly taken as 1.5 for slopes supporting structures or walls. For slopes adjacent to structures or for minor walls where slope instability would have a lesser effect in terms of safety considerations, the factor of safety may be taken as 1.3. Minimum acceptable factors of safety for the pseudo-static and post-liquefaction static cases are 1.1. We again performed analyses on each of the three cross-sections A-A', C-C' and D-D' based on II our understanding of the proposed grading(i.e. filling) of the site. Figures 9 through 15 • schematically represent each of the cross-sections along with the corresponding results of our stability analyses. As is evident from the figures, it appears that the combination of site grading and bank treatment proposed will provide for a static factor of safety against failure in the range of 1.2 to 1.3 (see Figures 9, 12, and 14). This level of stability is below that which is normally considered acceptable (FS = 1.5) for an engineered structure, but is believed to be appropriate in Final Geotechnical Report-3-Acre Park.docx 10 HWA GeoSciences Inc.