Laserfiche WebLink
January 3, 2025 <br />HWA Project No. 2021-159-21 <br />Geotechnical Engineering Report 19 HWA GEOSCIENCES INC. <br />Port Gardner Storage Facility <br />The slope stability factors of safety were evaluated with Bishop simplified, Janbu simplified, and <br />Spencer methods. <br />4.3.1 Static Condition Limit Equilibrium Analyses <br />The results of HWA’s LEA under static loading conditions along Geologic Profiles A-Aʹ and <br />B-Bʹ indicated factors of safety above 1.0 along the project site. However, the IBC requires a <br />minimum factor of safety of 1.5 under static conditions. Failure surfaces with factors of safety <br />less than 1.5 along Geologic Profiles A-A’ and B-B’ are presented on Figures F-1 and F-2 of <br />Appendix F. These failure surfaces extend about 16 to 34 feet behind the crest of the slope at the <br />existing seawall; existing structures and proposed improvements within 16 to 34 feet of the <br />seawall could experience modest lateral loading under static conditions due to slope creep. HWA <br />anticipates that existing structures and proposed improvements supported on deep foundations <br />can tolerate the imposed lateral loads due to slope creep and will not likely be damaged. <br />Construction activity (e.g., temporary loads and vibrations) within 16 to 34 feet of the seawall <br />may initiate slope movement or damage the seawall. HWA recommends that the contractor avoid <br />stockpiling material or mobilizing heavy equipment within 34 feet of the existing seawall. Where <br />construction activities require mobilizing equipment within the 34-foot seawall buffer zone, the <br />contractor should conduct a stability analysis based on the loads of the specific equipment to <br />assure that a minimum global stability factor of safety of 1.1 is achieved within the work zone. <br />Additionally, HWA recommends that the contractor monitor stability and movement of the <br />seawall while construction activity occurs within the 34-foot seawall buffer zone. <br />4.3.2 Pseudo-Static Condition Limit Equilibrium Analyses <br />Pseudo-static slope stability analyses model the anticipated earthquake loading as a constant <br />horizontal force applied to the soil mass. In HWA’s analyses, a horizontal seismic coefficient <br />equal to one-half of the peak ground acceleration (PGA) was implemented. Pseudo-static <br />stability along Geologic Profiles A-Aʹ and B-Bʹ was evaluated for the Cascadia design seismic <br />event (2,475-year return) as well as from smaller Nisqually type events (210-year return). The <br />results of the pseudo-static LEA with failure surfaces below 1.0 are presented on Figures F-3 <br />though F-6 in Appendix F. The analyses suggest that failure surfaces during a Cascadia design <br />seismic event extend as far back as approximately 330 to 390 feet behind the seawall, and failure <br />surfaces during a Nisqually type event extend approximately 75 to 240 feet behind the seawall. <br />Hence, slope stability mitigation methods as discussed per Section 4.4 of this report should be <br />considered for the proposed improvements within this area.