7505 SEAWAY BLVD Geotech Report 2022-10-25

November 11, 2016 HWA Project No. 2014-117-21 temporarily lose strength and behave as a liquid in response to moderate to strong earthquake shaking. Primary factors controlling the development of liquefaction include the intensity and duration of strong ground motions, the characteristics of subsurface soils, in-situ stress conditions and the depth to ground water. Based on the very dense nature of the soils encountered along the project alignment and the absence of ground water across the site, liquefaction will not be a design consideration for this project. 4.3 PLATFORM AND CENTER ISLAND CONSTRUCTION Project plans provided by Perteet, Inc., indicate a 6.5- to 24-foot wide pedestrian platform and sidewalk section, constructed with PCC, will be situated around the perimeter of the transit center. The plans also indicate a 150-foot wide triangular-shaped center island, constructed with PCC, will be positioned in the middle of the transit center. A single-story maintenance building will be located on the south side of the center island. 4.3.1 Subgrade Preparation Project plans indicate a significant portion of the existing fill mound currently occupying the eastern half of the property will be removed prior to construction of the center island, and platform sections. We recommend over excavating and replacing all undocumented fill from below the entire footprint of these structures. As grading plans indicate a significant portion of the mound is already scheduled to be removed prior to construction,we do not anticipate a significant amount of additional over-excavation will be required. Project plans indicate structures on the western portion of the transit center will be constructed above current grade. This portion of the site has not been covered by the fill mound, and the soils in this area generally consist of surface deposits of weathered glacial till, or medium dense to dense fill underlain by weathered glacial till. As previously mentioned in this report, a remnant HMA pavement structure was encountered at depths of 1.5 feet and 0.8 feet in hand- borings HH-1 and HH-2, respectively. We recommend excavating the remnant HMA pavement structure and a portion of the fill material below the pavement structure. The exposed subgrade should be thoroughly proof-rolled to identify poor subgrade areas after excavation. All organic material should be removed from beneath the entire footprint of the platforms and center island areas. Loose or soft soil, defined as being penetrable more than 1 foot with a 1/2 inch diameter rod pushed in under a 150 lb load(T-probe pushed in by hand), should be removed and replaced with"Structural Backfill" or be suitably compacted. Additionally,undocumented fill, saturated soil, soil significantly disturbed by construction methods, and otherwise unsuitable subgrade material may also need to be removed and replaced with"Structural Backfill". Where Crushed Surfacing Base Course (CSBC) and Crushed Surfacing Top Course (CSTC) are used, the materials should meet the gradation requirements specified in Section 9-03.9(3) of the WSDOT Standard Specifications (WSDOT, 2016). 2014-117 Final Geotechnical Report 8 HWA GEOSCIENCES INC.