2320 RAILWAY AVE INTERWEST CONSTRUCTION INC Geotech Report 2023-02-17

Interwest Construction Co.-Railway Avenue Improvements,Everett,WA Feb. 14,2019/Revised Feb. 7,2020 Geotechnical Engineering Report P1486-T18 3.4 Infiltration Conditions As indicated in the Soil Conditions section of this report, the project area is generally underlain by silty alluvial soils;with groundwater seepage typically being encountered at depths of 7 to 10 feet. Given the hydrogeologic conditions present within this region,and the proximity to the Snohomish • River,infiltration would likely not be feasible. Site produced stormwater should be diverted to the planned 54-inch storm line referenced in the Site and Project Description section of this report. • 3.5 Seismic Conditions Based on our analysis of subsurface exploration logs and our review of published geologic maps, we interpret the onsite soil conditions to generally correspond with site class E, as defined by Table 20.3-1 in ASCE 7, per the 2018 International Building Code(IBC). Using 2018 IBC information on the USGS Design Summary Report website, Risk Category I/II/III seismic parameters for the site are as follows: Ss= 1.261 g Sys = 1.135 g SDs=0.756 g S�=0.480 g S � =0.153 g SD1=0.768 g Using the 2018 IBC information, MCER Response Spectrum Graph on the USGS Design Summary Report website, Risk Category I/II/III, Sa at a period of 0.2 seconds is 1.14 g and Sa at a period of 1.0 seconds is 1.15 g. The Design Response Spectrum Graph from the same website,using the same IBC information and Risk Category, Sa at a period of 0.2 seconds is 0.76 g and Sa at a period of 1.0 seconds is 0.77 g. 3_6 Liquefaction Potential ' Liquefaction is a sudden increase in pore water pressure and a sudden loss of soil shear strength caused by shear strains, as could result from an earthquake. Research has shown that saturated, loose, fine to medium sands with a fines(silt and clay)content less than about 20 percent are most susceptible to liquefaction. Given the presence of extensive saturated, poorly consolidated soils underlying the proposed improvement area, we interpret site soils as having a high potential for liquefaction during a large-scale seismic event, which could lead to post-construction settlement. 4.0 CONCLUSIONS AND RECOMMENDATIONS Improvement plans for this project include the extension of Railway Avenue approximately 2,000 feet north of its existing terminus. A 12-inch water line and a 54-inch storm line will also be constructed as part of the improvements. We offer these recommendations: • Feasibility: Based on our field explorations, research and analyses, the proposed structures appear feasible from a geotechnical standpoint. • Excavation Considerations: General trench excavation, pipe bedding, and trench backfilling should be completed using the general procedures described in WSDOT Standard Specifications, Section 7-17, or other suitable procedures specified by the project civil engineer. As indicated on the exploration logs, the site soils are ' Migizi Group, Inc. Page 5 of 13