2202 100TH ST SW Geotech Report 2022-10-25

Subsurface Exploration,Geologic Hazard,and Stephens Everett Property Preliminary Geotechnical Engineering Report Everett, Washington Preliminary Design Recommendations 15.0 SHALLOW INFILTRATION FEASIBILITY The results of our subsurface exploration indicate that shallow stormwater infiltration will be constrained by limited vertical and lateral extent of the permeable and unsaturated Vashon recessional outwash sediments or the low permeability of the silty Vashon lodgement till (hardpan) present near the surface. The fine-grained nature of the Vashon glacial till acts as hydraulically restrictive layers, perching shallow groundwater. Consequently, the infiltrated stormwater will largely move laterally in the very shallow subsurface through the sandier portions of recessional outwash soils similar to interflow, and potentially emerging at the ground surface or in shallow excavations for future site improvements, potentially becoming a nuisance. It is AESI's opinion that infiltration into the sediments observed in our explorations is not feasible and shallow stormwater infiltration best management practices (BMPs) are not recommended at the project site, due to the low-permeability soils, expected seasonal shallow groundwater, and the likelihood of lateral seepage at the subject site, as previously discussed. 16.0 PROJECT DESIGN AND CONSTRUCTION MONITORING Our report is preliminary since only a preliminary project concept was available at the time this report was prepared. We recommend that we be allowed to review project plans when they are completed and to revise the recommendations presented in this report as needed. We are also available to provide geotechnical engineering and monitoring services during construction. The integrity of the foundation system depends on proper site preparation and construction procedures. In addition, engineering decisions may have to be made in the field in the event that variations in subsurface conditions become apparent. Construction monitoring services are not part of our currently approved scope of work. March 29,2019 ASSOCIATED EARTH SCIENCES,INC. TG/ms-180567E001-2-Projects\20180567IKEIWP Page 19