10521 19TH AVE SE Geotech Report 2022-10-25

Geotechnical Engineering Report Mixed-Use Everett 10521-19`h Ave. SE Everett, Washington March 23,2016 RN File No. 3109-001A Page 4 system and the low permeability layer. Our site explorations encountered dense to very dense glacial till at depths ranging from approximately 3 to 3.5 feet below the ground surface throughout the site. Therefore, in our opinion stormwater infiltration is not feasible for the site. Geologic Hazards Erosion Hazard: The erosion hazard criteria used for determination of affected areas includes soil type, slope gradient, vegetation cover, and groundwater conditions. The erosion sensitivity is related to vegetative cover and the specific surface soil types (group classification), which are related to the underlying geologic soil units. We reviewed the Web Soil Survey by the Natural Resources Conservation Service (NRCS) to determine the erosion hazard of the on-site soils. The site surface soils were classified using the SCS classification system as Alderwood-Urban land complex, 2 to 8 percent slopes (Unit 5). The corresponding geologic unit for these soils is till, which is in agreement with the soils encountered in our site explorations. The erosion hazard for the soil is listed as being moderate for the gently sloping conditions at the site. The elevator pit located within the planned structure is anticipated to extend below groundwater seepage elevations. A dewatering system should be expected, but will depend on finish elevations of the structure and elevator design. Seismic Hazard: It is our opinion based on our subsurface explorations that the Soil Profile in accordance with the 2012 international Building Code (IBC) is Site Class C with Seismic Design Category D. We used the US Geological Survey program "U.S. Seismic Design Maps Web Application." The design maps summary report for the 2012 IBC is included in this report as Appendix A. Additional seismic considerations include liquefaction potential and amplification of ground motions by soft soil deposits. The liquefaction potential is highest for loose sand with a high groundwater table. The underlying dense till soils are considered to have a very low potential for liquefaction and amplification of ground motion. Site Preparation and Grading The first step of site preparation should be to strip the vegetation, topsoil, or loose soils to expose medium dense or firmer native soils in pavement and building areas. The excavated material should be removed from the site, or stockpiled for later use as landscaping fill. The resulting subgrade should be compacted to a firm, non-yielding condition. Areas observed to pump or yield should be repaired prior to placing hard surfaces. The on-site glacial till likely to be exposed during construction is considered highly moisture sensitive, and the surface will disturb easily when wet. We expect these soils would be difficult, if not impossible, to compact to structural fill specifications in wet weather. We recommend that earthwork be conducted during the drier months. Additional expenses of wet weather or winter construction could include extra excavation and use of imported fill or rock spalls. During wet weather, alternative site preparation methods may be necessary. These methods may include utilizing a smooth-bucket trackhoe to complete site stripping and Robinson Noble, inc