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 111 I The project area is located in a bend of the Snohomish River, which comes within 100 feet of the northern terminus of the proposed water and storm lines. Extensive ponding was observed at surface elevations along the entire length of the proposed roadway extension,and the water table across the site often rises to within 7 feet of the surface. No other hydrologic features were observed onsite such as seeps or springs. 3.2 Soil Conditions We performed six auger boring explorations as part of this geotechnical study;one near the existing endpoint of Railway Avenue at the entrance gate to the parcel owned by Penske Truck Leasing,and five additional borings spaced roughly 400 feet apart along the proposed roadway and storm/water line extensions to the north. Our subsurface explorations encountered relatively consistent subgrade conditions along the length of the project area,consisting of a surface mantle of vegetative groundcover and gravel or quarry spalls, overlying alluvial sand and silt with abundant organics. The thickness of surface vegetation tends to decrease toward the north,and the volume of organics in the borings also decreased in the same direction. The organics encountered were typically wood chips or "hog fuel", often seen as thin beds within sandy or silty units. Occasionally, very thick pieces of timber were encountered at depth, possibly buried logs or branches. Boreholes B-1, B-2, and B-4 all encountered very similar units of brown/gray silt at depths between 10 and 11 feet. Soils near the surface were typically medium dense to very dense sand or gravel, transitioning to very loose or soft finer-grained deposits with depth. In the Geologic Map of the Everett 7.5-Minute Quadrangle, Snohomish County, Washington,published by the United States Department of the Interior(USDI) (1985), the project area is mapped as Qyal, or younger alluvium deposited during the Holocene. These deposits lie in and along present streams and are partly subject to seasonal flooding. The sediments consist mostly of unconsolidated, stratified,gray to brown clay,silt,and very fine sand to fine sand,with abundant organic material. The younger alluvium can be anywhere from 1 meter to possibly 20 meters thick and is mostly poorly drained. Peat deposits and buried logs and stumps are common in the flood plain of the Snohomish River. Our field observations generally conform with the site classifications prepared by the USDI. The enclosed exploration logs (Appendix A) provide a detailed description of the soil strata encountered in our subsurface explorations. 3.3 Groundwater Conditions We encountered groundwater seepage in each of our explorations,at depths between 7 and 10 feet. Given that our explorations were performed during what is generally considered the "rainy season",we do not anticipate that groundwater levels will rise higher than that which we observed. Regional groundwater levels will fluctuate with localized precipitation and geological conditions. Migizi Group, Inc. Page 4 of 13 .