4320 THOMSON AVE GEOTECHNICAL ENGINEERING REPORT Geotech Report 2023-08-10

GeoTest Services, Inc. 4320 Thomson Avenue, Everett, WA Surface Conditions March 17, 2022 Project No. 22-0285 The subject property occupies approximately 0.22 acres on the western side of Thomson Avenue in the Forest Park area of Everett, Washington. The property is bordered by single family residences to the north, west, and south, and Thomson Avenue to the east. The property currently contains an existing, one-story, 600 square foot residence that was constructed in 1951. This house is situated at the base of an approximate 40-foot-tall steep slope. The lower portion of this slope is situated on the subject property, and the upper half lies on the adjacent western property. The average grade of this slope is 47 percent. Vegetation on the slope consists of brambles, grasses, bushes, and small trees. A mature coniferous tree exists on the slope above the existing residence. This tree is vertical and shows no apparent indications of recent movement. Subsurface Soil Conditions As part of our previous study, subsurface conditions were explored by advancing two test pits with a tracked excavator and two Dynamic Cone Penetrometer (DCP) explorations on February 27, 2019. The test pit explorations were each advanced to an approximate depth of 10 feet below ground surface (BGS), while the DCP explorations were advanced to approximate depths of 4.5 to 7.5 feet BGS. Approximate locations of these explorations are plotted on the Site Plan/Bare Earth Image drawing (Figure 2). DCP explorations included advancing a steel rod through the site soils using a 35-pound drop hammer. The number of blows required to drop the rod 4 inches (10 centimeters) was recorded and correlated with the density of granular soil deposits and the consistency of cohesive soils. Soils encountered in the test pits generally consisted of 12 inches of loose, silty, organic topsoil overlying native, medium -dense, silty sand (Weathered Advance Outwash) to an approximate depth of 3 to 3.5 feet BGS. Underlying these soils was very dense, gravelly sand (Advance Outwash) that was encountered to the maximum explored depth of the test pits. Both DCP explorations encountered refusal conditions in the Advance Outwash soils due to the increased gravel content and very dense nature of these soils. General Geologic Conditions Mapped geologic information was obtained from the Geologic map of the Everett 7.5-minute quadrangle, Snohomish County, Washington (Minard, 1985), published by the United States Geological Survey. According to Minard, general geologic conditions at the site are mapped as Advance Outwash (Qva) and Till (Qvt) of the Fraser glaciation. The approximate boundary between these two soil units is on or adjacent to the subject property, based on Minard's map. Till (locally referred to Vashon Till or Glacial Till) is a nonsorted mix of all grain sizes deposited directly by the advancing glacier. Advance Outwash is a well stratified sand to gravel deposited 2