1815 HOLBROOK AVE Geotech Report 2025-03-06

June 9,2022 Page 2 of i5 Geotechnical Evaluation Vashon Advance Outwash underlies the till and consists of fine to medium grained sands. These soils are also dense below a weathered zone. Transitional Beds underlie the outwash and include silt and clay which is relatively stiff to hard below the weathered zone. Groundwater often becomes perched between the outwash and underlying Transitional Beds. Soil & Groundwater Conditions The geotechnical field investigation program was completed on April 6, 2022 and included three test pits. We returned on May 31, 2022 and drilled a hollow stem auger boring with a limited access drill rig to determine the soil conditions at greater depth as part of infiltration feasibility analysis. Disturbed soil samples were obtained during drilling by using the Standard Penetration Test (SPT) as described in ASTM D-1586. The Standard Penetration Test and sampling method consists of driving a standard 2-inch outside-diameter, split barrel sampler into the subsoil with a i4o-pound hammer free falling a vertical distance of 30 inches. The summation of hammer- blows required to drive the sampler the final 12-inches of an i8-inch sample interval is defined as the Standard Penetration Resistance, or N-value. The blow count is presented graphically on the boring logs in this appendix. The resistance, or "N" value, provides a measure of the relative density of granular soils or of the relative consistency of cohesive soils. The boring encountered about 12 inches of topsoil and grass underlain by approximately 20 feet of medium dense, fine to medium grained sand trace gravel (Advance Outwash). These deposits were underlain by a 1.5 feet thick layer of medium dense, silty-fine to medium grained sand trace gravel (Advance Outwash — finer grained interbed). This layer was underlain by dense to very dense, fine to medium grained sand trace gravel (Advance Outwash), which continued to the termination depth of the boring. The test pits encountered approximately 6 inches of vegetation and topsoil underlain by o to 1.5 feet of loose to medium dense, silty-fine to medium grained sand with debris (Fill). These deposits were underlain by medium dense,fine to medium grained sand trace gravel trace to with silt(Advance Outwash),which continued to the termination depths of the test pits. We have included several test pit logs performed by ESNW in a previous investigation at this property. They generally encountered soils similar to those encountered in our test pits. Groundwater was not encountered in the test pits. Groundwater was observed at 20 and 26 feet below grade in B-i during drilling. The final groundwater depth was 24 feet following completion of the boring. Water table elevations often fluctuate over time. The groundwater level will depend on a variety of factors that may include seasonal precipitation, irrigation, land use, climatic conditions and soil permeability. Water levels at the time of the field investigation may be different from those encountered during the construction phase of the project. Geologic Hazards Most critical area ordinances designate slopes with magnitudes greater than about 4o percent and vertical relief of at least io feet as potentially geologically hazardous (steep slope/landslide hazards). www.cobaltgeo.com (206)331-1097