1920 GRAND AVE Geotech Report 2022-10-25

Mr. Bill Messner ES-6132 October 5, 2018 Page 3 The referenced geologic map resource identifies Vashon advance outwash (Qva)and transitional beds (Qtb) across the site and slope. Vashon advance outwash is typically a well-sorted sand and gravel deposit, which was formed by streams issuing from the advancing ice sheet. The Vashon advance outwash commonly grades upward into glacial till (mapped east of Grand Avenue). Transitional beds are largely clay, silt, and very fine to fine sand, deposited in still or slowly moving water. In general, the referenced WSS resource identifies Alderwood series soils, either disturbed or undisturbed, across the site and surrounding areas. The Alderwood series was formed in glacial till plains. Based on the conditions reported at the boring locations, native soils on site appear consistent with the geologic setting outlined in this section. Groundwater was not reported at the boring locations during the November 2000 ECI fieldwork. It should be noted seepage rates and elevations fluctuate depending on many factors, including precipitation duration and intensity, the time of year, and soil conditions. In general, groundwater flow rates are higher during the winter, spring, and early summer months. Geologically Hazardous Areas We reviewed both the referenced City maps and EMC Title 19 to determine the presence of geologically hazardous areas (GHAs) on site. Based on our review, the following GHAs are present on site, all associated with the slope: high to very high and/or severe erosion hazard, medium to very high landslide hazard, and seismic hazard. Based on the observed sloughing and/or eroding, slope instability is shallow and can be remediated by the proposed system. Both the soldier pile and tieback installations will require isolated intrusions into the slope, which will be grouted, thereby increasing local slope stability. The combination of soldier piles and tiebacks will both protect the slope area from future weathering and erosion and stabilize the upper loose to medium dense soils. Considering these benefits, it is our opinion the proposed construction is appropriate from a geotechnical standpoint and will reduce the overall hazard associated with the mapped GHAs. Cantilever and Single-Tieback Soldier Pile Walls The shoring system should be designed to resist lateral soil pressure based on an active earth pressure condition given the proposed excavation depth. The following parameters may be used for shoring design: • Active earth pressure (level backfill) 35 pcf • Traffic and existing wall surcharge Negligible* • Passive earth pressure** 175 pcf(neglect upper three feet) * Existing pile-supported wall and related backfill is assumed negligible with respect to surcharge ** Passive earth pressure value may be applied over two-and-one-half pile diameters Earth Solutions NW,LLC