7000 HARDESON RD Geotech Report 2022-10-25

Geotechnical Engineering Report Community Transit Merrill Creek Operating Base 7100 Hardeson Road Everett, Washington February 28, 2018 RN File No. 3224-002A Page 2 We first visited the site on June 21, 2017 and observed several patches in the asphalt. Seepage was observed around some of the patches and in other areas where the asphalt was cracked. The seepage appears to be concentrated south and east of the administrative building as shown in the Site Plan in Figure 2. These observations were conducted after an extended period of dry weather, Geology Most of the Puget Sound Region was affected by past intrusion of continental glaciation. The last period of glaciation, the Vashon Stade of the Fraser Glaciation, ended approximately 14,000 years ago. Many of the geomorphic features seen today are a result of scouring and overriding by glacial ice. During the Vashon Stade, areas of the Puget Sound region were overridden by over 3,000 feet of ice, Soil layers overridden by the ice sheet were compacted to a much greater extent than those that were not. Part of a typical glacial sequence within the area of the site includes the following soil deposits from newest to oldest: Artificial Fill (af)- Fill material is often locally placed by human activities, consistency will depend on the source of the fill. The thickness and expanse of this material will be dependent on the extent of fill required to grade land to the desired elevations. Density of the fill will depend on earthwork activities and compaction efforts made during the placement of the material. Advance Outwash (Qva) -The advance outwash typically is a thick section of mostly clean, pebbly sand with increasing amounts of gravel higher in the section, The advance outwash was placed by the advancing glaciers and was overridden and well compacted by the glacier. The geologic units for this area are mapped on the Geologic Map of the Bothell Quadrangle, Snohomish and King County, Washington, by James P. Minard (U.S. Geological Survey, 1985). The site is mapped as being underlain by a deposit of Advance Outwash. Our site explorations encountered fill and Advance Outwash. Explorations We explored subsurface conditions within the site on September 20, 2017, by drilling five shallow borings close to the administrative building where seepage through the asphalt has been observed. We did not observe any groundwater seepage in those explorations. We returned to the site from October 24 to October 26, 2017, completed seven additional borings throughout the site and installed groundwater monitoring wells in four of the borings. We installed pressure transducers in three of the standpipes where we observed indications of groundwater seepage. The borings were drilled to depths ranging from 11.5 to 36.5 feet below the ground surface. Samples were obtained from the borings at continuous to 5-foot intervals using the Standard Penetration Test. This test consists of driving a two-inch outside diameter split spoon sampler with a 140-pound hammer dropping 30 inches. The number of blows required for penetration of three 6-inch intervals was recorded. To determine the standard penetration number at that depth the number of blows required for the lower two intervals are Robinson Noble, Inc.