8407 BROADWAY SECURE SPACE STORAGE Geotech Report 2025-04-24

Z i ppe rG eo Broadway Everett Expansion Geoprofessional Consultants Project No. 2601.01 July 7, 2022 direction,with the west portion at a higher elevation. Multiple storm drain catch basins are located down the center of the parking area and at various other locations on the site.Two concrete ecology block-type retaining walls are located at the east and south perimeter of the site. The east block wall has exposed heights ranging from about 1 to 7 feet. The south wall begins near the southwest corner of the existing stormwater vault and continues west to the southeast corner of an existing storage building. The south wall has exposed heights ranging from about 3 to 10 feet. The existing south retaining wall and stormwater vault lie at the crest of a relatively steep slope that descends downward to the south to the top of an existing cast-in-place concrete retaining wall that lies along the edge of the 1-5 southbound SR 526 exit route. Between the crest of the slope and the top of the existing retaining wall,the ground surface slopes at about 20 to 34 degrees (36 to 67 percent grade)with a total vertical relief ranging from about 10 to 12 feet. The height of the existing concrete retaining wall ranges from about 8 to 14 feet. Including the height of the existing ecology block retaining wall at the top of the slope, the slope itself, and the retaining wall located at the toe of the slope, the total maximum vertical relief from the crest of the slope to the toe of the existing concrete retaining wall is about 30 feet. Several evergreen trees and a ginkgo tree are planted and staked on the crest of the hill. Below the crest of the slope, the ground surface is covered with ubiquitous, nearly impenetrable blackberry brambles. SUBSURFACE CONDITIONS Published Geologic Mapping The site is located in what is referred to as the Puget Lowland. Geologists are in general agreement that the Puget Lowland was subjected to six or more glaciations. Each event deposited new sediment and partially eroded previous sediments. Most recently,around 17,000 years ago(Porter and Swanson, 1998), the Puget Lobe of the Vashon Glaciation advanced into the Puget Lowland, originating from the Canadian Rockies. The ice scoured the existing surface, eroding previous sediments and depositing new sediment. Meltwater streams emanating from the advancing glacier deposited sand and gravel (advance outwash). Beneath the glacier, an unstratified mixture of silt, sand, gravel, cobbles, and boulders (glacial till) was deposited. The glacier's weight compacted (overconsolidated) the glacial and remnant nonglacial soil. Later, as the glacier receded, meltwater from the glacier deposited recessional glacial outwash and lacustrine sediments. The recessional deposits are overlain by younger (Holocene-age), relatively loose and soft soil, including alluvium and man-made fills. According to Minard, the site is underlain by Quaternary Vashon glacial till (Qvt)and advance outwash (Qva)from the Vashon glacial advance(Minard, 1985). Troost (2005) describes Qvt as a compact diamict (gravel and sand in a silt matrix) typically three (3)to thirty(30)feet thick and very dense.The dense nature of glacial till soils does not allow stormwater to be infiltrated effectively(low permeability). The author describes the Qva as dense to very dense well- sorted sand and gravel. The US Department of Agriculture Natural Resource Conservation Service (NRCS) web soil survey maps the site as underlain by Alderwood-Urban land Complex, gravelly sandy loam to very gravelly sandy loam with 2 to 8 percent slopes. Relevant properties and qualities of Alderwood Complex soils are provided below. 2