909 51ST PL SW Geotech Report 2022-10-25

Geotechncial Engineering Evaluation NGA Project No. 104021.8 Garcia Property Additions June 22, 2018 Everett, Washington Page 3 Subsurface Conditions Geology: The geologic units for this site are shown on the Geologic map of the Everett 7.5-minute Quadrangle Snohomish County Washington, by James P. Minard (USGS, 1985). The site is mapped as glacial deposits of the Vashon Stade of the Fraser Glaciation, consisting of glacial till (Qvt), with ravine slopes mapped as underlying advance outwash (Qva). Glacial till is described as a nonsorted mixture of gravel, sand, silt, and clay in a dense condition. Advance outwash is described as clean, gray sand with pebbles and locally silty. Our explorations within the vicinity of the proposed addition generally encountered undocumented fill underlain by silty sand with gravel, consistent with the description of the glacial till at depth. Reconnaissance within the ravine slopes exposed gray -brown silty fine to medium sand below surficial colluvium, which is generally consistent with the description of the mapped glacial advance outwash deposits. Explorations: The subsurface conditions within the site were observed on May 24, 2018 by observing three hand auger explorations to depths between 5.5 and 9.3 feet below the existing ground. The approximate locations of the explorations are shown on the Site Plan in Figure 2. In addition, several shallow hand holes within the steep northeast -facing ravine slopes were completed to depths up to two feet in order to map the conditions on the slopes. A geologist from NGA examined the soils and geologic conditions exposed, obtained samples of the different soil types, and maintained logs of the explorations. The soils were visually classified in general accordance with the Unified Soil Classification System, presented in Figure 3. The logs of our explorations are attached to this report and are presented as Figure 5. We present a brief summary of the subsurface conditions in the following paragraphs. For a detailed description of the subsurface conditions, the exploration logs should be reviewed. In Hand Auger One, we encountered dense or better soil below approximately 0.4 feet of surficial silty fine to medium sand with trace fine gravel. Underlying soils consisted of tan to gray silty fine to coarse sand with gravel, which were interpreted to be glacial till. Hand Auger One was terminated due to dense conditions at a depth of 5.5 feet below the existing ground surface. In Hand Auger Two, located downslope of the proposed addition in the vicinity of the proposed deck, we encountered 8.3 feet of mixed silty fine to coarse sand with charcoal and organic debris, including decomposed roots. The soils were encountered in a variably dense condition from loose to medium dense. We interpreted these soils to be undocumented fill related to the orginial grading of the site. Underlying the fill, we encountered approximately 0.5 feet of silty fine to coarse sand with organic debris in a medium dense condition, which was interpreted to be weathered native glacial soils. These soils graded into gray silty fine to coarse sand with iron oxidation staining in a dense condition, which we interpreted to be glacial till soils. Hand Auger Two was completed at a depth of 9.3 feet below the existing ground surface. NELSON GEOTECHN/CAL ASSOCIATES, INC,