9 74TH ST SW 2024-08-06 - Laserfiche WebLink

Geotechnical Engineering Evaluation NGA File No. 1322621 Clark Garage Slope February 4,2022 Everett,Washington Page 2 1. Reviewing available soil and geologic maps of the area. 2. Visiting the site to reconnoiter surficial information as well as qualitatively assess existing slope stability conditions. 3. Exploring the subsurface soil and conditions on the slope within the site using hand-tool explorations, as necessary. 4. Providing our opinion on the presence of geological Environmentally Critical Areas on the site, as well as general recommendations for hazard mitigation, if applicable. 5. Providing general recommendations for drainage and erosion control. 6. Documenting the results of our findings, conclusions, and recommendations in a written geotechnical letter. SITE CONDITIONS Surface Conditions The property is roughly rectangular in shape and covers 0.22 acres in area. It is currently occupied by a single-family residence on the south-central portion. The site is bordered by a cul-de-sac of 74th Street SW to the south and residential properties on all other sides. The ground surface within the site forms a relatively level terrace on an otherwise gentle- to moderate east-facing slope but includes moderate- to steep east-facing slopes on the northeastern most corner, which descend toward neighboring properties eastward. The slope descends through the property from the west to the east at inclinations in the range of approximately 1 to 31 degrees (1.75 to 60 percent), as shown on Cross-Section A-A' in Figure 3. The top of the slope is supported by an approximately 3.0-foot-tall keystone retaining wall underlain by a rockery. Vegetation within the site consisted of grass yard areas while the steep slopes were more substantially vegetated with underbrush and scattered young to mature trees. We did not observe standing surface water throughout the site during our site visit on January 4, 2022. We also did not observe any groundwater seepage emitting from the site slopes or indications of deep-seated landslide activity or susceptibility during our site visit. Subsurface Conditions Geology: The geologic units for this site are shown on the Geologic Map of Everett 7.5 Minute Quadrangle, Snohomish County, Washington, by Minard, J.P. (USGS, 1985). The property is mapped as glacial till (Qvt). Glacial till can be described as an unsorted mixture of sand, silt, gravel, cobbles, and boulders deposited and compacted by an overriding glacier. Our explorations generally exposed surficial layers of topsoil and/or undocumented fill underlain by light brown, silty sand with varying amounts of gravel and cobbles which we interpreted to be consistent with the description of glacial till. NELSON GEOTECHNICAL ASSOCIATES, INC.