1311 W MUKILTEO BLVD 2022-06-29 - Laserfiche WebLink

Geotechnical Engineering Evalual NGA File No.1295621 Tsang Slope Restoration October 21,2021 Everett,Washington Page 3 18 Explorations: The subsurface conditions within the site were explored on September 8, 2021 by monitoring the excavation of one test pit and three hand auger explorations within the disturbed portion of the property to depths ranging between 4.0 and 12.5 feet below the existing ground surface. The approximate locations of our explorations are shown on the Schematic Site Plan in Figure 2. A geologist from NGA was present during the explorations,examined the soils and geologic conditions encountered, 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 4. The logs of our borings are attached to this report and are presented as Figure 5. We present a brief summary of the interpreted subsurface soil stratigraphy in the following section. For a detailed description of the subsurface conditions,the specific exploration logs should be reviewed. 1. Undocumented Fill:At the surface of each exploration we encountered brown to dark brown silty sand with varying amounts of organics, debris, and gravel in a loose to medium dense condition,which we interpreted as undocumented fill. The fill soils adjacent and behind the block retaining wall extended to depths of 7.0 feet below the existing ground surface, while hand tool explorations above and below the cut rockeries encountered fill to depths in the range of 1.5 to 4.5 feet below the existing ground surface. 2.Glacial Advance Outwash:Underlying fill soils in all explorations we encountered fine to coarse sand with silt and varying amounts of gravel to the depths explored. The deposits were encountered in a medium dense to dense condition,and we interpreted them to be native glacial advance outwash soils. Test Pit One terminated in the advance outwash soils at a depth of 12.5 feet below the existing ground surface. Hand Augers One through Three terminated in the advance outwash soils at depths of 4.0,4.0, and 5.0 feet, respectively. Hydrogeologic Conditions Groundwater seepage was not encountered in our explorations. If groundwater is encountered during restoration, we would interpret this as perched water. Perched water occurs when surface water infiltrates through less dense,more permeable soils and accumulates on top of relatively low permeability materials. Perched water does not represent a regional groundwater "table" within the upper soil horizons. Perched water tends to vary spatially and is dependent upon the amount of rainfall. We would expect the amount of perched groundwater to fluctuate seasonally, if present. NELSON GEOTECHNICAL ASSOCIATES, INC.