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Geotechnical Engineering Evaluation Miller Property Slope Stabilization Everett, Washington NGA File No. 1333322 July 1, 2022 Page 4 Subsurface Conditions Geology: Washington State's Puget Lowlands region has experienced multiple intervals of glaciation during the Pleistocene epoch. Continental glaciers periodically extended southward from what is now Canada and scoured the surface, leaving behind thick sequences of unconsolidated sediment and mostly burying bedrock outcrops throughout the region. The most recent glaciation, the Vashon Stade of the Fraser glaciation, likely occurred between 18,000 and 16,000 years ago in the area of the site. Regional geology is shown on the Distribution and description of geologic units in the Mukilteo quadrangle, Washington, by Minard, J.P. (USGS 1982). The site is mapped within advance outwash deposits (Qva), however other geologic units mapped in the near vicinity consist of Vashon till (Qvt) and transitional beds (Qtb), to the east and west of the site, respectively. Vashon till is generally described as a dense, gray diamicton—or mixture —of silt, sand, and gravel, deposited beneath the toe of the glacier. Advance outwash deposits are generally described as blue- to gray sand, pebble to cobble gravel, silt, and clay in varying amounts. These soils were deposited in front of the glacier during its southward advance. In areas where the advancing glacier had dammed streams and rivers, the advance outwash deposit is characterized by a predominantly silt and clay unit deposited in a lacustrine environment. Transitional beds are described by Minard as glacial and non -glacial deposits beneath advance outwash and consisting of mostly thick beds of gray clay, silt, and fine- to very -fine grained sand. Our explorations and observations within the areas of instability generally encountered a significant surficial mantle of saturated undocumented fill soils underlain by competent sand and gravel, consistent with the description of advance outwash at depth. In the explorations closer to the top of slope and within the slide body, fine-grained native deposits mantled the lower granular soils possibly indicating a lacustrine depositional environment as described above. Explorations: The subsurface soil and groundwater conditions within the site were explored with three geotechnical boreholes and four hand tool explorations within the landslide affected areas. Instrumentation at the site included installation of two inclinometers for long term slope monitoring. The approximate locations of our explorations are shown on the Schematic Site Plan in Figure 2. Geologists from NGA were 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 6. The exploration logs are presented in Figures 7 through 11. 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. NELSON GEOTECHNICAL ASSOCIATES, INC.