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Geotechnical Engineering Evaluation Miller Property Slope Stabilization Everett, Washington NGA File No. 1333322 July 1, 2022 Page 8 Landslide Mechanisms and Hazard General: The criteria used for evaluation of landslide hazards include soil type, slope gradient, and groundwater conditions. The south-central to southern portions of the site and near vicinity show recent evidence of landslide activity. In general, surface deformation has been characterized by extensional cracks separating cohesive blocks of soil within the upper portions of landslides and a compressional toe near the base of the landslide zone, proximal to the southern fence line. Along the ravine slopes below the main landslide, groundwater seeps and shallow skin slide activity was observed. Landslide hazard criteria present on the site, include previous failures and slopes with inclinations greater than 33 percent which intersect geologic contacts with a relatively permeable sediment overlying a relatively impermeable sediment and which contain springs or ground water seeps. Instrumentation: A total of two inclinometers were installed within the landslide affected areas, one of which was situated along the central portion of the subject site and the other on the adjacent property. Inclinometers were installed within Borings 1 and 2 at respective depths of approximately 30- and 20-feet below the existing ground surface. Inclinometers consist of 2.75-inch diameter casing installed into a borehole, which accept an instrument that is capable of measuring minute deflections of the casing. Typically, inclinometers are utilized to constrain slope movement over time. A series of readings are required to develop trends. Baseline readings for each inclinometer were collected on March 4, 2022. Cumulative displacement graphs will be developed for each inclinometer after subsequent readings are obtained. Mechanism: The landslide appears to consist of a 'deep-seated' type failure, a category which is characterized by movement greater than 6 to 10 feet, and can consist of coherent movement of soils along an overall subparallel-to surface failure plane. The recent slide activity shows signs of coherent masses of soil slumped downward to produce approximately three to eight -foot -tall headscarps along the south-central portion of the site. We anticipate that loss of lateral confinement will result in further movement along the same failure surface and backwards migration within previously disturbed deposits, especially in the event of seismic activity or significant precipitation events. Based on the geomorphic features observed, the landslide displays components of deep-seated rotational and translational slide activity, primarily impacting the upper wet to saturated, fine-grained undocumented fill soils documented in the hand tool boring and drilled borings overlying competent granular outwash soils. The recommendations provided in this report for slope stabilization and drainage improvements, including a pile wall and tieback anchors, should help protect the residence(s) and yard areas against ongoing, future activity on the slopes. NELSON GEOTECHNICAL ASSOCIATES, INC.