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Geotechnical Engineering Evaluation Miller Property Slope Stabilization Everett, Washington NGA File No. 1333322 July 1, 2022 Page 15 Slope Improvements and Drainage After the retaining wall installation has been completed, areas below the wall and the slope on either side of the wall should be improved by re -grading the over-steepened/loosened areas, removing the vegetative cover, placing suitable erosion control systems on the prepared areas, and re -vegetating with deep-rooted drought -resistant plants. The actual improvement methods for these areas will be highly depended on the conditions encountered during construction. We should work with your contractor to determine the best course of action at the time of construction. Areas of significant debris on the slope should be cleared of debris, covered with erosion control matting, and then planted with deep-rooted plants. For the upper portion of the landslide area, we recommend that the exposed soil be covered with heavy duty erosion control matting such as Tensar C350 Turf Reinforcement Mat, or equivalent. Prior to placing the matting, any areas of loose soils should be removed to expose dense native soil. The matting should be staked to the slope face with two- to three-foot long metal rebar that is either bent at the end or has a metal "T" welded to the end. The mat should be staked to the exposed soil at five feet on center. We should be retained to review and comment on this plan and observe the slope repairs. Final site grades should allow for drainage away from the top of the steep slope and the wall. If possible, the finished ground should be sloped at a gradient of three -percent minimum for a distance of at least 10 feet away from the top of the slope. In any case, all runoff generated above the slope, including paved areas, roofs, and yard areas should be collected and routed into a permanent discharge location at the bottom of the slope or an approved discharge location. The drains exposed in the landslide scarp should be further investigated during construction to determine the extents of the drains. We recommend that these drains be removed entirely from the slope and abandoned or, extended to the bottom of the slope or an approved discharge location. The remaining drainage associated with the site structures should also be investigated and fully understood to verify these drains are being directed to the bottom of the slope or a suitable discharge location away from the steep slope area. Restoration of the backyard area associated with Option A should consist of lightly re -grading and removing loose surficial soil and placement of structural fill, geofoam blocks, or other suitable backfill material approved by NGA. Backfill materials should generally be placed on benched excavations, as depicted on the proposed conditions slope stability analyses on Figures 14 and 15. If geofoam backfill is utilized, gaps should be maintained between blocks to allow drainage pathways through the backfill zone. Gaps can be filled with pea gravel or other free draining material. After desired grades are reached, we recommend vegetation is established. NELSON GEOTECHNICAL ASSOCIATES, INC.