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Geotechnical Engineering Evaluation Miller Property Slope Stabilization Everett, Washington Shoring Wall NGA File No. 1333322 July 1, 2022 Page 12 General: A shoring wall up to 10 feet in exposed height is recommended to fully stabilize the top of slope area and landslide impacting the residence. The wall should generally be located along the top of the steep slope or immediately along the headscarp, depending on the layout option chosen, and extend across the width of the property. The most feasible shoring system is a solider pile wall with tieback anchors. A solider pile wall typically consists of a series of steel H-beams placed vertically at a certain distance from one another (typically six to eight feet). The beams are usually placed in drilled shafts that are filled with structural concrete or a lean mix. The concrete shafts are typically embedded below the bottom of the planned excavation a distance equal to one to two times the height to be shored. The steel beams are extended above finished ground surface to provide shoring capabilities for the area to be retained. The beams are typically spanned by pressure treated timber lagging or concrete panels. The H-beam size, shaft diameter, shaft embedment, and pile spacing are dependent on the nature of the soils anticipated to be placed below the wall and at depth, wall height, drainage conditions, and the final geometry. Details regarding the proposed soldier pile wall are shown on the Schematic Soldier Pile Wall Detail in Figure 12. Tiebacks extending into the native stable soils behind the shoring wall will be needed to provide additional lateral resistance for the wall and reduce loading onto the lower slope. Soldier Pile Wall Design: The shoring wall should be designed by an experienced structural engineer licensed in the State of Washington. We recommend the piles be embedded a minimum of 15-feet into competent granular soils encountered at depth. Based on our explorations and measurements we anticipate overall pile embedment below the ground surface will be approximately 20- to 30-feet, depending on the wall configuration chosen. The wall should be designed to resist an active pressure acting on the piles and lagging for design of the soldier piles and should be calculated based on a triangular pressure distribution equivalent to that exerted by a fluid. In the table below, we have provided active pressures for wall configuration Options A and B, which represent a retaining wall placed along the southern fence line or a retaining wall placed along the headscarp, respectively. The provided active pressures account for seismic loading conditions. Wall Configuration Active Pressure (PCF) Option A 150 Option B 70 NELSON GEOTECHNICAL ASSOCIATES, INC.