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Geotechnical Engineering Evaluation Miller Property Slope Stabilization Everett, Washington NGA File No. 1333322 July 1, 2022 Page 16 Structural Fill General: Fill placed directly behind the wall should consist of 2-inch clean crushed rock placed as structural fill. Structural fill, by definition, is placed in accordance with prescribed methods and standards, and is monitored by an experienced geotechnical professional or soils technician. Field monitoring procedures would include the performance of a representative number of in -place density tests to document the attainment of the desired degree of relative compaction. The area to receive the fill should be suitably prepared by removing soft/loose soils and undocumented fill from the wall area. Materials: Structural fill should consist of a good quality, granular soil, free of organics and other deleterious material, and be well graded to a maximum size of about three inches. All-weather structural fill should be used at this site. All-weather fill should contain no more than five -percent fines (soil finer than U.S. No. 200 sieve, based on that fraction passing the U.S. 3/4-inch sieve). The use of on -site soils as structural fill is not recommended. We should be retained to evaluate proposed structural fill material prior to placement. Fill Placement: Following subgrade preparation, placement of structural fill may proceed. All fill placements should be accomplished in uniform lifts up to eight inches thick. Each lift should be spread evenly and be thoroughly compacted prior to placement of subsequent lifts. All structural fill placed behind the wall and pavement subgrade should be compacted to a minimum of 95 percent of its maximum dry density. Maximum dry density, in this report, refers to that density as determined by the ASTM D-1557 Compaction Test procedure. The moisture content of the soils to be compacted should be within about two percent of optimum so that a readily compactable condition exists. It may be necessary to over -excavate and remove wet soils in cases where drying to a compactable condition is not feasible. All compaction should be accomplished by equipment of a type and size sufficient to attain the desired degree of compaction. Drainage Considerations Where groundwater is encountered during slope regrading activities, we recommend that the contractor slope the bottom of the excavations and collect water into ditches and small sump pits where the water can be pumped out of the excavations and routed into an appropriate outlet. Existing drainage elements within the property should be investigated and improved as needed, such that all runoff generated within the property is collected and discharged to a suitable point of discharge away from the site slopes. This applies to the drain recommended behind the retaining wall. Alternatively, stormwater should be collected and routed to the toe of the slope via a surface mounted HDPE pipe terminating at a dissipator tee and splash pad. This option would require a drainage easement as the subject property does not extend to the toe of slope. We are available to provide further recommendations on such a system, if needed. NELSON GEOTECHNICAL ASSOCIATES, INC.