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Geotechnicai Engineering Evaluation Lyashenko Accessory Dwelling Unit Development Everett, Washington NGA File No. 1165620 April 10, 2020 Page 11 Slab -on -Grade Slabs -on -grade should be supported on subgrade soils prepared as described in the Site Preparation and Grading subsection of this report. We recommend that all floor slabs be underlain by at least six inches of free -draining gravel with less than three percent by weight of the material passing Sieve ##200 for use as a capillary break. A suitable vapor barrier, such as heavy plastic sheeting (6-mil, minimum), should be placed over the capillary break material. An additional 2-inch-thick moist sand layer may be used to cover the vapor barrier. This sand layer may be used to protect the vapor barrier membrane and to aid in curing the concrete. Pavements Pavement subgrade preparation and structural filling where required, should be completed as recommended in the Site Preparation and Grading and Structural Fill subsections of this report. Based on our explorations, approximately 4.0- to 7.5-feet of unsuitable fill materials were encountered within the proposed development area. Due to the significant depth of fill encountered, we recommend pavement areas be overexcavated by a minimum of 12-inches and replaced with crushed rock structural fill to avoid significant settlement issues. Some pavement settlement and cracking should still be anticipated as most of the undocumented fill would remain in place. The pavement subgrade should be proof -rolled with a heavy, rubber -tired piece of equipment, to identify soft or yielding areas that require repair. The pavement section should be underlain by a stable subgrade. We should be retained to observe the proof -rolling and recommend repairs prior to placement of the asphalt or hard surfaces. Utilities We recommend that underground utilities be bedded with a minimum six inches of pea gravel prior to backfilling the trench with on -site or imported material. Trenches within settlement sensitive areas should be compacted to 95 percent of the modified proctor as described in the Structural Fill subsection of this report. Trenches located in non-structural areas should be compacted to a minimum 90 percent of the maximum dry density. The trench backfill compaction should be tested. NELSON GEOTECHNICAL ASSOCIATES, INC.