10627 19TH AVE SE COOLEY SMILES DENTISTRY 2019-03-18

Geotechnical Engineering Evaluation NGA File No. 1008917 Cooley Smiles Commercial Development October 3,2017 Everett,Washington Page 11 Slab-on-Grade Slab-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. We recommend that the capillary break be hydraulically connected to the footing drain system to allow free drainage from under the slab. 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 is optional and is intended to protect the vapor barrier membrane during construction. Pavement Subgrade 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. The pavement subgrade should be proof-rolled with a heavy, rubber-tired piece of equipment, to identify soft or yielding areas that require repair. We should be retained to observe the proof-rolling and recommend subgrade repairs prior to placement of the asphalt or hard surfaces. Utilities We recommend that underground utilities be bedded with a minimum 12 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% 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% of the maximum dry density. Stormwater Infiltration The Department of Ecology 2012 Stormwater Management Manual for Western Washington, as amended in 2014 was utilized to determine the long term design infiltration rate of the site soils. According to this manual, on-site infiltration testing consisting of the Small-Scale Pilot Infiltration Test (PIT) was used to determine the long-term design infiltration rates. The subsurface soils generally consisted of silty sand with varying amounts of gravel and cobbles that we interpreted as native glacial till soils. We conducted three small-scale PIT infiltration tests within each of the associated parcels within the site as shown on the attached Schematic Site Plan in Figure 2. NELSON GEOTECHNICAL ASSOCIATES, INC.