1910 W MUKILTEO BLVD Geotech Report 2022-10-25

Geotechnical Engineering'Evaln • NGA File No.1238721 Spencer Garage Addition February 11,2021 Everett,Washington Page 10 Slab-on-Grade Slabs-on-grade should be supported on subgrade soils prepared as described`inthe 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. 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. Site Drainage Infiltration:We intended to conduct a Small PIT in accordance with the 2014 Stormwater Management Manual for Western Washington (2014 SWMMWW) within Infiltration Test Pit One located within the southeastern portion of the site as shown on the attached Site Plan in Figure 2. The test was deemed infeasible when moderate ground water seepage was encountered at 4.0-feet filling the pit with several inches of. water. Test pit caving was encountered at 5.0-feet. Infiltration Test Pit One measured approximately 4.0-feet long by 2.0-feet wide by 6.5-feet deep. Due to the presence of groundwater seepage within all of our explorations, and the silty, dense nature of the subsurface soils, it is our opinion that stormwater infiltration is infeasible within this site. All runoff generated on this site, including roof downspouts, yard and footing drains, and hard surfaces, NELSON GEOTECHNICAL ASSOCIATES, INC.