3610 W MUKILTEO BLVD Geotech Report 2022-10-25

Geotechnical Engineering Evaluation NGA File No. 1129119 Zahler Residence Development October 14,2019 Everett,Washington Page 11 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 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. The trench backfill compaction should be tested. Site Drainage Infiltration: The 2014 DOE SWMMWW 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. We conducted one small scale PIT as shown on the attached Schematic Site Plan in Figure 2. The subsurface soil generally consisted of gray silty fine to medium sand with gravel,which we interpreted as native glacial till soils. The test was conducted within a pit that measured 4.0-feet long by 3.0-feet wide by 4.0-feet deep. As soon as the Infiltration Pit 1 was excavated, the pit was filled with 12 inches of water and we began the soaking period of the PIT for approximately 6-hours. At this time, the water flow rate into the hole was monitored with a Great Plains Industries(GPI)TM 075 water flow meter for the pre-soak period. After the 6-hour soaking period was completed, the water level was maintained at a depth of approximately 12 inches, respectively, for one hour for the steady-state period. The flow rate for Infiltration Pit 1 stabilized at 0.035 gallons per minute (2.1 gallons per hour). This equated to an approximate infiltration rate of 0.28 inches per hour. The water was shut off after the steady-state period and monitored at least every 15 minutes for one hour. After one hour, the water level within the pit had decreased by 0.25 inches,resulting in a falling-head infiltration rate of 0.25 inches per hour. NELSON GEOTECHNICAL ASSOCIATES, INC.