215 DORN AVE Geotech Report 2022-10-25

Geotechnical Engineering Evaluation NGA File No. 1031018 Multi-Family Residential Development April 19,2018 Everett,Washington Page 11 on the attached Schematic Site Plan in Figure 2. The subsurface soils generally consisted of silty fine to medium sand with gravel,which we interpreted as native,glacial soils. Infiltration Pit One: We conducted a Small PIT within Infiltration Pit One, as shown on the Schematic Site Plan in Figure 2. The test was conducted within a pit that measured 3.6-feet long by 5.0-feet wide by 3.0-feet deep.We filled the pit with 12-inches of water at the beginning of the day and were able to maintain the water level for the duration of the soaking period for approximately 6 hours.At this time,the water flow rate into the hole was monitored with a Great Plains Industries(GPI)TM 075 flow meter for the pre-soak period. After the 6-hour soaking period was completed,the water level was maintained at 12.0-inches for one hour for the steady-state period. The flow rate for Infiltration Pit Two stabilized at 0.86 gallons per minute(51.6 gallons per hour).This equated to an approximate infiltration rate of 4.60 inches per hour. The water was shut off after the steady-state period and monitored at least every 15 minutes for an hour. At the end of the monitoring period,the water level within the pit had dissipated by 3.0 inches, resulting in an infiltration rate of 3.0 inches per hour. Infiltration Pit Two: We conducted a Small PIT within Infiltration Pit Two,as shown on the Schematic Site Plan in Figure 2.The test was conducted within a pit that measured 3.6-feet long by 5.2-feet wide by 3.0-feet deep.We filled the pit with 12-inches of water at the beginning of the day and were able to maintain the water level for the duration of the soaking period for approximately 6 hours.At this time,the water flow rate into the hole was monitored with a Great Plains Industries(GPI)TM 075 flow meter for the pre-soak period. After the 6-hour soaking period was completed,the water level was maintained at approximately 12-inches for one hour for the steady-state period. The flow rate for Infiltration Pit Two stabilized at 1.77 gallons per minute(106.2 gallons per hour). This equated to an approximate infiltration rate of 9.10 inches per hour. The water was shut off after the steady-state period and monitored at least every 15 minutes for approximately two and a half hours. At the end of the monitoring period,the water level within the pit had dissipated by 6.25 inches,resulting in an infiltration rate of 2.5 inches per hour. Long Term Rate: In accordance with the Table 3.5 of the Department of Ecology 2014 SWMMWW, correction factors of 0.8, 0.5, and 0.9 for CFv,CFt, CFm,respectively were applied to the field measured infiltration rate of 3.0 and 2.5 inches per hour, obtained from the falling-head portion of the testing in Infiltration Pits One and Two,respectively. A total correction factor of 0.36 was applied to the measured field infiltration rates obtained from the falling head portion of the tests to determine the long-term design infiltration rates. NELSON GEOTECHNICAL ASSOCIATES, INC. 94