10719 19TH AVE SE Geotech Report 2023-04-07

Geotechnical Engineering Evaluation NGA File No. 1014317 Zerzura Townhome Development November 9,2017 Everett,Washington Page 11 with varying amounts of gravel and cobbles that we interpreted as native glacial till soils. We conducted one small-scale PIT within the site as shown on the attached Schematic Site Plan in Figure 2. Infiltration Pit 1: Infiltration Pit 1 was located within the central portion of the site and measured 4.4 feet long by 2.75 feet wide, by 5.0 feet deep. The hole was filled with approximately 12.0 inches of water and this level was maintained for six hours for the pre-soak period. At the end of the pre-soak period,the water flow rate into the hole was monitored with a Great Plains Industries (GPI) TM 075 water flow meter for one hour. 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 1 stabilized at 0.061 gallons per minute(3.66 gallons per hour). This equated to an approximate infiltration rate of 0.48 inches per hour. The water was shut off after the steady-state period and monitored at least every 15 minutes for one hour. After 60 minutes, the water level within the pit had decreased to 11.75 inches, resulting in a measured infiltration rate of 0.25 inch per hour. Conclusions: Based on the very slow infiltration rates obtained from our on-site infiltration testing, and relatively silty nature of the site soils, it is our opinion that the native glacial till soils encountered throughout the property at depth are not conducive to traditional infiltration systems. However, shallow low-impact design infiltration systems in the form of pervious pavements,bio swales,or rain gardens may be feasible at specific areas within the site depending on the final layout. This should be further discussed with the civil engineer. We recommend that any shallow infiltration systems installed on the site incorporate overflow components directed to an approved system. In accordance with the Table 3.5 of the Department of Ecology 2012 Stormwater Management Manual for Western Washington, as amended in 2014, correction factors of 0.8,0.5, and 0.9 for CF,,, CFt, and the long-term conductivity loss factor, respectively were applied to the field measured infiltration rate of 0.25 inches per hour, obtained from the falling-head portion of the testing. A total correction factor of 0.36 was applied to the measured field infiltration rate obtained from the falling head portion of the test to determine the long-term design infiltration rate. Using the above correction factor,we calculated a long-term design infiltration rate of approximately 0.09 inches per hour. In our opinion, a long-term design infiltration rate of 0.09 inches per hour could be utilized to design the on-site shallow infiltration systems within the native silty fine to medium sand with gravel found on this site. NELSON GEOTECHNICAL ASSOCIATES, INC.