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Geotechnical Engineering Evaluation Lyashenko Accessory Dwelling Unit Development Everett, Washington NGA File No. 1165620 April 10, 2020 Page 12 Site Drainage Infiltration: We conducted one Small PIT within Infiltration Pit 1 located within the north -central portion of the site as shown on the attached Site Plan in Figure 2. The test was conducted in accordance with the 2016 Snohomish County Drainage Manual. The test was performed within Infiltration Pit 1 that measured 4.0-feet long by 3.0-feet wide by 6.0-feet deep. The pit was filled with 12-inches of water at the beginning of the day and maintained this level for 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 water flow meter for the pre-soak period. We monitored the water level during the steady state portion of the test within the infiltration pit. Checking the water level every 15 minutes, we observed the water level had remained at 12 inches for the steady state portion of the test with no inflow of water. After the steady state portion of the test, the test was concluded due to the water level remaining at 12 inches with no additional water added. Due to the presence of perched groundwater encountered at relatively shallow depths in our explorations throughout the site, and the results of our small -PIT test, it is our opinion that traditional stormwater infiltration within the site is not feasible. We recommend that stormwater generated within this site be directed into on -site detentions systems and ultimately into an approved discharge location as determined by the civil engineer. Low impact design infiltration systems, such as rain gardens, bio- retention planters, or pervious pavements could be used within the site if a suitable overflow component is included in the design. Alternatively dispersion systems may be feasible within the site. We recommend the base of any low -impact infiltration systems be advanced through the upper soils and be founded within the native glacial soils found at depth. Additionally, we recommend any low - impact design systems within the site have an incorporated overflow component directed towards an approved point -of -discharge. The stormwater management systems within this site should be sized and designed in accordance with the 2016 Snohomish County Drainage Manual. We recommend that any proposed stormwater management systems be located as to not negatively impact any proposed or existing nearby structures and/or properties and also meet all required setbacks from existing property lines, structures, and sensitive areas in accordance with the stormwater manual. NELSON GEOTECHNICAL ASSOCIATES, INC.