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 12 Long-Term Design Infiltration Rate: We referenced Table 3.3.1 of the manual to provide an adequate correction factor to apply to the infiltration rate obtained from the testing to obtain the long-term design infiltration rate. Correction factors of 0.50, 0.40, and 0.90 were utilized in this equation for CF,,, CFt, CFm, respectively. A total correction factor of 0.36 was applied to the measured field infiltration rate obtained from the falling head portion of testing conducted within Infiltration Pit 1 to determine the long- term design infiltration rate for the native soils across the site. The 0.25 inches per hour rate obtained from the falling-head period of the test was utilized as the overall measured field infiltration rate. Using the above correction factor and the measured infiltration rate of 0.25 inches per hour,we calculated a long-term design infiltration rate of approximately 0.09 inches per hour. In our opinion, traditional stormwater infiltration is not feasible at this site. However low-impact design infiltration systems, such as pervious pavements, rain gardens, or bioswale could be feasible. The low impact systems should include-an overflow. This should be determined by the civil engineer during the final design. If pervious pavement is utilized within the site,we recommend that an overflow component directed to an approved point of discharge be incorporated into the design. The permeable pavement surfacing should be designed in accordance with Snohomish County standards. We recommend that the permeable pavement surfacing be underlain by a minimum of 12 inches of clean crushed rock with competent native soils below. We recommend that any proposed low-impact infiltration systems such as pervious pavements be placed as to not negatively impact any proposed or existing nearby structures and also meet all required setbacks from existing property lines, structures, and sensitive areas as discussed in the drainage manual. In general, infiltration systems should not be located within proposed fill areas within the site(associated with site grading or retaining wall backfill)as such condition could lead to failures of the placed fills and/or retaining structures. We should be retained to review pavement design and oversee installation. We recommend that any infiltration system be extended down through any undocumented fill soils and founded within the native till soils. Based on our explorations,the native till soils should be encountered at depths in the range of 2.0 to 3.0 feet below the existing ground surface. We should be retained during construction to evaluate the soils exposed in the infiltration systems area to verify that the soils are appropriate for infiltration. NELSON GEOTECHNICAL ASSOCIATES, INC.