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 12 Using the above correction factor,we calculated a long-term design infiltration rate of approximately 1.08 and 0.90 inches per hour for areas in the immediate proximity of Infiltration Pits One and Two,respectively. In our opinion,a long-term design infiltration rate of 1.0 inch per hour could be utilized to design the on- site infiltration systems within the native,silty fine to medium sand with gravel found on this site at depth. We should be retained during construction to evaluate the soils exposed in the infiltration locations to confirm that the soils are appropriate for infiltration. The stormwater manual recommends a minimum three-foot separation between the base of an infiltration system and any underlying bedrock,impermeable horizon, or groundwater. We did not encounter groundwater or any impermeable layers to the extent of our subsurface soil explorations. Dense material observed below three feet within explorations did not appear to be impermeable based on our infiltration testing. In our opinion, the maximum seasonal high groundwater level appears to be greater than three feet below the bottom of the proposed infiltration trenches within this site,and as such,we do not anticipate that groundwater would impact the performance of the infiltration systems. Site Drainage Surface Drainage: The finished ground surface should be graded such that stormwater is directed to an appropriate stormwater collection system. Water should not be allowed to stand in any areas where footings,slabs,or pavements are to be constructed. Final site grades should allow for drainage away from the residences. We suggest that the finished ground be sloped at a minimum gradient of three percent,for a distance of at least 10 feet away from the residences. Surface water should be collected by permanent catch basins and drain lines,and be discharged into an appropriate discharge system. The overflow water should be directed to discharge into an appropriate location. Subsurface Drainage: If groundwater is encountered during construction, we recommend that the contractor slope the bottom of the excavation and collect the water into ditches and small sump pits where the water can be pumped out and routed into a permanent storm drain. We recommend the use of footing drains around the structures. Footing drains should be installed at least one foot below planned finished floor elevation. The drains should consist of a minimum 4-inch-diameter, rigid,slotted or perforated,PVC pipe surrounded by free-draining material wrapped in a filter fabric. We recommend that the free-draining material consist of an 18-inch-wide zone of clean(less than three- percent fines),granular material placed along the back of walls. Pea gravel is an acceptable drain material. The free-draining material should extend up the wall to one foot below the finished surface. The top foot of backfill should consist of impermeable soil placed over plastic sheeting or building paper to minimize surface water or fines migration into the footing drain. Footing drains should discharge into tightlines NELSON GEOTECHNICAL ASSOCIATES, INC. 95