10627 19TH AVE SE COOLEY SMILES DENTISTRY 2019-03-18

Geotechnical Engineering Evaluation NGA File No. 1008917 Cooley Smiles Commercial Development October 3,2017 Everett,Washington Page 13 resulting in an infiltration rate of 0.375 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 2014 WSDOE Stormwater Management Manual for Western Washington, correction factors of 0.7, 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 0.75 inches per hour, collected from the central PIT during the falling-head portion of the testing. A total correction factor of 0.315 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 0.236 inches per hour. In our opinion, a long-term design infiltration rate of 0.236 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. Site Drainage Surface Drainage: The finished ground surface should be graded such that runoff is directed to an appropriate stormwater collection system. Water should not be allowed to collect in any areas where footings, slabs,or pavements are to be constructed. Final site grades should allow for drainage away from structures. 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 structures. Surface water should be collected by permanent catch basins and drain lines,and be routed into an appropriate discharge system. 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 from the excavation and routed to a suitable discharge point. 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 NELSON GEOTECHNICAL ASSOCIATES, INC.