4730 COLBY AVE SEPA 016 - 027 - YMCA Geotech Report 2025-04-14

October 25, 2015 YMCA - Everett L&A Job No. 15-089 Page 16 Runoff Over Impervious Surfaces Storm runoff over impervious surfaces, such as roofs and paved driveways and parking areas, should be collected by underground drain line systems connected to downspouts and by catch basins installed in paved areas. Stormwater thus collected should be tightlined to discharge into a nearby storm sewer or a suitable stormwater disposal facility system, such as infiltration trenches. Building Footing Drain A subdrain should be installed around the perimeter footings of the multi-use building. The subdrain should consist of a 6-inch-minimum-diameter, perforated, rigid, drain pipe, laid a few inches below bottom of the perimeter footings. The trench and the drain line should have a sufficient gradient (0.5% minimum) to generate flow by gravity. The drain line should be embedded in washed gravel completely wrapped in non-woven filter fabric to within about 12 inches of finish grade. The remaining trenches may be backfilled with clean on-site soils. Water collected by the perimeter footing subdrain system should be tightlined, separately from the roof and surface stormwater drain line systems, to discharge into a stormwater sewer or suitable water disposal facility. Surface Drainage Water should not be allowed to stand in any areas where footings, on-grade-slabs, or pavement is to be constructed. Finish ground surfaces should be graded to direct surface runoff away from buildings. We recommend the finished ground be sloped at a gradient of 3 percent minimum for a distance of at least 10 feet away from buildings, except in the areas to be paved. LIU & ASSOCIATES, INC.