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

October 25, 2015 YMCA - Everett L&A Job No. 15-089 Page 20 The infiltration trenches/galleries should be at least 24 inches wide and at least 12 inches into the targeted advance outwash deposit. The side walls of the trenches (but not the bottom) should be lined with a layer of non-woven filter fabric, such as MIRAFI 140NS. The trenches are then filled with clean, 3/4 to 1-1/2 inch, washed gravel or crushed rock to within about 16 inches of the finish grade. The dispersion pipes should be constructed of 4- inch rigid perforated PVC pipes and laid level in the gravel or crushed rock filled trenches at about 3 feet below the top of trenches/galleries. The top of the gravel or crushed rock fill should also be covered with the filter fabric liner. The remaining trenches should then be backfilled with compacted onsite clean soils. The gravel or crushed rock fill should be placed in lifts no more than 10 inches thick in loose state, with each lift compacted to a non- yielding state with a vibratory mechanical compactor. The compaction and densification of trench fill is critical if it is to support driveways and parking lot. Stormwater captured over paved driveways/parking lot should be routed into a catch basin equipped with an oil-water separator before being released into the infiltration trenches/galleries. Rain Garden Construction Rain gardens may also be used to dispose stormwater onsite. Rain gardens should be set back at least 5 feet from property lines and 15 feet from adjacent building foundations and utility trenches with bottom of the rain garden ponds lower than adjacent building foundations and utility trenches. The slope of cut banks of the rain garden ponds should be no steeper than 2.5H:1 V, while the slope of fill embankments should be no steeper than 3H:1 V. Vegetation within footprints of fill embankments should be cleared and thoroughly grubbed and surficial topsoil and weathered soils should be completely stripped down to firm bearing soil. Exposed soil should be compacted to a non-yielding state and proof- LIU & ASSOCIATES, INC.