4620 SEAHURST AVE 2020-05-20 - Laserfiche WebLink

March 7, 2011 Mr. Paul Thompson!Kolbear, Inc. L&A Job No. 11-001 Page 15 recommend a design infiltration rate of 1.5 inches per hour be used for the design of infiltration trenches on these two lots. The infiltration trenches designed at this rate should have a factor of safety of at least 6.0. A typical section of infiltration trenches is presented on Plate 9. The trenches should be at least 2 feet wide and about 3.5 to 6.0 feet deep to reach the gravelly sand layer into which water is to be dispersed. The side walls,but not the bottom, of the trenches should be lined with a layer of non- woven filter fabric. The trenches should be filled with 3/4 to 1-1/2 inch, clean, washed gravel or crushed rock to within about 10 inches of the finish grade,with the top of gravel/crushed rock fill covered by the filter fabric liner. The remaining trenches should then be backfilled with clean onsite soil or imported sandy soil. DRAINAGE CONTROL Building Footprint Excavation Building footprint excavation for the proposed residences, if encountering groundwater seepage, should have bottom of excavation sloped and ditches excavated along the bases of cut banks to direct collected groundwater into sump pits from which water can be pumped out. A layer of 2- inch crushed rock should be placed over footing bearing subgrade soils, as required, to protect the spils from disturbance by construction traffic. This crushed rock base should be built to a few inches above groundwater level,but not less than 6 inches thick. The crush rock base should be compacted in 12-inch lifts to anon-yielding state with a vibratory mechanical compactor. Runoff Over Impervious Surfaces Storm runoff over impervious surfaces, such as roofs and paved driveways, should be collected by underground drain line systems connected to downspouts and by catch basins installed in LIU & ASSOCIATES, INC.