4624 SEAHURST AVE 2018-01-02 MF Import

March 7, 2011 Mr. Paul 'lhompson,/Kolbear. Inc. L&A Job No. I 1 -00 1 Page 1 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. :\ typical section of infiltration trenches is presented on Plate 9. The trenches should be at le. �t 2 feet wide and about 3.5 to 6.0 feet deep to reach the gravelly sand laver into which water is to be dispersed. The side walls, but not the bottom, of the trenches should be lined with a laver of non- woven filter fabric. the trenches should he tilled 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 graveUcrushed rock fill covered by the filter fabric liner. the remaining trenches should then be back -fulled 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 sunip 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 soils 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 crash rock base should be compacted in 12-inch lilts to a non -yielding state with a vibratory mechanical compactor. Runoff Over Impervious Surfaces Storm mnoff over impervious surfaces, such as roofs and paved dui, ew•ays, should be collected by underground drain line systems connected to downspouts anu by catch basins installed in LIU & ASSOCIATES, INC. �sa� /�f