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July 3,2017 <br /> Single-Family Residence <br /> L&A Job No. 17-079 <br /> Page 18 <br /> out A layer of 2-inch crushed rock should be placed over footing bearing subgrade soils, <br /> as required, to protect the soils from disturbance by construction traffic. This crushed <br /> rock base should be built to a few inches above groundwater level, but not less than 6 <br /> inches thick. The crush rock base should be compacted in 12-inch lifts to a non-yielding <br /> state with a vibratory mechanical compactor. <br /> Runoff over Impervious Surfaces <br /> Storm runoff over impervious surfaces, such as roof and paved driveway, should be <br /> collected by underground drain line systems connected to downspouts and catch basins <br /> installed in paved driveway. Stormwater thus collected should be tightlined to disperse <br /> over a gently-sloped well-vegetated area through an elevated spreader pipe system. <br /> Building Footing Drain <br /> A subdrain should be installed, around the perimeter footings of the residence. The <br /> subdrain should consist of a 4-inch-minimum-diameter, perforated, rigid, drain pipe, laid <br /> a few inches below bottom of the perimeter footings of the building. The trench and the <br /> drain line should have a sufficient gradient (0.5% minimum) to generate flow by gravity. <br /> The drain line should be wrapped in a non-woven filter fabric sock and completely <br /> enclosed in clean washed gravel. The remaining trenches may be backfilled with clean <br /> onsite soils. Water collected by the perimeter footing subdrain system should be <br /> tightlined, separately ftom roof and driveway drain lines, to discharge onto a rock spall <br /> pad installed at the gully bottom or into the preserved storm drain line of the existing <br /> house currently occupying the site. .. <br /> LIU & ASSOCIATES, INC, <br />