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<br />- 16 - <br />UNDERDRAINAGE <br />Retaining systems such as proposed for truck dump and stockpile <br />• <br />reclaim structures should ba provided with underdrains leading to a <br />gravity outfall unless the structure is designed to withstand hydrostatic <br />pressures. The base drain should consist of a minimum 4-loch diameter <br />pipe embedded in free draining gravel. A gravel layer at least 4 Inches <br />thick should be provided below the slab which connects to the base <br />drain. If the wall Is double formed, we also recommend a granular <br />backfill behind the wall except for the top 2 feet which should consist <br />of compacted on-s(ta clays. if seeps are encountered In the excavation, <br />a gravel drain should ba placed from that level to the base drain. <br />Whore walls are cast directly against the bedrock, some type of drain <br />should be provided. A small perforated plastic pipe or gravel-filled <br />void could be cast into the wall Co allow seepage to enter the underdraln. <br />Perimeter subdralns may also ba required in tNe stockpile and <br />train loadout areas to lower potential perched water levels below pro- <br />posed f{nal surface grades. In the loadout area, the present water <br />table is above the final grade. An interceptor drain extending into <br />bedrock should bo placed at the toe of the cut slopes. This drain <br />should wnsist of a minimum 4-inch diameter pipe surrounded by free <br />draining gravel. The gravel should extend to near the surface. Com- <br />pacted clay should cover the gravel at the surface. It may be necessary <br />to place gravel on grade to provide a trafflcable surface. <br /> <br /> <br />y. Z.-.i. ~ .~RU'~,i..~-,n4si.'.'.. aYz~..e: ..: ~~~..:. ,,}'i_fad~ r: t~ .._.. ~ . <br /> <br />i <br />1 <br />i <br />:.~ <br />:~ <br />n <br />-n <br />1 <br />