Laserfiche WebLink
Construction of the underdrain will begin ,just downstream from the WC <br />diversion point. A trench 2 feet deep and 3 feet wide will be excavated <br />parallel and In the middle of the smoothed channel. The trench will be dug <br />downstream to where the present access road (see Map 5-1) crosses the <br />channel. The trench will be lined with a filter fabric having a minimum <br />burst strength of 250 psi (see attached specification sheet) and filled <br />with pit run gravel averaging 2 inches in diameter. The ends of the fabric <br />will be overlapped as illustrated on revised Figure A-2. <br />The fabric wrapped trench and channel bottom will be covered with <br />blasted sandstone spoil from the wedge of Pictured Cliffs Sandstone <br />overlying the Lewis (C) seam. The progressive placement of the sandstone <br />will begin at the WC diversion point and proceed downstream to the access <br />road. The minimum sandstone thickness on the channel bottom will be 10 <br />feet and will extend from bank to bank in the drainage (see revised Figure <br />A-2). <br />As sandstone placement nears the access road, the road will be <br />relocated upstream onto the completed sandstone underdrain. Subsequent <br />trenching, fabric/gravel placement and sandstone placement will be <br />undertaken in the manner previously described. <br />The existing channel bank slopes bounding the constructed underdrain <br />are normally less than 2:1 (horizontal to vertical). For the purposes of <br />designing the minimum discharge area of the underdrain, a 1:1 bounding <br />slope was assumed (see revised Figure A-2). <br />2 <br />