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March 1998 Slttmp Invetttgation and Mitigation Report ~ Page /5 <br />8.0 DESIGN AND CONSTRUCTION SPECIFICATIONS <br />This section contains general design and construction specifications and presents a <br />recommended sequence of construction activities. <br />1. French and toe drains should be 12 to 18 inches wide, and be filled with ~/,-inch gravel to <br />within 18 inches of the ground surface. The upper 18 inches of the trenches should be <br />backfilled with native material including a layer of organic topsoil. The drains should <br />extend to a depth of at least two feet below the sliding surface of the slump, and should <br />daylight a[ a minimum 2 percent slope to assure positive drainage. The ends of [he drain <br />pipe should extend 5 feet beyond the exit of the slope. <br />2. The drain pipes should consist of 4-inch diameter slotted HDPE pipe, suitable to withstand <br />the loads imposed by 12 feet of trench backfill at 130 pounds per cubic foot. Slots should <br />be positioned around the circumference of the pipe. The portion of pipe that exits along <br />the ground surface should be without perforations. <br />3. The drain pipes do not need to be extended to the shallow surface drainage ditches located <br />mid-slope or downslope from the slump mass. MWA does not anticipate anything more <br />than an intermittent trickle of flow from any of the pipes. <br />4. The construction sequence should be as follows <br />• Construct the upper surface ditch. <br />• MWA is in the process of abandoning the temporary terrace ditches. <br />• Install the two toe drains to facilitate immediate toe drainage. <br />• Regrade the toe of the slump to prevent ponding and allow for positive drainage. <br />• Install the middle French drain to allow for drainage of the main slump mass. <br />• Install the upper French drain to allow interception of up-slope water. <br />• Regrade the head scarp and upper slump mass area to approximate original contour, <br />assuring that the regraded surface promotes positive surface drainage. <br />• Dispose excess materials as a buttress along the toe, but make sure the toe drains extend <br />downslope of the buttress material. <br />• Install the lower surface ditch. <br />• Replace topsoil and reseed the disturbed areas. <br />• Install permanent movement monuments. <br />These features are shown in Exhibit I-I.. <br />Montgomery Watson Americas ' PO. Box 774018 "Steamboat Springs, Colorado " 80477 (970J 879-6260 <br />W.lb)l~SlvmR NUib~~vnlSl.np Rrpn~ <br />Vaf/Vl JW <br />