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The slope failure is likely the result of a combination of factors including groundwater perched over the <br />claystone bedrock surface, placement of fill materials over the saturated natural soils and relatively steep <br />topography. NWCC believes that the perched groundwater condition has contributed to the failure <br />condition through increased loading and lowered soil shear strength values. Placement of fill materials and <br />removal of materials at the toe of the slide have also contributed to the failure. <br />Stabilization and Reconstruction Recommendations <br />NWCC believes there are several alternatives for stabilizing the slope and reconstructing the access road. <br />However, based on site constraints, depth to groundwater and bedrock, NWCC believes that a feasible <br />method for stabilization, reconstruction of the access road and minimizing future slope movement is to <br />construct a drilled pier retaining wall downslope of the conveyor and access road. In addition, regrading <br />the slope downslope of the wall within the failure area to a flatter configuration will substantially reduce <br />the risk of future slope movement. <br />The drilled pier retaining wall should be constructed along the northern edge of the access road alignment <br />and across the failed area in a northeast to southwest direction. Based on our discussion with the client, the <br />access road will need to be at least 12 feet in width. Therefore, the drilled piers should be located <br />approximately 15 feet north of the existing conveyor. NWCC recommends that the drilled piers be a <br />minimum of 24 inches in diameter and they penetrate the underlying bedrock materials a minimum of 10 <br />feet. The piers should be spaced at 3 feet on centers and the tops of the piers should extend at least 2 feet <br />above the finished grades to act as a guard rail. <br />NWCC does not recommend the placement of roadway fill materials in the access road until the piers have <br />been installed and the concrete properly cured. The placement of fill materials in the access road prior to <br />the installation of the piers could reactivate the slide causing additional distress to the conveyor. NWCC <br />recommends that the concrete achieve a compressive strength of at least 3,000 psi before placing the fill <br />materials. <br />Additional recommendations for the construction of the drilled piers are provided below. A pier wall plan <br />and details are provided in Figure #5. <br />1) Piers should be reinforced their full length with at least twelve #10 vertical steel reinforcing bars <br />and #4 horizontal steel loops spaced at 18 inches. <br />2) Piers should be properly cleaned and dewatered prior to steel and concrete placement. Concrete <br />should not be placed in more than 3 inches of water unless a tremie or pump method is used. <br />Job Nurnbel. 11-8970 <br />NWCC, Inc Pa 5 <br />Page .i <br />