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Slope Stability Analysis of the <br />Lower Bench, Logan Wash Mine <br />Introduction <br />A slope stability analysis of the lower bench slope at the Logan Wash Mine was conducted to <br />evaluate stability affects from construction of an infiltration trench on the lower bench. The <br />infiltration trench will allow infiltration of mine drainage that is currently collected and <br />discharged in a natural ephemeral stream bed. Currently, the Colorado Department of Public <br />Health and Environment ( CDPHE) requires monthly sampling and reporting for the discharge <br />under the Colorado Discharge Permit System (CDPS). A memorandum of agreement between the <br />Colorado Division of Reclamation, Mining and Safety (DRMS) and CDPHE allows for discharge <br />of mine water at an off - channel location, where infiltration of mine water will occur and not flow <br />on the surface to a natural surface drainage. Stability of the lower bench waste rock dump (lower <br />bench) was analyzed to evaluate stability before and after construction of a small infiltration <br />trench for management of mine water discharges. <br />Site Descrintion <br />The lower bench at the Logan Wash Mine appears to have been constructed by side casting mine <br />waste rock rubble from underground mine workings over the native grade. The waste rock rubble <br />consists of shale fragments and rock fines that were described visually per the Unified Soil <br />Classification System as well - graded sandy GRAVEL with COBBLES, trace silt and clay, <br />angular, non - plastic, gray -brown material. <br />Sections <br />The lower bench possesses an outslope angle of approximately 36 °, as obtained from a 2011 aerial <br />Lidar survey of the site. Figure No. 1 shows the Lidar survey with a cross - section line indicating <br />the location of the slope analyzed. The outslope angle represents the angle of repose for the mine <br />rubble, which is the angle of internal friction of a deposited cohesionless granular material when <br />the surface is on the verge of sliding. Figure No. 2 presents the cross - section analyzed and shows <br />both the fill and assumed native grade surfaces. <br />Native material at the Logan Wash Mine site is the Parachute Creek Member of the Green River <br />Formation. The weathered exposures form natural vertical ledges and cliffs and slopes at an <br />approximate 40° angle of repose as measured from an existing USGS topographical map (Figure <br />No. 3). The 40 0slope forms the interface between the mine rubble fill and the native slope. <br />Locally the Parachute Creek Member is horizontally bedded. <br />Material properties <br />The angle of repose is the minimum angle of internal friction of a granular material achieved in <br />the loosest state. Confinement of the material increases the angle of internal friction. Confining <br />