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stresses increases as depth of the fill increases. Data presented by Hough, 1957' indicate that <br />under moderate confining stress, the angle of internal friction increase up to 4 °over the angle of <br />repose to create a peak value. Accordingly, the angle of internal friction for the lower bench fill <br />will be assumed conservatively to be 2° greater than the observed angle of repose, or 38 °.Field <br />measurements have shown the lower bench dump slope is, in fact, 38 °. In addition, the mine <br />rubble fill material will be conservatively considered a cohesionless material, with no cohesive <br />strength. Also, the fill material is considered free draining and no pore pressure build -up will <br />occur during shear movements. <br />The native shale is a fine- grained, horizontally bedded and anisotropic, marlstone shale. Shear <br />strength is provided by both internal friction and cohesive strengths. Literature values were <br />consulted to obtain an estimated shear strength value of 41 ° at 1788 psi2 for the native shale <br />material. No groundwater (saturated conditions) currently exists within the native or fill <br />materials. <br />Stabilitv Cases Analvzed <br />The general solution of slope stability using a 2 -D limiting equilibrium solution was <br />assessed with the PC STABL 5 computer code. The PC STABL 5 code is a DOS version <br />of the STABL computer algorithm that has been widely accepted for more than 30 years <br />to analyze geotechnical slope stability. The program was initially presented at Purdue <br />University and has gained wide acceptance in the geotechnical community since 1975. <br />The PC STABL 5 computer code calculates the factor -of- safety (FOS) value against <br />instability by the method of slices. Circular shaped failure surfaces are analyzed with the <br />modified Bishop method and the simplified Janbu method is used to analyze irregular <br />shaped block and wedge -type failure surfaces. Circular failure surfaces were analyzed for <br />slope failures within the fill material and block and wedge -type failures were used to <br />analyze the fill /native interface. Copies of computer output files are appended to this <br />report. <br />Stability of the existing lower bench slope under dry conditions without introduced water <br />was initially analyzed to allow comparison to the slope with the addition of an infiltration <br />gallery. The infiltration gallery is expected to receive an average inflow rate of <br />approximately 2' /z to 3 gallons per minute. A conservative water surface is assumed to <br />form over time from the discharge. Because of the free - draining nature of the fill <br />material, discharge waters are assumed to flow in a vertical fashion with little lateral <br />spreading until the native interface is reached. It is assumed that once infiltrating water <br />reaches the native interface, the water flows along the interface. Initiation of the water <br />surface is allowed to occur with an approximate 18 -foot width at the surface. Three <br />1 Hough, B.K., 1957. Basic Soils Engineering, The Ronald Press Company, New York, NY. <br />2Chong, H.P., Chen, J -L, Dana, G. and S. Sailor, 1984. 'Triaxial testing of Devonian Shale ", ASCE, Journal of <br />Geotechnical Engineering, 110(10):1491 -1497. <br />