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8. Recommendations <br /> The recommended minimum FOS for the mine highwalls within the South and West <br /> Quarries has been established at 1.25 by MLRB Policy 30 and is considered the minimum <br /> FOS for rock in mine highwall situations. Appropriate design of highwalls for slope stability <br /> requires accurate characterization of geological conditions within the South Quarry and West <br /> Quarry including: <br /> • Rock type <br /> • Structural conditions within the rock(discontinues such as foliations,joint sets, <br /> intrusions, and shear zones) <br /> • Rock strength and weathering <br /> • Highwall orientation <br /> The recommendations presented below are based on results of the stability analyses, <br /> subsurface explorations, geologic mapping, and observed conditions by GEI. <br /> 8.1 Highwall Recommendations <br /> The evaluations suggest that highwall stability is strongly controlled by rock type and the <br /> condition of discontinuities. Compositional changes between the hard, durable <br /> graniticibiotitic gneiss and the softer, weaker sillimanitic gneiss have a significant impact on <br /> the overall rock mass strength and the measured factor of safety in local and global stability <br /> evaluations. The orientation and condition of discontinuities also has a significant impact on <br /> kinematic failures, an assumption that is supported by visual observations and preliminary <br /> stereonet evaluations. <br /> 8.1.1 Recommendations for Rock Mass Stability <br /> Results of slope stability analyses indicate that the proposed configuration for the main bench <br /> to highwall geometry of 30H:70V generally produces an acceptable factor of safety <br /> (FOS>1.25) in granitic or biotitic bedrock, but with notable exceptions in the local stability <br /> case at El. 6,650 and when the rock mass is composed of sillimanitic gneiss. The local <br /> stability case indicates an increase bench width of 40 ft is necessary at El. 6,650 to meet <br /> stability criteria and should be incorporated into mining plans. <br /> Site mapping by GEI indicates that a significant amount of sillimanitic gneiss could be <br /> encountered in both the South Quarry and the West Quarry, and Aggregate Industries should <br /> be prepared to modify mining plans if thick units of the sillimanitic gneiss (>5ft) are <br /> observed at the bench face during excavation. If sillimanitic gneiss is observed at the bench <br /> face the area should be considered unstable, and a revised configuration with an increased <br /> horizontal bench width should be used in that location until a site-specific geotechnical <br /> evaluation can be conducted. Until a site-specific evaluation can be made, we recommend <br /> 40 <br /> Aggregate Industries—Morrison Quarry GEI Consultants, Inc. <br /> AM-07 Geotechnical Stability Exhibit November 2020 <br />