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As open pit coal mines venture to previously unchartered depths, the importance of ensuring the geotechnical slope stability of high spoil piles becomes critical. However, a reasonable understanding of material parameters under the high stresses involved is yet to be achieved. Of particular interest is the necessity and effectiveness of testing scalped specimens in the laboratory and applying the results obtained to actual mine spoil piles. In an attempt to better understand the implications of particle size on shear strength parameters of coal mine spoil materials, samples were retrieved for testing from four mine sites in Queensland and New South Wales. The sampling covered a range of spoil types from fresh, well-cemented sandstone to weakly-cemented, weathered clay-rich rock. Laboratory direct shear strength testing was undertaken in shear boxes of two sizes; 60 and 300 mm, to investigate the effects of scalping. Specimens were placed loose and tested at their as-sampled moisture content and in a water bath. This was aimed at better simulating mine site conditions in which initially relatively dry spoil is placed loose and is subsequently wet-up by rainfall infiltration. The paper presents the shear strength parameters obtained for spoil specimens tested in the laboratory, and compares these with data from the literature and recommended shear strength parameters, which has implications for assessing the geotechnical slope stability of coal mine spoil piles.
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