2015 Unsaturated Shear Strength of Coal Mine Spoil

Coal mine waste rock (spoil) is a weak granular material derived from rocks of sedimentary origin. There are plans for mining operations to construct piles of spoil up to 400m high. Under the consequent burial stresses, the spoil materials undergo substantial compressions, characterised not only by compaction but also by particle breakdown/disintegration. This leads to significant decreases in void ratio with consequent increases in the degree of saturation (Sr) at constant water content. In this study, a series of direct shear tests was undertaken on a typical mine spoil derived from Permian-aged sandstones and mudrocks, compressed under stresses of 1 and 3MPa, with water contents from 0 to 17%. The samples, with different water contents but compressed under the same stress, all attained similar void ratios, but with degrees of saturation ranging from 25 to 100%, and corresponding suctions from 140MPa to 6kPa. When sheared, these differences in suction produced a difference in measured shear strengths of up to 400%, with a consistent trend of decreasing shear strength with decreasing suction. By fitting the measured shear strengths to an extended Mohr-Coulomb shear strength framework, both φ and φb are found to be highly non-linear and suction dependent. Interestingly, when shear strength is plotted against suction on a log scale, the relationship is approximately linear. There is an observed tendency for friction angles to decrease with decreasing suction, and this is shown to be consistent with values back-calculated from spoil pile failures, with the “unsaturated” value of 28° found in the literature being consistent with the values interpreted in this work