2015 A Trilinear Shear Strength Envelope For Coal Mine Spoil

Currently, linear Mohr failure strength envelopes are commonly used in the Australian coal industry for shear strength estimation of spoil dumps within certain bounds of normal stress related to historical experiences. Mostly, the linear envelopes appear to be reliable. However, deviation from linearity is fundamentally important for slope design and stability analyses performed for higher values of normal stress, as the true available shear strength may be considerably lower than is estimated from linear models. This is becoming increasingly relevant as spoil dumps are being constructed to unprecedented heights. This paper presents the results of an experimental study in which a strong Permian coalmine spoil from the Hunter Valley, Australia was tested under both saturated and unsaturated conditions, in a large (720 mm 720 mm 600 mm) direct shear machine over a wide normal stress range, up to that expected for very high spoil dumps. A trilinear Mohr failure envelope was developed from tests performed on 23 specimens, with normal stresses up to 4600 kPa. The secant friction angle values from each test were found to display a corresponding trilinear trend with increasing normal stress. The trilinear behaviour were evident in results from both the unsaturated and saturated samples; however, the linear segment boundaries were lower for the saturated test data. The trilinear behaviour is attributed to differences in the shearing mechanisms that occur under the different stress conditions.