A new ring shear apparatus for the determination of the residual shear resistance of remoulded brown coal

Victorian brown coal is a typical intermediate geomaterial, whose behaviour falls between that of soft rock and engineering clay. The intact material exhibits lower permeability and higher tensile strength compared to overconsolidated clay. Large, shallow open cuts are used to mine the coal in the Latrobe Valley, Victoria, Australia, predominantly as fuel for Victoria’s power stations. For batter design the brown coal is treated as clay with high values of cohesion and friction angle. Composite rotational and block sliding is a recognised failure mode for this material and it is apparent from recent observations that failure risk increases with time. During long-term movements of the brown coal behind and below the batters it is anticipated that the material will be crushed and remoulded along sliding surfaces as a result of progressive pre-failure displacements. After periods of decades, the assumption is that for large sections along any incipient failure surface, shear strength will be at or close to the residual shear strength of the material. Thus. it is important to understand whether this assertion is correct and the processes leading to the weakened state. A new ring-shear test apparatus has been designed to determine the variation of shear strength of this material for a range of shear strains under essentially drained conditions. In this paper the design considerations and the resulting form of the test apparatus are presented. The equipment is also applicable to the testing of the interseam clays, silts and fine sands that are also found in the mines.