Review of the historical data characterizing Latrobe Valley brown coal consolidation behaviour

Victoria, Australia contains 25% of the world’s known brown coal reserves. Much of the coal is located in thick seams overlain by a thin veneer of sands and clays. Three brown coal open-cut mines are presently operating in the Latrobe Valley. As a result of the mining activity, ground movements can be significant within and around the mines. Slope stability has to be managed during all mine development phases. Recent stability issues at some mines have highlighted a need to review the historical geotechnical information and data with a view to guiding future data collection. To support this purpose, the existing understanding of the geomechanical properties of the brown coal has been revisited, with emphasis on impacts of unloading. The coal has low density (specific gravity<1.2) and high water content (>60% by volume). Given this background, historical data for 49 consolidation tests on one of the major mined upper coal seams, the Morwell seam, were extracted from the historical records. Additionally, 44 consolidation tests for interseam materials were also extracted for the same area. These data have been analysed by re-evaluation of the consolidation data. The analysis shows that the behaviour of brown coal differs from that of typical engineering soils, in that at lower load the stress v. strain gradient is higher than for greater loads. At higher loads the stress v. strain gradient is almost constant. These observations indicate that reassessment of the unloading parameters for slope design in the open cuts is required.