Open Cut » Geology
Project C15033 involved detailed theoretical and fieldwork-based investigations of a perplexing geotechnical hazard for open cut coal mining. Sudden composite-style failures of highwalls typically occur while coal mining activities are taking place at pit floor level. Within the Queensland coal mining industry the documented consequences of such failures have ranged from production delays and coal losses to equipment damage and minor injuries to personnel, but there is always the potential for multiple fatalities. On the other hand, such sudden events are also very rare. The risks associated with composite highwall failure therefore remain high, no matter what level of mitigation is deployed.
The purpose of the project was to investigate extension strain fracture processes within highwalls, and in particular to determine whether the Extension Strain Fracture Criterion (ESFC) proposed by Stacey (1981) could be used as a predictor of fracturing that could develop into a composite failure process. All highwalls deform in response to the stress relief of excavation with movements typically in the range of tens of millimetres for typical highwall batters directly above coal seams. Current technologies for monitoring highwall response to excavation are based upon measurement of wall movements, and the trigger levels for movements that may develop into failure were of primary interest for the research project.
The project involved detailed review of many composite highwall failures, from which a number of common features were identified. It was clear that most failures occurred during coal mining, and that there was no relationship between overall pit depth and failure incidence. It was also clear that the failure frequency rate had not changed significantly despite introduction of many technological advances in mine planning and operations. Specific geological structures, often "hidden" behind the highwall face and not identified by exploration, were frequently identified as contributing factors.
Detailed numerical modelling studies were undertaken and these identified some level of brittle extensional fracturing as inevitable in overburden stripping and coal mining for any highwall configuration. While initial stresses played a role in the level of fracture development, it appeared that rock mass blockiness and jointing were controlling factors.
Field investigations were planned for Saraji Mine which has had a history of composite highwall failures. Initial geotechnical drilling was undertaken to characterise the rock mass. A microseismic monitoring array was designed, specified, and purchased in order to measure the locations, magnitudes, and timing of fracture processes within the rock mass during coal mining. The Saraji fieldwork was curtailed after difficulties in coordination with mining activities were compounded by damage to microseismic equipment.
The field investigations were transferred to Coppabella Mine which also has had a history of composite highwall failures. Array installation was successfully completed but the array was intersected by a fault which caused a wedge failure that damaged part of the array during overburden stripping. Coal mining was successfully monitored by the array, but highwall movement measurement using a Groundprobe Slope Stability Radar (SSR®) was only partly successful due to power supply issues. The fieldwork data was supplemented by a nearby corehole which provided detailed rock material property data, and by face mapping which provided detailed rock structure orientation and distribution data.
Detailed numerical modelling was undertaken of the Coppabella experiment site. The modelling was deliberately undertaken using a commercially available 2D finite element code, Phase2, in order to investigate and demonstrate the limitations and capabilities for use in practice. The numerical model incorporated bedding-parallel planes of weakness as well as representations of across-bedding joints using the joint network facility within Phase2. The results of the modelling demonstrated that significant brittle extensional fracture was inevitable during stripping and coal mining, and that the initial stress state was far less significant for overall movement levels in comparison with the effects of the orientations of the joints and weakness planes. Full representation of the brittle fracture process was not possible, and is still the subject of ongoing rock mechanics research. However, the modelling demonstrated that a certain level of stable movement could be predicted with movements in excess of such predictions being prima facie triggers for potential composite failure.
Mine operators may choose to maintain a visual monitoring regime for managing highwall instability hazards. Visual monitoring is inadequate for conclusive detection of trigger levels for potential composite failure. The alternative, using radar or other accurate and repeatable measurement technologies, is more than capable of identifying precursor movements. Reliable trigger levels for progression to a failure process may be obtained by site-specific modelling, or by applying the guidelines set out in the concluding sections of this report.