Author name: Sam Jansch

As spoil dumps get higher, particularly in strip mining where most overburden is placed in-pit, consequences of slope failure become disproportionately greater.

Current understanding of the shearing behaviour of spoil for stability design has involved a combination of laboratory-scale diagnostic testing and engineering judgment. This is a relatively empirical approach that provides a linear shear strength envelope for materials known to exhibit non-linear behaviour, particularly under high confining stresses. A shortcoming to the diagnostic testing is that oversize particles are usually scalped to accommodate the device capacity. The influence of prototype-size particles on the geomechanical behaviour of mine spoil is not truly captured.

In response to concerns about overestimating the shear strength and stability of high spoil dumps, and current plans for coal mine dumps to exceed 400m in height, there is a need to rationally define the stress-strain behaviour of more characteristic spoil masses under representative compressive and
shearing loads.

A Large Direct Shear Machine (LDSM) has been designed at The University of Newcastle to generate reliable stress-strain data on large samples of coal measures spoil (0.72m x 0.72m x 0.6m) subjected to loads representative of very high dumps (~3.5MPa). This paper reviews current methods for predicting shear strength parameters in the context of very high spoil dumps, and presents an overview of the design considerations of the DSM.

Open pit strip coal mining requires a reliable highwall slope design for a safe and economic large-scale bulk mining process. When slope stability issues arise a flexible approach to mine design based on geotechnical analysis is used to assist the operation to manage geotechnical hazards.

Highwall slope instability related to large pit-ward dipping thrust faults occurred in a large strip coal mine in the Bowen Basin, Queensland. The ability to identify the location of thrusts and predict potential instability proved difficult. A series of highwall instability events developed into a 700m-long tension crack located 80m from the crest, resulting in a large block of the highwall creeping towards the active open pit workings. The risk of further highwall instability was high with the potential for premature closure of the pit and loss of significant coal reserves at a time of high global demand for quality coking coal. The geotechnical setting, the failure mechanism identified by detailed geotechnical investigation and the geotechnical engineering approach to managing risk associated with this issue while continuing mining operations are described.

The softwall method comprises a highwall slope design where the rock mass is blasted beyond the pit limit to disrupt rock defects. The softwall design was successfully adopted as the preferred geotechnical slope design to manage potential highwall instability in the final mining strip for the pit. The economic benefit compared to the conventional hard excavated highwall was also evaluated. This further demonstrated the softwall to be an
appropriate slope design method for geotechnical risk management for large scale coal mining operations.

Open pit strip mining is a dynamic process, requiring a flexible approach to geotechnical management. This paper describes two case studies involving highwall and lowwall slope instabilities which occurred in coal mines during 2008 in the Bowen Basin, Queensland. The first case study details a sequence of highwall instability events that developed into a 700m long by 50m wide block of the highwall moving towards the active open pit workings over a 6 month period. The second case study describes a box cut-type lowwall failure that occurred when developing a new mining block. The geotechnical setting, the identified failure mechanisms, the geotechnical back analysis and the geotechnical engineering approaches to managing these issues while undertaking mining operations are described.