Wines, DR, Hulls, I, Woods, E & Creighton, A 2013, 'The use of numerical modelling, slope monitoring and operational procedures to manage slope deformations at the Ranger 3 pit', in PM Dight (ed.), Slope Stability 2013: Proceedings of the 2013 International Symposium on Slope Stability in Open Pit Mining and Civil Engineering
, Australian Centre for Geomechanics, Perth, pp. 611-624, https://doi.org/10.36487/ACG_rep/1308_40_Wines
Elevated slope movements were observed on the northeast wall of the Ranger 3 pit during excavation of the final pit shell. Both prism and radar monitoring indicated that the movement rate and magnitude were greater in this area when compared to surrounding areas, and periods of acceleration were evident. Several months after the initial onset of movement, tension cracks were observed behind the upper pit crest. At that stage, several benches were yet to be mined at the base of the wall, and concern existed regarding the safety of operations below the moving wall.
A calibrated three-dimensional numerical model was selected as the prime risk management strategy for mining the balance of the attainable ore in Pit 3. A detailed structural geological study was initially undertaken to provide reliable inputs for the modelling. The modelling was integrated into the overall risk management process, with the model being constantly updated based on the observed slope behaviour and the rock mass conditions being exposed at the toe of the slope. Ongoing and detailed calibration between the model behaviour and the comprehensive slope monitoring data was performed to provide a reliable understanding of the mechanism of movement, and to assess the likelihood of slope failure. Possible failure scenarios were also examined as a part of the risk management strategy, and strict operating procedures were implemented to minimise the risks associated with mining under an actively moving pit wall.
Although the modelling indicated that ongoing slope movements could be expected, slope failure was not predicted, provided that final pit excavation would be completed, as planned, prior to the upcoming wet season. In light of these modelling results, mining continued in Pit 3 based on the original mine plan, and final pit excavation was successfully completed in late 2012. Backfilling of the pit commenced soon after. After initial discovery of the tension cracks, some consideration had been given to incorporating a step-out in the lower part of the wall in an attempt to stabilise the observed movements. The successful risk management process, including detailed numerical modelling, comprehensive slope monitoring and strict operational procedures, allowed the original mine plan to be achieved, and therefore avoided the significant loss of ore associated with the proposed design change.
This paper summarises the numerical modelling methodology and results, the monitoring methods and data and the operational procedures that were used to successfully manage final pit completion.
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