Brown coal open pit, Yallourn Eastfield, Victoria, FLAC modelling: Morwell River diversion channel-southern batters buffer width

Yallourn Mine, 130 kilometres south-east of Melbourne, supplies brown coal to Yallourn W power station which generates 22% of Victoria’s electricity. The mine is bounded to the south by a man made diversion of the Morwell River, constructed in the 1980s. The initial geotechnical designs located the mine batters 300 m from the river diversion. The worst case scenario from the initial geotechnical modelling was that, if the mine was excavated too close to the diversion, mining induced strain could cause cracking of the diversion which in turn could induce high water levels and failure of the mine batters, potentially leading to catastrophic failure and uncontrolled flow into the mine.

When the mine had developed to total depth it became clear that the actual movements and ground strains were significantly less than previously predicted and that there was an opportunity to reduce the buffer width and increase coal reserves. In the recent past, numerical modelling has been recognised as a powerful tool for efficient analysis and economical design in Civil Engineering projects. In 2000 further analysis was carried out using the limit equilibrium inhouse program (BSTAB) in conjunction with numerical analysis (FLAC). This analysis resulted in a reduction of the buffer width by approximately 50% and enabled deferral for approximately 20 months of major expenditure associated with moving the river to access the coal beyond the river diversion. This paper discusses the results of the stability analysis and model calibration and compares subsequent ground movements against the model predictions.