A new protective destressing technique in underground hard coal mining

Abstract

Mining-induced stress accumulates in the immediate excavation borders to propose a risk of sudden and violent failure in the form of high-velocity ejection of rock fragments. Rockburst has become a major stability problem in hard rock mining as underground operations progress deeper down the earth. Likewise, coalburst exhibits a similar failure mechanism in relatively lower depths due to its low strength but high brittle characteristics. Commonly, stress manipulation techniques have been used to mitigate the risk of coalburst. Destress blasting offers a fast and significant decrease in stress concentration within the coal seam compared to the alternative destressing techniques. Stress accumulation shifts away from the excavation face and the fractured zone performs as a cushion between the overstressed region and the working panel. This study evaluates the conventional destressing techniques and proposes a novel method based on the simulations of an underground longwall mine located in New South Wales, Australia. Discontinuous rock media motivated the use of 3-dimensional Distinct Element Method (DEM) for modeling. Discrete Fracture Networks (DFN) were tested in terms of their capability to degrade the rock strength both on lab and field scale simulations. Later, an alternative method was developed to implement the fractured rock mass by eliminating the computational difficulties of DFN. Destress blasting was evaluated in terms of the location and roof stability measures. Despite its significance, blasting creates a violent and sudden impact on the rock mass, which is hard to control. Coalface undercutting was proposed as a new destressing technique. Undercut dimensions required for an effective stress-free cushion were studied in terms of numerical simulations. Later, a practical tool was developed by ‘Adaptive Neuro Fuzzy Inference System’ (ANFIS) in order to predict the effective undercut dimensions. Finally, some practical application guidelines were established for the new method.