3D Rock failure modelling was used to perform a coal pillar stability analysis comparing normal and angled Gas Drainage Stub (GDS) design during the approach of a longwall. This paper aims to benchmark the response behaviour of rectangle and triangle shape fender pillars respectively formed during longwall retreat into normal and angled GDS designs. The results of this study are intended to provide a general hazard awareness of potential layout/support issues relevant to GDS design rather than providing a unique recommendation or overall mining solution.
Using FLAC3D, computer modelling of the rock failure process simulated pillar loading and pillar failure of the fender pillar that is created between the GDS at the tailgate end of the longwall. A standard gas drainage stub, angled at 90° to the roadway and another angled at 70° stub was simulated to provide comparison between normal and angled GDS design scenario. The modelled strata section was based on typical moderate strength sandstone/siltstone roof and floor strata. A variety of geometrical parameters were varied in the models to assess sensitivity of pillar stability, in particular anticipated peak strengths were compared against a range of reducing pillar sizes, representing the reduction of the fender pillar size during longwall extraction.
Pillar shape was determined to be a major influencing factor with respect to pillar strength. It was found that the triangular fender pillar created from the GDS driven at an angle to the tailgate ultimately results in reduced pillar strength and softer post failure modulus as compared to rectangular pillars of similar widths. This reduction in overall strength is mainly attributed to the geometry of the triangular pillars (i.e. a smaller load bearing area) and their reduced ability to generate internal confinement. Although the triangle pillar yields at a further distance away from the longwall face, the impacted zone is focused over a smaller area of the tailgate corner rather than over the entire total length of the stub.