
Falls of ground are the single greatest hazard faced in an underground coal mining environment and with the industry continually driving to improve safety, any enhancement in ground control understanding is valuable. Chain pillar cut throughs have been observed to enhance overburden weighting effects. This is likely as a result of the gap in support from the chain pillar as the cut through is limited to localised ground support. While the impact of longwall alignment with cut throughs has been discussed, it is yet to be quantifiedThis research aimed to determine if there is a quantitative impact from longwall alignment with cut throughs on weighting events and longwall stability as indicated by shield pressure and shield closure rates. The analysis involved the use of hydraulic shields sensor data in Longwall Visual Analysis (LVA) software to analyse Time Weighted Average Pressure (TWAP) and convergence data inbye and outbye of the cut through location. The data set consisted of 65 cut throughs on the tailgate side of three Longwall Top Coal Caving (LTCC) panels. The analysis involved an assessment of the impact of cut through location on TWAP, convergence and weighting event location. Following this, an investigation into key geological features influencing longwall stability was conducted.The results showed that there is no correlation between cut through location and increased TWAP or convergence at the mine site. Increases in TWAP were found to be most dependent on the caving of massive strata at the site which occurred at regular intervals along the panel. An accidental finding of the study was that there is an increase in TWAP at the cut through location at 40 m across the panel from the tailgate side. This is very likely a result of side abutment loading from the adjacent goaf.Increases in convergence were noted as being typically linked to increased zones of TWAP. However, it was also observed that there were conditions under which high convergence was recorded that corresponded to low pressure areas. This was likely as a result of shields being unable to support overburden loading and thus yielding. Furthermore, it was found that while there are weighting events that correlate with cut through location this is likely a result of the typical weighting interval coinciding with a cut through location.Geological features were found to have likely increased the overall stress distributions on Longwall 8 due to the presence of a dyke through the panel. The other panels had no major geological structure and so longwall stability was more dependent on periodic weighting effects and longwall operational factors. The analysis highlighted no consistent impact from cut throughs on longwall stability. It is thought that this is very likely due to the influence of chain pillars in combination with roadway support measures and the absence of caving on the 18 m adjacent to the tailgate. Furthermore, mine design factors such as the staggering of the maingate and tailgate cut throughs may have reduced potential cut through impact.The concluding statement of the analysis was that at the mine site there is no correlation between longwall alignment with cut throughs and weighting events. It was found that periodic weighting events are more influential on longwall stability than cut through location. Cut throughs may coincide with weighting event location however this is due to a typical weightinginterval not the cut through presence itself. Furthermore, there is a side abutment load at 40 m from the tailgate side of the panel. Finally, it is likely that chain pillar influence in combination with roadway support and the absence of LTCC on the tailgate side reduces cut through influence.Key recommendations arising from the study include firstly the compilation of data from multiple sites to confirm the observed lack of influence of cut throughs on longwall stability. Secondly, analysing data from a conventional longwall operation should be conducted as it has been noted that there is reduced weighting in typical caving environments compared to LTCC and so cut through influence may be more prevalent. Finally, a numerical modelling assessment should be done in conjunction with shield monitoring data and roof extensometer data to determine the influence of weighting events on the cut through roadway itself.The findings of this research are applicable to the mine site itself and other mines with similar weighting and geological environments. The findings will improve mine safety standards as they will allow geotechnical engineers to focus attention on the most critical ground control mechanisms with the understanding that cut through location alone will not cause longwall stability issues.
School of Mechanical and Mining Engineering, 09 Engineering
School of Mechanical and Mining Engineering, 09 Engineering
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