Underground » Mining Technology and Production
This main aim of the project was to investigate issues associated with longwalling in weak mining conditions and in particular, ground response when mining through faults. The objectives of the project were:
- By combined longwall support and microseismic monitoring, devise a means to readily understand changes in the interaction between strata and supports in faulted zones and regions of weak conditions more generally.
- Assess actual mining experiences from mines that have mined through fault zones to understand the need for grouting and other actions given the geological context of these faults.
A review of past fault consolidation projects from various mines in the Bowen Basin was also undertaken. The aim of the review was to devise a means to assess faulted areas to determine the need for consolidation and their likely impact on mining performance.
A measurement trial and program of microseismic monitoring of mining through a fault was carried out in LW105, Moranbah North mine. Analysis of the fault pre-consolidation program including comment on grout takes, pressures and penetration was undertaken. Longwall support monitoring and face stability analysis was used to examine overall strata-support interaction processes.
The objective of the CSIRO microseismic monitoring study was to investigate the feasibility of using geophones installed on the ground surface to detect ground response characteristics associated with mining through the fault. It was found that locations of significant rock fracturing could be determined with high accuracy in plan view but poorly in depth. In general, the number of seismic events recorded on the ground surface was about 1/3 of those recorded in the deep boreholes, due to considerable seismic attenuation near the ground surface.
The majority of seismic events recorded on the ground surface did not have clear first arrivals, but are good enough for event counting analysis. No significant anomaly in event rates was found during mining of the fault zone in LW105. Similarly, no significant concentration of the events was found near the grouted fault zone. This is probably due to both the weak signals likely to emanate from the fault zones and/or the lack of any movement on the fault(s) during mining.
Analysis of grouting data indicated a rough correlation between grout pressure and grout take and was found to depend on the other major variable, the permeability of the strata and/or fault(s). Water test data can give an indication of the permeability characteristics of the fault and was used to obtain a measure of the conductivity of grout. The analysis provided a means to estimate an effective radius of grout penetration. An estimated effective radius of 1.9m for the grout bulb compared well with the 4m drill hole toe spacings used in the grout program.
Analysis of longwall support pressure data was undertaken to investigate strata – support interaction in the fault zones. Specific characteristics of the data that were investigated included:
- Variation in chock performance characteristics in pressure plots
- Variation in strata response in loading rate plots
- Loading rate analysis to delineate weighting periods
Longwall support monitoring and load rate analysis shows considerable promise in providing a predictive capability for assessing caving characteristics and load transfer across faults. In particular, the analysis could be closely correlated with the position of the fault(s). In areas where poor roof was encountered, the load rate plot showed almost no load transfer across the fault zone. When considering the relatively large grout take, the results of the support load rate analysis and the overall mining performance, fault stability is likely to have been significantly improved due to the pre-consolidation effort.