Underground » Mining Technology and Production
This project has conducted fundamental research into the weighting behaviour of thick and massive strata units in close proximity to longwall extractions, the effect on mining, and means of effective control (primarily restricted panel widths). The impetus for the project was the catastrophic roof control problems that occurred on LW5 at Newstan in 1994 and 1995 which brought into question the proposed mining layouts of both Newstan and the neighbouring West Wallsend Colliery.
Both mines have used a number of different panel widths under varying geological conditions, with intensive overburden monitoring being used to establish the mode of behaviour of the massive strata and the potential for incrementally widening future panels. The aim has been to maximise the amount of coal extracted for each panel whilst eliminating uncontrollable Periodic Weighting effects.
The project has successfully facilitated a panel width increase from 90 m initially to the current maximum width of 150 m in a number of increments, with no uncontrolled Periodic Weighting or consequent roof falls occurring during extraction.
The total mining experience under thick massive strata at the two collieries has been formulated into a "classification" which relates massive strata thickness and panel width to the resultant weighting behaviour of the unit. This has been used in the incremental widening of extraction panels and will probably allow further increases to around 160 m in the near future. Other longwall faces at other mine sites (Australia and world-wide) with massive strata related problems have been incorporated into the classification and reasonably good agreement has been found with the Newstan / West Wallsend experience.
The results of the monitoring program have shown that the conceptual model of massive strata weighting used in justifying the control approach of restricted panel width is essentially correct.
Other important impacts on mine planning and management under massive strata conditions have become apparent during the course of the project, namely abnormally high tailgate loading conditions in areas adjacent to bridging strata, the effects of channel edges and also that of major windblasts.
The project objectives were to:
- develop and prove techniques by which narrow longwall face widths can be maximised, yet still prevent major roof control problems from occurring due to the uncontrolled weighting of massive and thick channel type sandstone /conglomerate units
- assess the effect (if any) of using wider longwall panels (at depths of cover up to 300 m)
- examine the effect (if any) of weighting of the overburden on gate road roof and rib behaviour and stability
Due to the proposed mining geometry for Longwalls 11 and 12 at West Wallsend Colliery, it became apparent that a comparison could be made between longwall face widths of 200 m and 240 m (in areas not effected by the massive channel) under relatively similar strata conditions. Whilst the proposed monitoring studies were quite specific to the area of channel affected extraction, comment can still be made on this issue on the basis of general mining experiences associated with both face widths.
In addition, as a result of a program of roof stability monitoring and strata management that is routinely used at West Wallsend, the opportunity to compare roof stability and control during extraction associated with a wide range of panel widths and caving conditions was recognised and comments will be given on this issue as part of the report.
In addition to the objectives stated at the outset of the project, the problem of major windblasts during extraction with narrow face widths became apparent at Newstan Colliery and a substantial amount of effort and resource was committed (by Newstan) to the understanding of the geological and mining conditions under which they occur and also in their prior prediction using micro-seismic monitoring. Whilst a full description of the results of that work will not be given herein, a brief summary will be made.
Two other aspects worthy of mention have become apparent during the course of the work at both collieries, namely tailgate roadway roof and rib control associated with minimal caving in the previous panel(s) and also general roof stability in roadways located beneath the edge of the channel unit. Both issues have resulted in ground stability problems at both the host mine sites and are by-products of the presence of the channel and/or the chosen mining geometry.
Conclusions
At the commencement of this project in the middle of 1995, there was no available experience or guidance on the issue of utilising longwall panel width as the control for massive strata of sufficient thickness to pose a significant risk to roof control on the longwall face. This project therefore represents a world's first study in what is a vital design aspect for two current Australian longwall mines with potentially a number of others facing similar strata conditions in the future.
The methodology of the project has been to extract initially at conservatively narrow panel widths and incrementally widen future panels based on the success of previous extractions panels. In conjunction with these "experimental" longwall panels, overburden monitoring in the form of powered support behaviour, micro-seismic monitoring and surface subsidence measurements has been conducted as well as a significant amount of core drilling to define the nature and variations within the massive channel units.
The project has facilitated an increase in useable panel width from 90 m (initial panel width on LW6 at Newstan) to 150 m (used on LW's12 to 16 at West Wallsend) and proven that uncontrollable weighting of massive strata is prevented in that range. However, when using panel widths at the top end of that range, rigorous face management is required in order to ensure that less significant weighting conditions do not cause roof control difficulties through aggravation by operational influences (e.g. a stood face for an extended period under heavy loading conditions).
The current state of knowledge suggests that further panel width increases are possible whilst still preventing uncontrollable weighting from occurring, although increments of panel width increase must reduce on each occasion that an increase is implemented. The next panel width attempted will probably be in the order of 155 to 160 m.
Monitoring data obtained during the project (in particular the micro-seismic data) has shown that the conceptual model of massive strata behaviour used to justify the control approach of panel width is essentially correct. The model has been re-defined to incorporate the recognition that the key issue in massive strata behaviour is the point (with respect to the face) at which it becomes unstable and mobile rather than simply the onset of fracturing within it. Any massive strata that becomes unstable at a point ahead of the face will naturally rotate and subject the face to potential roof control problems through the formation of multiple sets of high angle fractures in the measures above the face.
Conversely, instability in massive strata which commences behind the face above the goaf will have no significant impact upon face roof stability.
The nature of the measured micro-seismicity within the massive strata provides a very strong link between its behaviour up to the onset of instability and the concept of a Voussoir Beam (structural engineering analytical model that describes the stability and failure of a blocky beam). The significance of the internal compression arch within a Voussoir Beam in preventing massive strata becoming unstable ahead of a retreating face is clear.
Channel conditions resulting in a bridging condition above the face give rise to high magnitude compressive failure within the massive strata unit some distance behind the face above the goaf or no failure at all. The former is evidence of internal failure within the Voussoir Beam which if it continues to a sufficiently advanced state will result in complete instability and caving of the strata unit.
If instability commences ahead of the longwall face (through vertical shear failure at the beam abutments), the behaviour of the now mobile massive unit in a longwall sense is then governed by the principles of a failing cantilever with its fulcrum point some distance ahead of the face.