Underground » Mining Technology and Production
This project had the basic objective of demonstrating that powered support monitoring data could be of use in face management and operational decision making on longwall faces. The project has shown quite clearly that this is the case and has made clear recommendations as to how the data needs to be processed in order to be of use. By recourse to specific examples at both Newstan and Appin collieries in relation to roof falls on the longwall face and gas-outs respectively, positive guidance as to how that information may be interpreted has been given.
The INSITE System
A prototype data processing and presentation component of Longwall International's INSITE system has been developed during the project, but as yet, the accuracy of the algorithm used by the software is insufficiently proven. It is expected that as several collieries at present are using or are intending to use this system, the reliability and accuracy of the system will become apparent.
The Strata Weighting Model
On a more fundamental basis, the two investigations conducted by this project have allowed the conceptual model of strata weighting that was first postulated by ACIRL (1993), to be strongly substantiated and further refined, to incorporate other more subtle features that were not evident in the more general monitoring data of that first project. As a first, an initial relationship has been presented that links the nature of near seam massive strata units to the potential for roof falls to occur on the face. This is a departure from much of the previous work on longwall face support whereby the focus has been the rating of the powered support.
Newstan
The work at Newstan showed quite clearly that there are weighting conditions resulting from very thick, near field massive strata units that can significantly increase the likelihood of roof falls on the face. Despite quite competent immediate roof conditions above the face at Newstan, the weighting process caused substantial and very costly delays to face production.
The only control of such strata appears to be the use of panel width in order to change its mode of behaviour and protect the face area from the weighting effects of the strata.
A very strong link between geology, powered support monitoring data, weighting and face problems has been established. It is evident that an insufficiency of detailed geological information (and interpretation) prior to mining can be supplemented by powered support monitoring information to give a real-time picture of how the overburden is behaving.
The exact reason as to why the overburden is behaving in a certain manner, at a given point in time, during the production phase, may not be known. Nevertheless significant geological changes or anomalies will show up in the monitoring data and the face management process can be tailored to take account of it. During the course of the Newstan investigation, unexpected and unexplained short-term changes to the weighting of the overburden occurred, the knowledge of which allowed the appropriate decisions to be made to the economic benefit of the colliery.
Appin
The Appin investigation has generated links between two distinct weighting mechanisms of the overburden and high levels of gas ingress onto the face, one being a result of thick massive strata units in the overburden, the other being related to localised vertical structural features in the rock mass. It is apparent that other factors are also involved in the problem and that the relationship between an adverse weighting occurrence and a gas stoppage being a result is not unique. This does not detract from the use of weighting considerations in the investigation and management process, but means that these other factors also need to be considered.
The data which has been generated, interpreted and applied is quite limited. The interpretations which have been made are based on the best understanding at present, but only time and further more detailed studies will show how accurate or inaccurate they are.
The use of micro-seismic monitoring techniques is recommended to improve our understanding of the fundamental processes that accompany longwall mining. These techniques can pinpoint the nature and location of the fracturing process above a longwall face which determine the weighting phenomenon described in this report.
3D seismic surveys have recently been used at Clarence Colliery and Newstan and West Wallsend Colliery will soon have a comprehensive seismic monitoring system in place. Furthermore, large scale investigations at both Appin Colliery and North Goonyella Colliery on similar lines are planned. It would appear therefore that the results of this project will be put to the test in a variety of locations in the very near future.
Conclusions
The strata control related events and monitoring information from LW5 at Newstan are unique in the history of Australian longwall mining and probably worldwide. The face was affected by a wide range of geological conditions that were dominated by the presence of a massive sandstone/conglomerate channel up to 45m thick, situated in close proximity (5 to 25m) to the seam horizon. This had a significant impact on longwall face conditions.
Overall, the project has achieved the objectives that it set out with. From the results of the investigations, it is definitely worthwhile making powered support monitoring data processing and presentation, in a useable format, an automatic part of the data presentation package at surface.
The follow-on work being undertaken at both Newstan and West Wallsend collieries will use powered support monitoring as one of the tools in the monitoring process in conjunction with seismic monitoring of the overburden.
However if the use of such monitoring is to become routine, it will need to be in an automatic format if it is to be embraced by mine operators. The first attempt at this has already been incorporated into INSITE and during subsequent exercises, it is hoped that the robustness or otherwise of the algorithm will be evaluated.
The experiences of LW5 at Newstan and LW25 at Appin have shown that there is a fundamental process occurring above a longwall face, which in certain combinations can result in very difficult mining conditions. This process is mirrored strongly in powered support loading behaviour. It is also evident that the data and knowledge of the weighting mechanism at work in real-time that potentially can be gained from powered support monitoring, is a useful tool in the face management process for both problems examined by the project. On more than one occasion on LW5 at Newstan, the information was used to make a decision subsequently proved to be correct, which could not have been made had the data not been available.
Providing powered support monitoring systems are further developed to a point whereby relevant information is presented in an instantly useable format in a highly reliable and proven manner, there is no reason why they cannot become an operational tool in the face management process where weighting conditions that might cause difficulties exist.
A prototype system Is currently available as a result of modifications made to INSITE during this project, but it is felt that there is still a deal of system proving required before it could be considered as a fully integrated operational tool.