Underground » Strata Control and Windblasts
Massive strata overburden units are known to influence support loading on longwall faces. Past studies of the conglomerates in NSW and sandstones in Qld have identified factors such as unit thickness, proximity to coal seam, immediate roof strength and panel width that may all play a role in support loading and in the development of adverse ground conditions. Other controls such as cut height, cutting method, hydraulic supply, leg pressure control parameters and yield settings can also influence ground behaviour.
It is well known that face stoppages and/or differences in retreat rate can affect mining performance. What is unclear is the influence of time related strata response on support loading and its relationship to the factors such as those described above. There is a need to develop a view to understanding the relative changes in behaviour from one set of conditions to the next.
The interaction between longwall supports and the surrounding strata is a complex phenomenon. At present neither empirical nor numerical models can adequately capture the critical factors required to predict strata response. However, recent advances in the ability to analyse longwall monitoring data such as that developed by Longwall Visual Analysis (LVA) provide a potentially large and valuable data source to quantify time related factors. It also provides a means in which to assess how operational practice can influence shield behaviour.
Through previous ACARP projects we have developed an approach to characterise ground conditions using borehole geophysical logs. One aspect of this is the Geophysical Strata Rating (GSR), a rating scheme devised for coal bearing strata. Using geophysics data provides a high density and cost effective means of gathering geotechnical information that enables development of 2D and 3D models of strata characteristics. The project aimed to take advantage of GSR estimates to provide a practical means to classify or identify features that affect caving behaviour.
Project objectives
The main objective of this project was to analyse various longwall operations using the GSR to characterize the strata, assess the likelihood of weighting and then correlate this with the various outputs that can be provided by LVA. A significant advantage of integrating GSR and LVA datasets is to allow a 3D spatial understanding to be developed between strata characteristics and various support loading related parameters. Data from three sites were used for the study namely, Moranbah North, Dendrobium and Newlands Mines as they represented a range of conditions and locations.
Through a combination of strata characterisation, interrogation of monitoring data, statistical analysis and geotechnical modelling the aim was aim to identify the key factors affecting longwall ground response and their relationship to caving behaviour. Outcomes were anticipated to include a measure of the likelihood of weighting events, potential for development of face cavities and criteria to assist in future longwall support design, strata characterisation and face management.
Main findings and conclusions
A caving chart has been developed based on a combination of previous experience in longwall support assessment, strata characterisation, leg pressure data analysis and caving behaviour. The chart provides a link between strata conditions, stresses, panel layout and anticipated support loads via design thresholds that are related to roof convergence. The intent is to provide a means to assess the risk of cavities in the immediate roof and/or the risk of heavy weighting from the overlying roof units.
Three case studies are provided that test the capabilities of the caving chart in terms of differences in strata conditions and panel layout. It is also demonstrated that whilst the risks were similar, the causes of instability were different. At Moranbah North, the influence of different support types over similar conditions with a weak coal roof and supercritical layout were investigated. It is suggested that geotechnical factors had a considerable influence, which have driven a change in system design that subsequently appears to have enabled operational improvements.
At Dendrobium, an interbedded roof in a deeper, sub-critical layout was investigated. Instability appears to have been partly driven by the presence of weak interbedded roof below a channel but also probably due to a longwall system working at the limit of its capability. This has probably resulted in an uncertain operational environment leading to further instability. At Newlands, it appears that geotechnical conditions, i.e. relatively shallow conditions with strong coal roof have been made more difficult through poor system availability and associated limitations in operational response.
In reviewing the data it is evident that the caving chart can provide the broader setting for design, risk assessment and planning whilst longwall monitoring data can be used to disseminate key parameters from the daily records. A direct convergence estimate using leg pressure data will obviously be augmented in future by convergence monitoring. This will be a useful advance.
Convergence estimates from any particular shield alone however is unlikely to provide the necessary detail for a reliable real-time cavity risk indicator. A more sophisticated algorithm that uses each part of the load cycle in conjunction with continuous load rate and direct convergence measurement will be required for short-term longwall face stability assessment. The algorithm will also need to incorporate factors such as number and effect on adjacent supports, standing time and the influence of varying set pressure on each load cycle. This is likely to require site-specific assessments that take account of operating practice, longwall support configuration and prevailing ground conditions in order to provide a reliable quantitative outcome.
Cavity risk indicators such as the CRI or other parameters are a welcome addition to longwall TARPs. In this study a caving chart has been developed that aims to provide the setting in which different TARPs may apply. In doing so it is hoped that it will assist in defining relevant controls albeit operational or otherwise.