Underground » Mining Technology and Production
Underground augering has the potential to recover isolated, irregular pockets of coal that would ordinarily be discounted from reserves as being uneconomic or impractical to extract using the main mining system. A key design issue is the stability of narrow coal webs between auger holes and overall panel geometry.
Accordingly, the overall objective of this project was to d evelop credible guidelines for the design and operational management of safe and productive underground augering layouts utilising a systematic process of initial extraction design, performance monitoring, back-analysis and re-assessment ?
An analytical methodology was developed for the initial design of augering panels, which recognises the contributions of the various structural elements (ie coal webs and intra-panel pillars), as well as likely overburden behaviour.
The project involved monitoring ground behaviour during augering, with the intention of monitoring several sites with varying geotechnical environments and developing guidelines from these to assist in future layout design. This approach is appropriate where the mining layout involves the complex interaction of several components that cannot be readily simplified to the extent necessary for numerical or physical models to play the primary role. For reasons unrelated to geotechnical aspects, only one site was secured within the project time frame. Consequently, the project has utilised the results from a Southern Colliery augering trial, coupled to the outcomes of numerical and physical modelling tests.
The auger mining operations themselves were carried out by a Joint Venture ( Coal Recovery Australia Pty Ltd ) between Cutting Edge Technology Pty Ltd and SBD Services Pty Ltd.
The underground trial indicated that empirical design methodologies involving pillar strength equations coupled to abutment angle models can be used to design stable augering layouts. Although the designed hole configuration was not fully achieved, there is, nonetheless, a suggestion that a layout so determined will be conservative, holding out the possibility of future optimisation on the basis of actual performance. Monitoring and re-appraisal in the context of a formal strata management process are critical to the success of any such approach, particularly in terms of optimisation.
The two-dimensional UDEC numerical modelling code was used to model augering webs, but seemed to underestimate the stability of an auger mining panel, while over -estimating the strength of individual auger webs. Physical tests appeared to give a realistic quantification of the size effect (ie the beneficial effect on web strength of the hourglass shape when compared to a rectangular web of equivalent minimum width). The tests suggest that determining the strength of an hourglass web by increasing the strength of an equivalent rectangular web by 25% would be a justifiable step at this stage (ie this is a conservative preliminary conclusion for design purposes). It would be worthwhile to conduct further physical tests on differing materials and with various configurations to improve the quantification of the shape effect.