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Investigation Into Roadway Roof Support Design Using the Geophysics Strata Rating

Underground » Strata Control and Windblasts

Published: March 15Project Number: C22008

Get ReportAuthor: Terry Medhurst | PDR Engineers

This project was aimed to investigate methods of using roof monitoring data in combination with geophysics based models of strata conditions for roadway roof support design. The project has enabled development of an analytical model/graphical technique that provides a measure of both support load and roof convergence which can be matched and updated against roof monitoring data. The model is based on beam-column principles and incorporates bending, horizontal loading and shear. Using this model, estimates of roof convergence for various heights of softening (or surcharge loading) above a roadway can be obtained for a given support pressure. The model relies upon inputs from the Geophysics Strata Rating (GSR), roof bolt pull-out stiffness/load, H:V stress ratio and UCS.

 

Our proposal was to undertake a preliminary investigation to assess the viability of the concept at a few sites to determine if a more comprehensive design methodology can be developed. This has been achieved by analysis of data from three sites, namely Crinum, Oaky North and Grasstree.

 

Crinum Mine - the approach was applied in a very weak roof environment under a wide span, longwall installation road setting. An ability to analyse staged excavation and support installation was demonstrated along with the ability to assess the requirement for remedial support in a high convergence environment.

 

Oaky North Mine - unforeseen roof control problems were encountered in the development of some Mains headings. A comprehensive study was undertaken that showed that GSR modelling could identify the problem roof areas. The associated support assessment was able to demonstrate the difference in support response in stable and unstable zones. Timing of support installation, grout requirements and support type were all identified as key factors in the analysis.

 

Grasstree Mine - the effect of changing roof type, depth and structure were investigated. A significant database of roof convergence data was analysed. The stability analysis showed how relatively small changes in roof conditions can lead to quite different outcomes, particularly for stiff immediate roof strata.

 

Through analysis of the various examples, relationships were identified between roof conditions, depth, bolt placement, bolt length, support density and timing of installation. In particular the project has been able to quantify the relative roles of primary and secondary support and thus provide an opportunity to optimise the support cycle, particularly in weak roof.

 

The approach provides the ability to investigate support behaviour in relation to different types of roof conditions and support strategies. This flexibility, whilst powerful for investigating different behaviour mechanisms, also requires some consideration in its application. Detailed stability analysis in combination with convergence monitoring has many obvious benefits. In some instances, the height of softening, choice of roof interval and associated trigger point may be easy to define. In other cases it will not, such as that governed by complex stress/structure affected zones or unidentified blocky roof behaviour.

 

This research has provided some indicators at to what may guide various inputs. For example, the GSR/σv ratio and particularly the GSR/σh ratio appear to be a useful first pass indicator of potential conditions when no other information might be available. In combination with GSR models, additional tools could be developed in combination with stress maps to allow better prediction and hazard planning assessments.

 

The approach appears valid in weak roof and/or high stress environments where the roof deformation behaviour is readily applicable to the underlying analysis framework. In this regard, the influence of laminated and/or micaceous type roof conditions is of interest. The current research has identified that GSR analysis is able to detect the influence of clays on roof conditions. However it appears that the use of cross-plots may provide an additional input that may help to define roof that is particularly susceptible to stress related influences. Further work is required in this area, but initial indications suggest that when clay content exceeds a specific threshold, i.e. 40% to 50% in a laminated structure, then the roof may be more susceptible to failure. Further work is required to determine if such behaviour should be included in a GSR estimate or as an additional factor to be used in a support assessment.

 

The ability to develop the methodology into a site based approach has obvious advantages, but this requires further testing. In particular, areas that require further work include:

· Dependence on modulus estimates and associated properties in relation to the beam formulation requires further testing;

· Applicability of suitable stress concentration factors to be included in the analysis with reference to both development and longwall;

· Choice of parameters for intersections;

· Factors affecting the definition of "laminated" or "micaceous" weak roof and associated impact on analysis.

 

Some of the major mining companies are undertaking their own GSR analysis for planning purposes. As a result, the use of GSR for roof support assessment is already being tested to some extent by some mines. Further research is proposed to extend this approach to a more general framework and design methodology applicable to all underground mines.

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