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Assessment of Convergence Based Roof Support Design For Longwall Abutment Loads

Underground » Strata Control and Windblasts

Published: February 19Project Number: C24015

Get ReportAuthor: Terry Medhurst | PDR Engineers

The aim of Stage two of project C24015 was to develop a roof support design approach that takes account of differing roof conditions, effect of support type and stiffness that can be used for mine design and in the strata management process. An analytical framework was developed for roadway development that provides a measure of both support load and roof convergence which can be matched and updated against roof monitoring data. It is based on beam-column principles and incorporates bending, immediate roof failure and shear. The model relies upon inputs from the Geophysical Strata Rating (GSR), roof bolt characteristics including pull-out stiffness/load, in-situ stress ratio and unconfined compressive strength (UCS).

One aspect arising from the original study was how to deal with stress changes and include them in the roof beam model. This requires consideration of depth of cover, roadway orientation, horizontal stress regime, local mining induced concentration effects and differences in roof quality. At the completion of stage one the extension was proposed to assess, calibrate and extend the analytical roof beam model for longwall abutment loading conditions.

A review of available roadway instrumentation data was completed, and a series of 3D modelling analysis was undertaken to examine stresses about longwall panels. This analysis was combined with previous studies of longwall panel stresses (ACARP C20031) and other experience means to assess factors affecting abutment loading. These factors were then used to determine stress inputs to the beam-column analysis.

Conditions with a maingate stress concentration were also investigated. It was found that roof stability was highly dependent upon whether stress concentrations were aligned with the major or minor principal stress direction. The analysis showed that stress rotations, lateral stress relief, the magnitude of the minor principal stress, degree of bedding and its angle to the stress can all be as important as the magnitude of the maximum mining induced stress across the roadway. In some cases, cancellation effects can occur and in other cases, high levels of stress and damage can develop. This needs to be considered when assessing roof support performance and in determining stress related inputs for support design.

Tailgate loading is known to be highly dependent upon the magnitude of shear stresses that can develop. Maximum shear stress is related to the magnitude of differential stress (σ1 - σ3) and thus the amount of stress relief (σ3) is important. Tailgate loading conditions with major and minor stress notching were analysed. The analysis suggested that side abutment factors for tailgates generally range between 1.4 and 1.8 depending upon panel orientation. However, the presence of additional stress concentration effects can significantly alter this general condition by influencing the magnitude of differential stresses than may or may not develop. An increase in the minor principal stress can reduce the potential for roof shear, where notching in a direction that can increase the major principal stress is likely to have the opposite effect.

Stress inputs to the beam-column analysis are provided based on estimates of the GSR/σm ratio. Stress concentration factors are provided to determine the maximum stress (σm) for roadway development, which also include abutment load factor adjustments for longwall conditions. Workflows are presented for serviceability assessment and shear strength design.  

For serviceability, roof convergence is estimated in relation to both height of softening and installed support. A series of ground response curves (GRC), i.e. roof convergence vs support load, can then be plotted for different heights of softening.  Acceptable levels of roof convergence can then be determined in relation to support capacity.  

Shear strength analysis is based on the concept of shear flow induced by beam bending. The beam section properties are obtained from GSR analysis, which is then used to derive an estimate of Factor of Safety (FoS) against bedding plane shear when divided by the shear strength of the bolts. The approach does not address shear failure mechanisms associated with failure along geological structure or key block style failure mechanisms.  

Bending is often a pre-cursor to the development of more complex progressive failure mechanisms that occur in coal mining environments. In soft rock/high stress environments both strength and serviceability criteria are required to assess the high deformation conditions that often develop. The methodology provided here attempts to incorporate these factors in an analytical framework that can be used as part of the strata management plan.

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