Underground » Strata Control and Windblasts
Recognition of horizontal stress concentrations about longwall panels was made during the 1980's and 90's. The nature and factors which influence the stress concentration has become more apparent with further
experience and study of the phenomenon in a wider range of geotechnical environments.
The work contained in this report is a summation of the initial work conducted up to the 1990's and a review of additional monitoring, observation and computer modelling of the phenomenon.
Overall, the stress concentration concept as initially delineated has been confirmed and better defined in terms of influencing factors.
The factors that have been found to influence the nature and magnitude of the stress concentration typically relate to the actual geometry of the caved zone about the longwall gate end, the relative magnitude of the horizontal stresses and the elastic properties of the strata about the coal seam.
The stress relief adjacent to a longwall panel has been found to be significantly influenced by the youngs modulus and poisons ratio of the strata surrounding the coal seam. In thick coal or soft materials, the stress relief is not well developed, whereas in stiff materials the stress relief very well developed.
The horizontal stress concentration is created by stress redirection about the caved zone of the longwall. This caved zone may have variable dimensions depending on the geological nature of the overburden. Monitoring and observation demonstrated that the existence of a massive and strong sandstone in the immediate roof may inhibit, caving close to the face and as such the stress concentration effect may not occur. In cases of weak laminated material, the caving may be enhanced close to the face. If gateroad deformation occurs then the geometry of the gate end can be locally modified and cause increased stress concentration effects.
The ratio of the horizontal stress components (stress ratio) has been reviewed for Queensland and NSW. The ratio is typically 1.4-1.7 however local variation occurs due to structural and tectonic features.
The maximum stress concentrated about a longwall may be directed oblique to the gateroad. The stress component which appears to have the greatest impact on roadway deformation is that which acts perpendicular to the gateroad. Graphs of the maximum stress and the component perpendicular to the maingate (tailgate) are presented in the report.
Back analysis of a number of mine sites using field monitoring data and computer modelling of roadway deformation has provided validation of a method of predicting the deformation which would result from various mine layout options and stress regimes. It was found that the average roof displacement in a gateroad was satisfactorily predicted by modelling of the strata section with the anticipated stress concentrations and reinforcement pattern.
It was also noted that there is variability within the monitored data which is a reflection in variability within the geotechnical and stress environment.
An approach for optimisation of the mine layout with regard to stress concentration effects on the maingate and tailgate of a longwall has been presented. This approach is based on the optimum roadway stability during development and minimisation of the stress concentrations during longwall retreat.
In general, the layout of gateroads is a function of the severity of deformation to be experienced for the various angles between the gateroad and the stress field. However, in cases where significant deformation is expected, the maingate conditions are optimised in a layout for which the angle of the roadway to the maximum in situ stress direction is typically in the range of 0 to -25 degrees. It is noted that in cases of thick coal or soft strata, the relative benefit of directional layout is reduced, however, this needs to be looked at in terms of the overall stability of the roof and floor.
Computer modelling of the stress concentrations about the longwall panels has provided a good correlation with the updated field data. The updated relationship of the maximum principal stress relative to stress angle θsr is presented.
Modelling of the roadway deformation which relates to such stress concentrations has been shown to be an appropriate method to provide a quantitative assessment of the roadway stability and reinforcement options for a range of potential layout options. Such computer methods are continuously improving and are seen as the best approach to assess site specific mine layout options.