The design of longwall gateroad roof support for roadway development and panel extraction has typically relied on past panel experience. If strata conditions and depth of cover are reasonably uniform, this approach has merit. However, as geological or geotechnical conditions vary the need for a more rigorous design is required to provide safe working conditions and the calculations and justification for compliance and statutory purposes. Current roadway roof support design approaches have limitations, which have restricted their applicability. Analytical methods are available and are certainly applicable for more massive sedimentary rock-masses, but may not be appropriate when rock-mass yield occurs due to high mininginduced stresses. The application of complex post-yield numerical modelling in a periodic and routine design process for excavation support is currently a contentious issue, which requires better justification and industry understanding of its limitations. Complex post-yield numerical modelling is challenging, time-consuming and expensive, and typically outside the capability and resources of mine site geotechnical engineers. It does, however, have its place and will continue to make important contributions. A design method has been developed incorporating the major geometrical, geological and geotechnical parameters required to specify roof support. It is a hybrid numerical and empirical method called Gateroad Roof Support Model (GRSM), where specification of roof support comes from charts or equations. Empirical design techniques are currently used in Bowen Basin mines and should be used to provide a complementary design. They do not explicitly consider all the parameters and mining situations that GRSM does. In some aspects, GRSM may be considered as a progression of these current empirical methods. GRSM defines suggested roof support densities by linking a rock-mass classification with an index of mining-induced stress, using a large empirical database of Bowen Basin mining experience. Inherent in the development of GRSM is a rock-mass classification scheme applicable to coal measure strata. From numerous schemes assessed, two were considered appropriate. Coal Mine Roof Rating (CMRR) is an established and robust coal industry standard, and Geological Strength Index (GSI), as it provides rock-mass geomechanical properties. GSI would only be required if additional numerical modelling was conducted. An approximate correlation between GSI global rock-mass strength and CMRR has been presented. An elastic three-dimensional numerical model has been established to calculate an index of mining induced stress, for both roadway development and longwall retreat. The model anticipates height of strata caving and fracturing and the process of goaf reconsolidation. To effectively use GRSM it is important to be able to quickly and accurately calculate a stress index, without having to resort to a numerical model. Equations to calculate stress index have been developed for two situations; roadway development and longwall retreat. Installed roof support must be characterised and quantified, both to establish the empirical database and to specify suggested support densities and patterns for a design. An industry standard method of quantifying roof support was adopted as a base template (GRSUP). General design charts, utilising all data points and long-tendon support were also constructed for roadway development and longwall retreat. The logistic regression analyses considered three independent variables; roof classification, stress index, and various modifications to GRSUP. The logistic regression analyses indicate that an improved quantification of installed support can be gained by simple modifications to the standard formulation of GRSUP. Initial stable-failure boundaries were determined mathematically using an optimal statistical solution from the logistic regression. The position of the stable/failed boundary can be changed depending on design criteria and specified risk. Considering the probability of a stable outcome more appropriate and conservative stable/failure boundary can be defined. This initial version of GRSM provides suitable estimates of required roof support for both roadway development and longwall retreat in the maingate belt-road. It appears to be suitable for a range of immediate roof conditions, including coal. The design method is uncomplicated to apply when using the supplied equations, and can be readily set-up in spreadsheet form. Calculated GRSUP is only a first-pass assessment, and this is all it was ever intended to be. Design optimisation always needs to occur, based on local conditions and experience.
| Identifer | oai:union.ndltd.org:ADTP/252421 |
| Creators | Lawrence, William John Charles |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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