Underground » Strata Control and Windblasts
A longwall gateroad roof support design method for roadway development and panel extraction is demonstrated. It is a hybrid numerical and empirical method called Gateroad Roof Support Model (GRSM), where specification of roof support comes from charts or equations. GRSM defines suggested roof support densities by linking a rock-mass classification with an index of mininginduced 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. Coal Mine Roof Rating (CMRR) is an established and robust coal industry standard, while the Geological Strength Index (GSI) may also be used if rock-mass geomechanical properties are required. An initial correlation between GSI global rock-mass strength and CMRR is presented.
An elastic three-dimensional numerical model was 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 effect 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 derived from the numerical modelling have been developed.
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 is adopted as a base template (GRSUP). A statistical analyses indicated that an improved quantification of installed support can be gained by simple modifications to the standard formulation of GRSUP.
General design charts were constructed for roadway development and longwall retreat based on logistic regression analyses, which considered three independent variables; roof classification, stress index, and various modifications to 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 readily defined.