Analytical Tools For Managing Rock Fall Hazards In Australian Coal Mine Roadways

This report provides a reference source for the design of ground control measures in coal mine roadways using analytical methods.  Analytical methods are based on identifying potential failure modes from knowledge of the geology, followed by simple analyses based on hand calculations, spreadsheets and design charts.  It is intended that this report will complement other publications that provide empirical and more intensive numerical approaches.

Coal mining is conducted in layered materials - different rock types, different strengths, different layer thickness, different joint spacing.  Underground coal mines openings are rectangular, and at least in Australia, the roadway axes are parallel to the dominant discontinuity sets - bedding and joints.  The stress field on development has the principal axes parallel to the roadway axes, and there can be major changes in and around extraction panels.

These geometrical factors require a different approach to the empirical rock mass rating approaches used in metal mines.  Fortunately, the comparatively simple geometry allows analytical methods based on identifying collapse modes associated with jointed and bedded material.  This ability to analyse the relatively simple geometries allows a more anticipatory approach to ground control management including anticipation of geological conditions, prediction of collapse modes, the design of support or reinforcement system, and the monitoring of ground conditions for exceptions.  The approach provides more robust outcomes and limits the demands on the observational method.  

Collapse models are provided for roof and rib.  The roof models recognise that different collapse modes can apply in different stress fields - high, intermediate, and zero compressive stresses.  The rib models draw analogies to rock slope stability and also the impact of high vertical stresses.

Methods for determining support or reinforcement requirements are provided.  Suspension of collapsed masses is identified as the basis for roof support in both very high and zero compressive stress regimes.  Reinforcement of bedding discontinuities is advocated for intermediate compressive stresses.  For the ribs, restraint of coal blocks defined by pre-existing joints or by mining induced fractures is required.