Joint Structure and Coal Strength as Controls on Rib Stability

The assessment of the stability of coal ribs needs to consider not only the impact of the cleating and jointing in the coal but also the possible onset of failure of the coal itself. The same planar, wedge and toppling failure modes seen in pitwall slopes can be present in underground roadways. A 20o offset of the roadway from the strike of the through-going joints reduces the fall hazard, but a 35o offset may be required to reduce delays in installing support. Mining-induced fractures (MIF) in coal may be a manifestation of brittle rock behaviour and its preferential location at the top and bottom of the rib and its continuing development outbye of the face can be explained by a dominantly vertical stress field within the coal.