| Summary: | Student Number: 0420801P
Master of Science in Engineering.
Faculty of Engineering and the Built Environment === Underground observations at a coal mine indicated failure of the immediate roof
above the bords while pillars were observed to be intact. To determine the
underlying causes of roof failures, careful observations and photographic
recording of occurrences of roof failures have been made. Rock samples of the
immediate shale roof were collected for laboratory testing to determine the rock
strength and deformation properties. Numerical simulations were carried out to
analyse stress and strain distributions and also to attempt to explain the guttering
process. Mapping of roof failures showed that these took place mainly towards the centre of the roadways. The roof failures, termed “roof guttering”, were observed to
occur violently and with little warning. Occurrence of roof guttering had a
negative impact on production. Some panels are abandoned, production times have increased and safety of workers is compromised. The mine authorities
initially thought that roof guttering was caused by shear failure of the rock mass.
Roof bolts are therefore used as a means of primary support. No improvements
have been observed. Increasing the size of pillars has not solved the problem
either. It has only increased the amount of coal left in the pillars without any
improvements in reducing roof failures.
Stress measurement results carried out in 2001 showed that high horizontal
stresses exist at the mine. The immediate shale roof was observed to be weak.
Laboratory testing showed that the shale rock is transversely isotropic. Numerical
modelling results indicated that there are insignificant stress concentrations
towards the centre of the roadway using the elastic and transversely isotropic
elastic models. Stress concentrations were predicted at the roof-pillar contact area.
It is therefore expected that failure should initiate and occur at the roof-pillar
contact area. The Mohr-Coulomb and Mohr-Coulomb strain softening models
predicted shear failure at the roof-pillar contact area. The two models over
predicted the depth and under predicted the width of failures. The extension strain criterion predicted correctly the depth and width of failures
although the failures were predicted at the roof-pillar contact area while the
observations indicated failure mainly towards the centre of the roads. Initiation of failure was predicted ahead of the coal face at the centre of the road position using the extension strain criterion. Although none of the constitutive behaviours predicted correctly the observed
underground failures the extension strain criterion has shown the best agreement.
Guttering that occurred at the roof-pillar contact was modelled successfully using
the extension strain criterion. The extension strain criterion predicted initiation of
failure ahead of the coal face at the road centre position. It is possible that fracture
initiation could be taking place in this location ahead of the coal face, and, on
blasting the rock that has been fractured falls forming a gutter at the centre of the
road.
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