Observations and analyses of roof guttering in a coal mine

Underground observations in a coal mine have indicated failure of the immediate roof above the bords. Careful observations and photographic recording of occurrences of these roof failures, termed 'roof guttering', have been made. The occurrence of guttering results directly in roof instability, and indirectly, due to the interaction of the guttering with natural planes of weaknesses in the immediate roof rock. In this paper, relevant observations of roof guttering, possible mechanisms of formation of roof gutters, laboratory testing for determination of material strength and deformation properties, and numerical analyses to explain underground observations are described. Mapping of roof failures showed that these took place mainly towards the centre of the roadways. Roof guttering was observed often to occur violently and with little warning. occurrence of roof guttering has a negative impact on production-some panels are abandoned, production times are increased and safety of workers is compromised. The numerical stress analyses showed that, although none of the constitutive behaviour criteria used predicted the observed underground failures correctly, the extension strain criterion gave the best agreement. Once the root cause of roof guttering is understood it may then be possible to contain failure by optimization of excavation shape and positioning, or through the design and installation of an effective and efficient support system. The consequence would be improved safety, which should result in maximum extraction of the mineral reserves in a safe manner. The material in this paper contributes towards the understanding of stress distributions around bord and pillar geometries, the associated stress induced instability observed in a coal mine, and a better understanding of the guttering phenomenon.