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Geotechnical studies of retreat pillar coal mining at shallow depth Cullen, Michael


This thesis presents the results of research into geotechnical aspects of retreat pillar coal mining at shallow depth (less than 100m). The fieldwork component was carried out over a four-year period at the Quinsam Coal Mine. The geotechnical aspects investigated were excavation stability, excavation support, pillar design, gob cave prediction, and subsidence. These were all considered critical aspects of safe and cost effective mining. Geotechnical design at the Quinsam Coal Mine was initially carried out using existing design tools. For the most part these tools were developed at moderate to deep depth mines (greater than 100m). The suitability of these tools at shallow depth mines was previously not known; as such the primary objectives of this research were: 1) to determine if the existing geotechnical design tools were applicable at shallow depth, 2) to develop new design methods where the existing tools were found to not be suitable, 3) to improve safety, productivity and costs at shallow depth retreat pillar coal mines. Suitable existing tools where found for design of ground support, pillar size, excavation size, and gob cave prediction. Existing tools where not found to be suitable for prediction of caving and surface subsidence. The cave height at shallow depth was determined to extend much higher than predicted by existing methods. The extent of the ground surface affected by subsidence was determined to be much less than that predicted by existing methods. New tools for predicting caving and subsidence have been developed based on measurements at the Quinsam Coal Mine supplemented with numerical modelling. The distinct response of the rock mass to mining at shallow depth is attributed to the magnitude and orientation of the induced stresses. At shallow depth low compressive and tensile stresses are more likely to occur, and more likely to extend for a greater distance into the rock mass above the excavation. Although this work is based on studies carried out at the Quinsam Coal Mine it is believed that the findings can be applied to other shallow depth coal mines in similar geological environments. Since adopting many of the recommendations and design procedures developed through this work, the Quinsam Coal Mine has achieved significant improvements in safety, productivity, and costs.

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