Structural effects on the strength of New Zealand coal

Abstract

To define the behaviour of rocks under stress, certain material properties are required to characterise deformation and failure. The laboratory unconfined compressive strength (UCS) and triaxial behaviour forms the basis of many failure criteria and pillar strength equations. It has been well established that the UCS of rocks is greatly influenced by sample size and structural discontinuities. This is particularly true for coal which has a microscale fracture pattern (cleat) throughout its fabric. The aim of this study was to investigate the relationship between strength and structure and relate this to field performance. Results from laboratory strength tests with petrographic and thermal analysis were collated with other test data on the coal. Samples were selected from a thick, sub bituminous coal seam in the Waikato Coal Measures. This coal is a relatively weak compared to strength data from published sources and it has a defined cleat system. The coal was found to vary in composition throughout the seam section with strength differences partially attributed to material variations as well as preparation and structural controls. The thick seams are mined by a modified room and pillar method which leaves slender yield pillars on retreat. These pillars are essentially large-scale UCS samples. In the field the geometry of the pillars was recorded on completion of mining and the cleat was mapped where possible. It was found that the laboratory results and the historical case study information correlated well with pillar strength formulae. However when compared to the slender yield pillars it did not predict performance. Numerical modelling also failed to represent the observed behaviour. This suggests a different mechanism is involved which is possibly structurally controlled, where the intact strength of the coal appears to be less important than the structure and geometry of the excavation.