A study of the mechanical behaviour of coal for pillar design

A series of triaxial compression tests was carried out on 61, 101, 146 and 300 mnl diameter samples to investigate the effects of scale on the mechanical behaviour of coal. The "inherent variability" of laboratory coal strength data was found to be related to the degree of cleating or brightness of the coal samples. Using this observed relationship, a method was developed for estimating in situ coal seam strength bused upon the "intact" properties of dull coal samples and seam brightness profiles. The peak strength criterion for in situ coal uses the parameters sigma(c), m and s of the Hoek-Brown empirical strength criterion for rock masses. Unloading cycles were used to investigate the yield behaviour of the coal samples. The results show that, depending on the magnitude of the confining stress, the failure mechanism of coal alters significantly with a change from an axial splitting to a shearing mechanism. Measurements of the volumetric strain response also indicate an absence of scale dependence in the deformation behaviour of the three larger sample sizes. Unloading cycles were used as a means of investigating the development of recoverable (elastic) and irrecoverable (plastic) components of strain, The irrecoverable strain loci, for the lar ger sizes, were taken to be representative of the in situ deformation response of coal. (C) 1998 Elsevier Science Ltd All rights reserved.