A STUDY OF THE MECHANICAL-BEHAVIOR 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 t he mechanical behaviour of coal. The ''inherent variability'' of labor atory coal strength data was found to be related to the degree of clea ting or brightness of the coal samples. Using this observed relationsh ip, a method was developed for estimating in situ coal seam strength b used upon the ''intact'' properties of dull coal samples and seam brig htness profiles. The peak strength criterion for in situ coal uses the parameters sigma(c), m and s of the Hoek-Brown empirical strength cri terion 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 coa l alters significantly with a change from an axial splitting to a shea ring mechanism. Measurements of the volumetric strain response also in dicate an absence of scale dependence in the deformation behaviour of the three larger sample sizes. Unloading cycles were used as a means o f investigating the development of recoverable (elastic) and irrecover able (plastic) components of strain, The irrecoverable strain loci, fo r the lar ger sizes, were taken to be representative of the in situ de formation response of coal. (C) 1998 Elsevier Science Ltd All rights r eserved.