Numerical analysis of progressive fracture and associated behaviour of mine pillars by use of a local degradation model

Fracture of mine pillars is a progressive process that comprises, within the material of the pillar itself, the creation of cracks, alteration of the local material properties, disturbance of local stress fields, extension of local failures and the formation of fracture planes. The physical occurrence of these processes leads to nonlinear, macroscopic mechanical behaviour of fracturing pillars and an associated loss of load-carrying capability.

A local degradation model recently developed by the authors has been applied in plane strain analysis of the progressive failure of mine pillars. In terms of isolated fracture processes and the corresponding macroscopic mechanical behaviour it is shown that the model produces results that are similar to field observations. For example, the fracture of a pillar with a low width-to-height (w/h) ratio is shown to begin with the initiation of random local failures and to progress through extension and coalescence of these failed sites to slabbing or spalling of the ribsides, until ultimately large, through-going fractures develop. Associated with the development of these processes is a macroscopic, nonlinear mechanical behaviour.

The influence of both the w/h ratio and interface friction on pillar fracture was investigated. For w/h ratios between 0.5 and 2.0 the strength predicted from simulation with the degradation model was found to be in good agreement with the predictions given by best-fit empirical pillar-strength formulae. For pillars with w/h ratios greater than 2.0, however, the degradation model predicts higher strength, which may suggest that empirical formulae are over-conservative in this region.

Interface friction between the pillar and the immediate roof and floor has been found to exert a major influence on the development of fracture planes. With frictionless interfaces sets of fractures develop within a pillar at a constant inclined angle and constant spacing. With a non-zero interface friction angle fractures develop along curved planes, the distance between individual fracture planes decreasing towards the pillar centre. The curvature of the fracture planes increases as the interface friction angle increases.