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

Abstract

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. Pillars in underground miningPillars are utilized by all underground mining methods as major structural elements-either temporary or permanentto maintain the integrity of underground excavations, such as rooms or roads, and hence allow for the safe extraction of ore.

Numerical Studies on the Failure Process of Heterogeneous Brittle Rocks or Rock-Like Materials under Uniaxial Compression

“…In this approach, Weibull distribution function was used to present the elemental heterogeneity similar to RFPA and innovatively a degradation index was introduced to incorporate the confinement effects. The approach was successfully applied to failure process of rock specimens in laboratory experiment [ 63 ] and some rock projects in engineering scale [ 64 ]. The results showed that the confinement effect can be well reflected in this approach, e.g., stress-strain relations that evolve from brittle to ductile as well as fracture patterns that evolve from axial splitting to shear to dilatancy as confinement increases [ 63 ].…”

A review of the state of the art in modelling progressive mechanical breakdown and associated fluid flow in intact heterogeneous rocks

“…This model has successfully been implemented to simulate the fracturing process of rock in mine pillars [64], laboratory scale rock specimens in compression [65], and excavation of a circular opening [66]. In all cases the model produces convincing fracture patterns and realistic complete stress-strain curves corresponding to different stress states.…”

“…Progressive mechanical breakdown of rock can be simulated using statistical modelling in conjunction with CDM when implemented within such commonly used continuum based numerical methods as FEM and FDM [60][61][62][63][64][65][66]. One such example is that developed by Tang et al [60], whereby the microstructural heterogeneity of the rock material is represented by values of elemental strength and elastic modulus, both of which are described by the Weibull distribution function [59].…”

Development of a hydro-mechanical local degradation approach and its application to modelling fluid flow during progressive fracturing of heterogeneous rocks

“…This model uses an elemental constitutive relation to describe degradation in both the strength and stiffness of rock, with the result that failure occurs randomly and progressively within a volume of rock. The model has successfully been implemented to simulate the fracturing process in both mine pillars [17] and laboratory scale rock specimens in compression [18]. In both cases the model produces convincing fracture patterns and realistic complete stress-strain curves corresponding to different stress states.…”