Numerical modelling of rock mass bulking and geometric dilation using a bonded block modelling approach to assist in support design for deep mining pillars

Abstract

As underground mining operations move deeper and the stresses encountered become more adverse, mine pillars are experiencing increased stress-induced damage and rock mass bulking. This has led to significant challenges in designing effective support systems, leading to costly mine development, production delays and support maintenance and rehabilitation requirements. Efforts to assess rock mass bulking have relied almost exclusively on the use of continuum modelling techniques, employing plasticity theory to predict the depth of brittle failure and associated post-yield deformations. However, rock mass bulking in high stress environments is largely driven by brittle fracturing resulting in geometric dilation, which involves shearing and buckling of spalled rock pieces as they deform into the excavation. Continuum techniques are inherently limited in their ability to model this behavior. This paper presents the results of research investigating the application of discontinuum-based numerical modelling techniques for assessing rock mass spalling and bulking in highly stressed pillars. The 3-D distinct-element program 3DEC is used to develop a bonded block modelling approach, referred to here as 3DEC-BBM. The method developed allows for the explicit modelling of brittle fracturing, spalling and geometric bulking, and the corresponding support responses, in reaction to the simulation of different mine stress paths and operation scenarios. Several different support strategies are investigated to test their effectiveness as a means to improve displacement-based support design in highly stressed pillars. These are based on the simulation of an extraction-level drawpoint pillar for a block cave mining operation that is exposed to the abutment stresses from an undercut passing over it. Based on the results, three key findings are presented and discussed: (1) Adding a small support pressure has a significant impact on the performance of the pillar. However, the benefits gained by further increasing this pressure diminish and have a less dramatic impact on spalling damage and bulking with increasing effort. (2) It is crucial to install appropriate support before the undercut passes over the extraction level pillars. The depth of damage and amount of bulking can increase significantly during pillar unloading following passing of the undercut. Extra confinement applied to the failed pillar skin allows it to carry more load and thus provide more confinement to the pillar core, helping to limit further damage and bulking. (3) The addition of support pressure through shotcrete and rock bolting performs similarly to “perfectly” installed steel sets . Thus, a heavier support system will not necessarily result in better pillar performance.