Based on recent advances in modelling the post-yield behaviour of brittle rock, the authors have developed a calibrated inelastic model of the 7910 level (2.4 km depth) at the Creighton Mine in Sudbury, Ontario, Canada using data collected from the monitoring of pillar dilatancy. While this calibrated model represents a state-of-the-art continuum approach for capturing the progressive development of yield and stresses in mine pillars, alternative state-of-practice approaches (elastic and perfectly plastic material models, for example) represent potentially acceptable options for practical application. The purpose of this study is to examine the influence of constitutive model choice on stress paths throughout the pillar system on the mining level of interest. The strengths and limitations of various modelling approaches are compared and contrasted, and recommendations are provided for future pillar modelling studies.
When using continuum models to evaluate the behaviour of rock pillars in a highly stressed mine environment, there is no broadly accepted approach. Perhaps the most common approach is to use a mine-wide elastic model to evaluate which pillars will take on higher loads. An alternative approach which is frequently employed is to use laboratory compression testing data and rock mass classification information to estimate rock mass strength parameters (for example, the method of Hoek et al. 2002). In this case, it is most common to use a perfectly-plastic strength profile (no strength loss following yield), since it is often difficult to constrain post-yield parameters with a high degree of certainty.
A more complex but more physically realistic approach than those described above is to use a cohesion-weakening-friction-strengthening (CWFS) strength model, which captures the evolution of the strength of a brittle material over the course of inelastic deformation (Martin 1997, Diederichs 1999, Hajiabdolmajid et al. 2002, Diederichs 2007, Zhao et al. 2010, Walton et al. 2014, Walton, 2014). This model captures the lack of influence of confining stress on the initiation of cracking in-situ, as well as the extreme influence of confining stress that develops as cracks extend. Also important to the development of yield in this case is the influence of post-yield dilatancy. Because of the complexity of the behaviour captured by this type of constitutive model, it is often not used in favour of simpler models that can be more easily constrained by available data.
The purpose of this study is to examine how significantly the choice of constitutive model can influence predictions of yield and stress path in a system of pillars. The study has been performed using a case study from the Creighton Mine in Sudbury, Canada.
