Limitations of failure criterion for predicting stope instability. Penny Stewart Beng(Mining) PhD

2 Numerical Modelling  ‘Equivalent continuum’ based numerical models incorporate failure criterion (rock mass tensile failure cutoffs).  Eg. FLAC, FLAC3D, Phases2d and Map3d  This presentation provides an explanation as to why rock mass failure criterion are poor predictors of stope failure/instability.  South Crofty case back analysis to assess accuracy of prediction

3 An Equivalent Continuum (after Hoek and Brown, 1980)

4 South Crofty Map3d Predicted Failure zone Grey area  3 < MPa Predicted Failure zone Span 250 m 60 m Height Width 3 m  3 < -0.69MPa (Hoek Brown empirical failure criterion)

5 Analysis of Numerical Modelling results  Why do the failure criterion based numerical models over predict failure zone?  Possible explanations include;  Just because a rock mass has yielded, it doesn’t necessarily mean it will collapse.  Modelled tensile stresses are not developed in reality - stress redistributed by slip along discontinuities  Have the assumptions of equivalent continuum been met ? anisotropic behaviour ?