Limitations of failure
criterion for predicting
stope instability.
Penny Stewart Beng(Mining) PhD
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
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An Equivalent Continuum
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(after Hoek and Brown, 1980)
South Crofty Map3d Predicted Failure
zone
Span 250 m
Grey area 3 < -0.69 MPa
Predicted Failure zone
60 m Height
Width 3 m
3< -0.69MPa
(Hoek Brown empirical
failure criterion)
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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 ?
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