Questions and answers: intact rock 75ADisplacementVolumeDisplacementto a positive slope. The specimen continues to dilate at an increas-
ing rate until shear planes form, which allow the specimen to break
into separate mobile pieces, with the result that the dilatation rate is
reduced.46.2 What causes a rock to break when it is compressed uniaxially
to failure? Is it when the stress reaches a certain value or when the
strain reaches a certain value? Or is it when some other parameter
reaches a critical value, such as the energy input per unit volume, or
the microcrack density per unit volume?
Ad2 Over the years, many failure criteria have been proposed, and all
the parameters mentioned in the question have been used to develop
failure criteria at one time or another. Because the collapse of the rock
microstructure is a complex process (see Q and A6.1), it is not clear
which is the correct parameter, or combination of parameters, to use
in a failure criterion. The short answer to the question is that no one
knows the answer. As a result, the most widely used failure criteria are
empirical criteria based on the stress components (especially the Mohr-
Coulomb and Hoek-Brown failure criteria).
46.3 The geometry of the linear Mohr-Coulomb envelope is such
that a number of useful relations between strength parameters can