Every material in nature starts to yield after a certain stress or strain level, and rock or rock mass is a no exception. The terms of elasticity or visco-elasticity are replaced by the terms of elasto-plasticity or elasto-visco-plasticity once material behaviour deviates from linearity. The relation between total stress and strain or strain rate tensors can no longer be used and every relation must be written in incremental form. As a result, the constitutive laws are derived using some assumptions, which include yield functions. Therefore, several yield criteria are proposed for rocks. In this article, the authors compare and discuss the applicability of various criteria used in Rock Mechanics and point out their features and shortcomings in view of experimental results. It is shown that Hoek-Brown’s criterion as one of best criteria is quite questionable in view of experimental results.


The stability assessment of rock engineering structures is always of great concern. It is well known that every material in nature starts to yield after a certain stress or strain state and rock or rock mass is a no exception. Therefore, any stability assessment procedure for rock engineering structures would definitely require the evaluation of initial stress state in rock mass and the parameters for its constitutive law. The terms used for constitutive laws of rocks and rock masses are elastic, viscous, visco-elastic for linear behaviour and elasto-plastic or elasto-visco-plastic for non-linear irrecoverable behaviour. Furthermore, additional terms used to describe non-linear behaviour are yielding, failure, strain hardening, softening and flow. Yield (failure) criteria are important elements for mathematical expressions of constitutive laws. The authors briefly present the fundamentals of constitutive laws and point out the function of yield (failure) criteria in their developments. Furthermore, their features and shortcomings are discussed in view of experimental results.

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