Rock failure criteria were used to predict rock strength and to characterize deformation behavior of rock mass or intact rock under different loading conditions. Hoek and Brown failure criterion was considered as one of the most popular failure criteria, which is widely used in rock engineering. Beside all the advantages, the original description of this criterion does not include anisotropy in strength in an explicit manner. Therefore, this criterion has been modified (directly and indirectly) to take into consideration rock anisotropy. Such Hoek-Brown failure criterion modifications need additional parameters and functions, which can be deduced from lab test results. The authors explain these modifications and validate the proposed criterion by using published data of sedimentary and metamorphic intact rock samples. Finally, the authors study the correlation between these different models, which represent rock anisotropy either directly or indirectly.
Most of rock types have inherent anisotropy planes such as bedding planes, foliation, or flow structures, which introduce anisotropy in their strength and deformational properties. The consequential difficulties, which were faced at studying rocks, are their inherent characteristics of discontinuity, anisotropy, inhomogeneity and inelasticity (Hudson 2000).
Inherent anisotropy is considered as a major characteristic for the rock behavior, in particular for both types of rocks; metamorphic rocks, because of foliation and schistosity, and sedimentary rocks, because of bedding planes (Saroglou et al. 2008). An anisotropic rock has different properties in different directions. This nature is governed by different factors: for example, minerals forming the rocks, the orientations of the mineral crystals and the interaction between different grains (Ismael et al. 2014). From a mechanical point of view for rocks, anisotropic nature of rocks causes differences in rock strength with respect to the orientation of loading and inherent planes of weakness (ß).
Hoek and Brown criterion of failure is one of the main failure criteria in rock mechanics, which concerns on rock strength prediction. The empirical H – B failure criterion is based on three parameters (m, s and a) which are material constants and σc the compressive strength of the material.
The H – B failure criterion considers anisotropic behavior for both, intact rock and the rock mass to some extend indirectly and directly. Hoek & Brown (1980) proposed a modification to the failure criterion considering rock anisotropy. Colak & Unlu (2004) presented variations of rock parameter mi for intact transverse anisotropic rock in H – B failure criterion. On the other hand, Saroglou and Tsiambaos (2008) discuss a modification, where direct modification added to the original H – B failure criterion to account for the anisotropic nature of rocks. Finally, Bagheripour et al. (2011) proposed a complement to the H – B criterion in order to derive the strength of anisotropic rock from strength of the corresponding truly intact rock.
In the following part, the authors review the most important modifications to incorporate rock anisotropy into H – B failure criterion. In addition, these modifications will be applied to gathered lab test data and results will be statistically evaluated.
