A micro-mechanics-based model is developed to investigate microcrack damage mechanism at the stages of rapid stress drop and strain softening of brittle rock under rotation of the principal stress axes. The frictional sliding crack model is applied to analyze microcracks nucleation and propagation. Effects of rotation of the principal stress axes on microcracks nucleation and propagation and coalescence, as well as the inelastic strain increments and shear deformation localization of brittle rock, are investigated. It is shown that the dynamic damage constitutive relationship and the failure strength of brittle rock are sensitive to rotation of the principal stress axes.
The sequencing and advancement of a tunnel face leads to the disturbance and redistribution of the primary in-situ stress field under excavation condition. This disturbance and redistribution involve both changes in magnitude and orientation of the stress-field tensor in the proximity of the tunnel boundary. Moving away from the tunnel boundary, the stress tensor eventually returns to its initial in-situ state. Given the controlling influence that stress magnitude and orientation results in the development of brittle fractures, strength degradation and instabilities of rock mass, the analysis of such changes has become standard practice in most rock excavation designs (Eberhardt 2001).
The analysis of excavation-induced stresses has, in the past, been primarily restricted to change in magnitude of the stress-field tensor (Abel & Lee 1973; Chen et al. 1999; Kyung & Yong 2006; Jaeger et al. 2007; Li & Michel 2009). However, it is inadequate that change in magnitude of the stress-field tensor is only taken into account if effects of the principal stress axes rotation are considered to be significant. In the case of an advancing tunnel face, effects of the principal stress axes rotation have been shown to be an important factor, especially with respect to induced stress concentrations and rock strength degradation. Better understanding of effects of the principal stress axes rotation promises benefit in many areas from rock mechanics to slope and underground engineering and earthquake prediction. It is essential and important to understand how microcracks nucleate and propagate and coalesce under rotation of the principal stress axes in order to provide better understanding of brittle rock fracture process that occur in the slope and underground engineering fields. In the paper, the analyses concentrate on effects of the principal stress axes rotation on brittle fracture propagation, induced damage and rock strength degradation.