With the development of deep underground rock engineering, high slope rock engineering, dam base rock engineering and nuclear waste disposition project, the stressed rock mass in most engineering structures in rock is either saturated with water or is subjected to high humidity levels. The problems of the pressure effect including pore water pressure and confining pressure on time-dependent deformation of rock is of great importance in the upper crust. Here we present a constitutive creep model to describe the time-dependent deformation of brittle rocks under different constant pore pressures and confining pressures. First, we formulate the hydro-mechanical coupled time-dependent model and validate the model with experimental data. We then present and discuss the results of the brittle creep simulations. Finally, we discuss the underlying mechanism for the transition to accelerating creep in heterogeneous brittle rocks using the relative roles of the renormalization, the stochastic strength field, the progressive localization, and the transition from tensile to shear events at different stages of damage evolution.
When formulating the model in mathematical language, various levels of complexity should be incorporated into each component, with the accuracy and versatility of the model depending on the refinement of the description of each component. For a model used to investigate hydro-mechanical time-dependent deformation of a stressed rock, the coupled effect between the medium deformation and hydraulic components is very important. The model is based on the theory of elastic-damage mechanics and assumes that the damage is elastic and isotropic. The model accounts for material heterogeneity through a stochastic local failure stress field, and local material degradation using an exponential material softening law (Amitrano & Helmstetter 2006). The maximum tensile strain criterion as well as a modified Mohr-Coulomb criterion with a tension cut-off are adopted as two failure thresholds in the model. The tensile strain criterion is preferential since the tensile strength of rock is commonly far below its compressive strength (Jeager et al. 2007). This approach makes it possible to simulate the transition from distributed damage by tensile microcracking to damage where microcracks can interact, coalesce, and ultimately form a shear fracture. The model also describes the temporal and spatial evolution in the medium during the progressive damage process.
