Theoretical and numerical modelling of the anisotropic behaviour of jointed rocks

Abstract

In nature, various forms of rock anisotropy are widely pre-existing discontinuities such as bedding planes, joints, weak layers and cleavages. Rock anisotropic characteristics are in general critical for the stability of surface or underground rock excavations. The goal of this work is to investigate the anisotropic behaviour of jointed rock masses using theoretical and numerical modelling methods. In particular, discrete element modelling using flat jointed bonded particle model (FJM) was used in this research. A systematic micro-parameter calibration method for FJM was proposed first in this work to overcome the limitations of traditional approach, which essentially is a time-consuming tedious trial and error process. The relationships between the FJM micro-parameters and constitutive parameters, as well as macro-mechanical rock properties were first established through dimensionless analysis. Sensitivity and regression analyses were then conducted to quantify their relationships, using results from numerical simulations. The proposed method was demonstrated to be robust and effective based on the macro-mechanical property validation of four different types of rocks. The application of FJM to capture the load rate-dependent mechanical properties of rock materials was investigated. The results were cross-validated with experimental measurements, which indicated that FJM can model the dynamic behaviour of rocks from quasi-static to medium strain rate range. FJM, in combination with smooth joint model (SJM) used to model discontinuities, were then used to study the dynamic behaviour of rocks with a persistent joint at different orientations. A strength prediction model for dynamic UCS of a specimen containing a persistent joint at different orientations were proposed and the coefficients of the proposed equation were quantified based on numerical simulation results. The proposed model was shown to be capable of predicting VII the rate-dependent UCS of a jointed rock. Finally, the strength reduction of a jointed rock was further investigated using the statistical damage model approach based on the commonly used Weibull distribution, where the Jaeger’s and modified Hoek-Brown failure criteria were incorporated in the derived model. The proposed damage model was validated using published experimental data and numerical simulation results of FJM. Results indicated that parameter m only depends on strain parameter k, which is directly proportional to the increase of the failure strain, while parameter F0 is indirectly related to the strength of the jointed rock. In addition, joint stiffness can be easily incorporated in the proposed damage model, which has significant influence on the damage variable D, damage evolution rate Dr and rock mass deformation modulus. Outcomes of this research help us to understand better the influences of discontinuities on the mechanical behaviour of jointed rock masses.