Numerical modeling of unstable rock failure.

Abstract

Rockburst is an unstable and violent rock failure and it is a hazardous problem in deep underground mines and civil tunnels; it imposes a great danger to safety of workers and investment. Many factors that influence rockburst damage have been identified. In many rockburst case histories, the presence of geological structures such as faults, shear zones, joints, and dykes has been observed near excavation boundaries but their role in rockburst occurrence is still not fully understood. A good understanding of the role of geological structures on rockburst damage is important to anticipate and control rockbursts and constitutes the focus of this thesis. In this research an explicit finite element tool (Abaqus-Explicit) is employed to study unstable rock failure and rockburst processes. First, uniaxial compression tests were simulated to confirm the suitability of the adopted numerical tool for simulating unstable rock failures. Two indicators namely Loading System Reaction Intensity (LSRI) and the maximum unit kinetic energy (KEmax) were proposed to distinguish between stable and unstable failures in laboratory testing conditions. Unstable rock failures under polyaxial unloading conditions were also simulated. The influences of loading system stiffness, specimen‘s height to width ratio, and intermediate principal stress on rock failure were investigated. Next, material heterogeneity (in terms of strength and deformability) was introduced into the models using Python scripting to enhance the efficiency of Abaqus for modeling geomaterials. Numerical simulation results showed that heterogeneous models resulted in more realistic failure modes than homogeneous models. The effect of material heterogeneity on rock failure intensity in unconfined and confined compression tests was investigated. It was observed that when two materials have the same peak strength, the heterogeneous model had more released energy than the homogeneous model due to differences in the failure mode. The tensile splitting failure mode of the heterogeneous model released more energy than the shear failure mode of the homogeneous model. Then, the role of geological weak planes on rockburst occurrence and damage near the boundary of tunnels was studied using 2D models. Initially, a tunnel without any adjacent weak plane was modeled. Then a fault with different lengths, inclinations, and distances to the tunnel was added to the models and its effect on rock failure was simulated. The velocity and the released kinetic energy of failed rocks, the failure zone around the tunnel, and the deformed mesh were studied to identify stable and unstable rock failures. The simulation results showed that the presence of a fault near a tunnel could induce rockburst if the fault is positioned and oriented in such a way that it promotes high stress and low local loading system stiffness. Finally, a rockburst that occurred in the Jinping II drainage tunnel in China with an observed nearby fault was simulated. The modeling results captured the field observation of rockburst damage and confirmed that the presence of weak planes in the vicinity of deep tunnels is a necessary condition for the occurrence of rockburst. The finding from this research constitutes a better understanding of unstable rock failures in both laboratory and in situ. The insights gained from this research can be useful for rockburst anticipation and control during mining and tunneling in highly stressed grounds.