Failure criterion of hard rock and laboratory research of rockburt
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In deep excavation, typical instability subjected to the high induced stress is commonly controlled by the crack initiation, propagation, and accumulation in the rock mass. The surrounding rock mass in the region of stress concentration has a great potential to fail. So these processes are generally observed around excavations in highly-stressed brittle rock in forms of spalling, slabbing and even violent rock burst. So the research presented in this thesis was undertaken to investigate the mechanisms responsible for the stress-induced brittle fracturing issue. Crack initiation strength, CI which is defined as long-term strength, and crack interaction strength, CD which is defined as the short-term strength of the brittle rocks have a significant impact on determining the in-situ behavior of the rock mass. So the laboratory testing plan was carried out by systematic uniaxial compressive tests and triaxial compressive tests to simulate and estimate the brittle failure process and damage thresholds in the laboratory scale. Both Acoustic Emission method and the strain measurement method has been applied and compared for the accuracy and subjectivity of damage threshold detection. Investigation in the thesis has also emphasized on the influence of confinement and loading duration on the determination of damage thresholds and failure prediction. Besides, as a preparation of the physical model test, two dimensional FEM modelling has been carried out to simulate the failure process around circular openings using the approach of removing the failure part step by step. However the results show that the main limitation of this modelling is the overestimation of the failure zone, it still can act as the indicator for a better understanding of failure mechanism around the tunnel.