Study On Rockburst Mechanism Under Stress Gradient By Simulating The Excavation In Underground Engineering

With the development on mining site, hydropower engineering and tunneling project to deep rock mass gradually, deep brittle rock stress redisturbution by excavating, causing local stress concentration, stress gradient in surrounding rock increases and rock burst accompanied also gradually increases. There are few researches about the impact of stress gradient after excavation under different stress environments and stress paths on rock burst disasters, and rock burst test is also present on small size specimen uniform unloading stage. To study effect of gradient loaded on rock burst, the paper analyzes the force changes under different stress environments and stress paths on rock in the underground construction, summaries the main forms of surrounding rock stress field distribution caused by deep rock excavation, applies typical unloading paths of stress gradient, combined with independent research and development of rock burst loading device for specimens in lab, and analyses the influence of different stess gradients on rock burst. It also uses numerical simulation and experimental models validation to study mechanism of rock burst under different stress gradients and stress environments. The main achievements can be expressed as follows.(1) Through the stress theory analysis of rock mass elasticity solution in tunnel excavation influence area, by using the software of FLAC3D to simulating the tunnel excavation in different excavation depth and lateral pressure coefficient, the rock stress gradient distribution trends caused by tunnel excavation in different excavation depth and lateral pressure coefficient were analyze. Study showed that, when the excavating face of tunnel pass tested rock, tangential stress of tunnel wall rock increased dramatically, radial stress reduced obviously, surrounding rock under great stress gradients. Stress distribution in stable surrounding rock got by numerical simulation experiences the same trend of stress distribution derived by theoretical formulas.(2) The laboratory test chooses the model materials which meet the rock burst proneness options to cast specimens, uses the self-developed YB-A rock burst loading device to conduct gradient loads rock burst tests on the top of large size specimens in four different loading-unloading paths, and analyzes the rock burst morphology of unloading surface under the three conditions of specimens at different lateral pressure coefficients、different top loading when unloading and different loading rates. specimen suffers relatively lower load value at the top when one side unloading, and in the same stress gradient further loading, the specimen’s top stress gradient is relatively lower when rock burst occurs; if specimen suffers higher loading rate at the same loading path, the specimen’s top stress gradient is relatively lower when rock burst damage occurs.(3) Analyzing the influence of rock burst under the four loading-unloading paths on the stress gradient changes at the top, the characteristics of rock burst under the four loading-unloading paths have been studied. CT imaging assay pre and post of loading-unloading and clastic rock burst fractal analysis had been conducted through analyzing the strain gauge deformation of in different loading paths. Finally rock failure process had been obtained near the unloading face during process of rock burst. That was energy absorption, unloading face compaction, tension failure in the middle of unloading face and rupturing into plate. The damage zone and compaction area are clear showing by CT imaging figure. The loading velocity of the specimen in the same surrounding pressure is larger, the range of rock burst when unloading in single face is smaller, the vertical load of rock burst is smaller, the boulder yield of specimen is less, the fractal dimension is larger relatively.(4) Based on the principle of energy dissipation, the changing process of elastic energy density of measuring points had been calculated and tracked by using3DEC embedded the fish language. Simulation test of the specimen model in the loading-unloading process was carried out with stress gradient similar to laboratory test, in order to validate and supplement the laboratory test results. Simulation results showed that the unloading surface had been destroyed in the shape of local spalling under high confining pressure with the gradual increase of stress gradient, even block injection during the process of rock burst. The numerical experiments and the laboratory test resulted with the same path are in good agreement.