Fractal Behaviour And Characteristic Analysis Of Micoseismic Informations In Nucleation Process Of Immediate Rockbursts In Deep-Buried Tunnel

Rockbursts are also called impact ground pressure events whose mechanism is that elastic potential energy which is stored in the rock mass is released suddenly under high stress conditions. Economic loss and staff casualties are often caused by such rockbursts._How to warn of, and control the risk from, high-intensity rockbursts has become a major issue in deep underground engineering. The stress wave released upon the generation of damage in the rock mass carried micro-seismic information about that very event. Therefore the evolution mechanism of micro-seismic information arising from rockbursts was deemed worthy of research.Based on hundreds of rockbursts of differing intensities, and much micro-seismic information, from the deep diversion tunnels of Jinping II hydropower station which were excavated by conventional tunnel boring machine (TBM) and drill-and-blast methods, damage to the surrounding rock masses increased with the number of rockbursts. Those arising from different excavation methods were compared; the distribution of microseismic events was analysed by fractal theory. The key conclusions were:(1) A real-time micro-seismic monitoring method was established for both TBM and drill-and-blast excavation methods. Those factors influencing the rockbrusts in deep tunnel excavations were analysed by numerical method and their micro-seismic information. Above all, the stress paths for different excavation methods also differed, and on this basis contrasting the analyses over their ranges of activity, zones of influence, and energy distributions, the magnitude of micro-seismic events was show to have differed depending upon excavation method. The work proposed a method for understanding how micro-seismic information and the stability of the surrounding rock were influenced by choice of excavation method.(2) The micro-seismic information from rockbursts in deep tunnels excavated by drill-and-blast and TBM methods was analysed. The behaviour of micro-seismic information during immediate rockbursts, and under different excavation methods, differed and as such were analysed. This conclusion not only determined the excavation method causing each rockburst, but also provided a basis for their prediction and prevention.(3) The self-similarity of micro-seismic information, and a fractal method for the analysis of event time, position were proposed. During drill-and-blast excavation method, the micro-seismic events temporal fractal dimension values increased during immediate rockbursts but were significantly reduced before the rockburst occurred, the micro-seismic event temporal fractal dimensions were bigger if the intensity of the rockburst was stronger. The micro-seismic events spatial fractal dimensions in different excavation methods have the same evolution law, the spatial fractal dimension values decreased and were minimised upon rockbursting. The temporal and that of the spatial evolution by microcracking during immediate rockbursts were revealed by fractal theory.(4) A fractal method for the analysis of energy-released was proposed. The evolution law of micro-seismic events energy fractal dimensions in different excavation methods are also not same:The energy fractal dimension values increased continuously before a rockburst occurred and then increased to a critical value when the rockburst was imminent during drill-and-blast method excavation; however, the energy fractal dimension values for TBM excavation were greater than those found with drill-and-blast excavation. They could describe mild and moderate rockbursts at the working face.(5) Do energy fractal research to different type and intensity immediate rockbursts during drill-and-blast method excavation, the result not only indicate indicated that the micro-seismic energy fractal behaviour during the evolution of immediate strain rockbursts and immediate strain-structure slip rockbursts were distinguishable, but also response the relationship between energy fractal dimension to the intensity of rockbursts and the number of structural surface. The characteristics of micro-seismic energy distribution were analysed according to the physical significance of their energy distribution's fractal dimensions, and the reason why their energy fractal dimension are different were also explained according to the influence of the surface plane.