Study On Hydraulic Fracturing Mechanism Of Rockmass And Its Application Of Water Outburst In Mine

In the field of rock mechanics, hydraulic fracturing is the process of injecting pressured water into the sealed bare borehole to induce tensile fracture of rock. The hydraulic fracturing of rock is the unstable failure process of rock that is subjected to hydraulic pressure, characterized by the formation, growth and coalescence of microcracks, as well as the initiation and development of macroscopic faults.In rock engineering, such as mining, underground and tunnel engineering, interactions between in-situ stress and seepage pressure of groundwater have an important role. In particular, seepage-induced rock instability problems, such as when water outbursts from a coal-mine floor or a tunnel, are all associated with the basic principle of hydraulic fracturing. Therefore, the study of induced rock hydraulic fracturing is important, not only for characterizing the rock-failure mechanism associated with the coupled hydromechanics, but also for solving the seepage-induced failure problems encountered in engineering. These issues have been among those at the forefront of the most actively studied in rock-mechanics science in recent years.Mining excavations may induce the re-distribution of the in-situ stress field as well as cause damage and fracturing in surrounding rock. The damage and the new cracks induced by mining excavations may enhance the permeability of the wall rock significantly. If there exists an underlying aquifer with high pressure around the stopes, water outburst from the mine roof, from the floor or through faults may occur to threaten the mining production and workers. Outbursts are among the five major disasters during coal mining in China (i.e. leaks of poisonous gases, coal and gas outburst, water outburst, mining-induced rockburst and collapsing of mine stopes). Water outburst in a mine has the same mechanism as hydraulic fracturing and they are both related to the rockmass instability induced by coupled stress-seepage-damage interactions. The characteristics of crack propagation driven by hydraulic pressure are examined in this paper with numerical simulations and experiment from the viewpoint of coupled flow-damage interaction. Furthermore, the engineering problem, i.e. water-outburst from a coal-mine floor, is simulated to determine the failure mechanism induced by the coupled seepage-stress effect during underground mining. Based on thess research, the effective stress coefficient was introduced in traditional water-inrush to modify it, which is of theoretical and practical value in the understanding of the mechanisms of groundwater outbursts. The work in the paper can be summarized as follows:1. From fluid-solid coupling theory, it mainly discussed that the effective stress coefficient influence on the initial fracture pressure of hydraulic fracturing, and the theoretical formula was proposed which considered effective stress. Through the test of hydraulic fracturing with small scale artificially specimens, the correctness of the theoretical formula was verified.2. Based on the simulation tools of RFPA2D-flow and RFPA3D parallel programs, the2d and3d models of hydraulic fracturing were built, respectively. The condition of initial fracture and the influence law of crack propagation under different parameters were analyzed by simulation research. Through the research, the influence law was quantified which among the effective stress coefficient and the initial, breakdown pressure.3. According to an engineering example of water outburst in Fan Gezhuang coalmine, the whole process of the initiation of fractures, extension and formation of groundwater outburst pathway was simulated and studied. With the analysis of damage distribution, stress and seepage field evolution, the process of the evolution from water-resisting strata to inrush pathway about the floor of complete mudstone under the condition of mining disturbance and hydraulic driving is clearly revealed, and the water rush pathways are determined. Furthermore, the effects of water outburst under different water pressures in sandstone are discussed. The laws of crack propagation and water outburst under the different mining conditions were obtained4. Based on the principle of effective stress, the physical meaning of effective stress coefficient changing in the damage process of rockmass was discussed. And the influence factors of water-inrush from coal floor were analyzed. The concept of effective stress coefficient considering fluid-solid coupling conditions was put into the traditional water outburst coefficient method to amend it. According to the different seepage characteristics in the coal mine floor, the calculation method and field test method of correction water outburst coefficient were put forward, which have significant theoretical and practical senses for understanding the essences of water outburst induced by rock failure, the connotation of water outburst coefficient, and the guidance of seepage control design by grouting.