Microscopic Fracture Mechanisms Of Hard Rock And Rock Burst Prediction Criterion Under H-M Coupling Effects

Hard rock mass is prone to induce rock burst during the construction of deep buried tunnel engineering in high geostress areas.A single physical field of rock mechanics properties has not been able to study deep hard rock tunnel rock burst problems when deep hard rock mass is in the complicated geological condition caused by a combined role of high geostress and high underground water pressure.And the destruction of the rock mass is usually caused by macro cracks,which is induced by internal microscopic cracks initiation,propagation and eventually interpenetration under the action of external load.The study of rock mass crack extension,evolution law and damage mechanism from microcosmic point of view is the key to evaluate the rock mass stability correctly.Therefore,on the basis of mechanical test results,the PFC numerical simulation of Ba Miao tunnel hard sandstone is carried out.It is obtained the microcosmic failure mechanism by studying the stress-strain curve,macro damage deformation and failure characteristics,microscopic cracks extension and energy evolution law of sandstone under H-M coupling through the comparative analysis method.On this foundation,the initial correction for energy index method of rock burst criteria is suggested,the main results are as follows:(1)The whole rock X-ray powder diffraction experiment and Polarizing microscope rock slice experiment have been done to study the micro structure characteristic of sandstone.The study shows that the main mineral components of sandstone are quartz,calcite,plagioclase and clay minerals.The most concentration of minerals particle size distribution is 0.4mm,mainly dominated by quartz minerals,the strength of rock is high.Geometric configuration and properties of the mineral grains is refined by GBM-SJM model,which can well materialize the anisotropic characteristics of hard rock in the micro perspective and more truly simulate sandstone sample.The microcosmic effect of seepage water pressure inside the rock is simulated through the introduction of “domain” and “pipe” to analyze microscopic fracture mechanisms of hard rock under H-M coupling(2)According to conventional triaxial compression test under H-M coupling Effects,the crack initiation strength of hard rock increases as the seepage water pressure increases,the deformation and fracture characteristics of rock by hydraulic is more obvious at yield stage.The micro-tension cracks are more and transgranular cracks occur earlier under effects of H-M coupling.The proportion of intergranular tension cracks in total number of cracks is larger as the seepage water pressure is higher,and small tension cracks can be observed along the main macroscopic shear plane after the propagation and coalescence of cracks,the microscopic fracture characteristics of rock has a tendency to transition from plasticity to brittleness.The distribution of kinetic energy of rock is more concentrated under H-M coupling Effects,the fracture mode is similar to the "main shock type" in acoustic emission test,dropped rapidly after the peak,and the greater the pressure,the more obvious this phenomenon,indicating that seepage water pressure promotes the crack initiation and propagation,the stored energy is higher for failure state,the release of energy is more rapid,the damage is more intense,the greater the seepage water pressure,the more obvious this phenomenon is,namely stronger brittle features of rock.The above features show that: with the seepage water pressure increases,the microscopic mechanisms of sandstone embodies a characteristic of enhancing brittle features and weakening plasticity characteristics of rock,leading to the possibility of sandstone rock burst occurring increased.(3)Unloading triaxial tests compared with conventional triaxial test under H-M coupling,the peak intensity is low,growth rate of crack number after the peak is faster,and because the lateral restraint weakened,presents of transgranular cracks and shear cracks decreased,the proportion of the number of tension cracks rising,the impact kinetic energy distribution is more concentrated,"peaks" is also less,indicating that under stress unloading path,destroy hard rock is more intense,brittle characteristics is stronger.As the penetration of water pressure increases,the more the performance of the more obvious features,further demonstrates the microscopic fracture mechanism of sandstone that the characteristics of brittle is stronger an characteristics of plastic is weaker,and enhance the possibility of sandstone rock burst.(4)Under H-M coupling,the confining pressure for micro-cracking impact is mainly reflected that with the increasing of confining pressure,lateral restraint reinforced of hard rock enhances,hard rock crack initiation strength increases,the tension cracks along the grain occurs more easily between different particles,on the other hand,tension cracks have also been reduced,shear cracks increase,characterized by micro-fracture brittle weakened and plastic strengthened.(5)The rock fracture can be seen as the phenomenon of evolution and development of its internal energy and leads to destroy,the criterion of rock burst based on energy variation can represent the process of rock failure more really.Using PFC built-in energy function to correct criterion of energy impact indicators of rock burst,and according numerical test results,the author attempted to modified discriminant index of energy under the H-M coupling effects.Criterion corrected convinced that the seepage water pressure can promote occurrence of rock burst,and as the seepage water pressure is greater,the possibility of rock burst is higher.