Study On The Mechanical Behavior And Failure Mechanism Of Fissured Rock Under Triaxial Compression

Rock mass was created by the long-term geotectonic movement,which was internally embedded with a variety of defects,such as fault,jonits,holes and fissures and so on.Due to the existence of these defects,the obvious characteristics of rock mass structure are heterogeneity,discontinuity and anisotropy.Fissures are in common existance in rock mass engineering and fractured rock mass is generally under triaxial loading.Fissure geometry layout and loading condition are important factors that influence on the mechanical behavior of fractured rock mass.As a result,the reaserch on the mechanical behhavior of fractured rock mass under triaxial compression has greate guiding significance for the rock mass engineering stability evaluation.In this study,conventional triaxial compression tests were carried out on pre-fissured cuboid specimens by using a GCTS RTX-4000 high-pressure and high-temperature triaxial rock testing system,and the self-designed strain measurement for rectangular specimen.This study focuses on the mechanism of fissured sandstone,which was suppoted by the National Natural Science Foundation of China(51179189,51734009)and Natural Science Foundation of Jiangsu Province for Distinguished Young Scholars(BK20150005).The research object was sedimentary sandstone.Laboratory experiment,numerical simulation and theory analysis were involved in this study.The following main results can be derived:(1)Conventional triaxial compression tests were carried out on intact sandstone and sandstone containing a single fissure by using a GCTS RTX-4000 high-pressure and high-temperature triaxial rock testing system,and the self-designed strain measurement for rectangular specimen.The experimental results show that the strengths of the sandstone containing a single fissure increase linearly with confining pressure increasing and the failure modes of sandstone containing a single fissure under conventional triaxial compression change from tensile failure to shear failure gradually.Combined with 3D CT images,it can be seen that the crack distribution is complex in x direction under uniaxial compression,while it is similar in x direction under triaxial compression.The damage fracture mechanism of sandstone containing a single fissure under triaxial compression was revealed.This research can provide a reference for the stability evaluation of fractured rock mass.Introducing fissure initial damage variable into the statistical damage constitutive model of rock mass,the damage evolution law of fissured rock mass under triaxial compression was interpreted based on this constitutive model.(2)On the basis of the research on the damage fracture mechanism of sandstone containing a single fissure,conventional triaxial compression tests for sandstone specimens containing two coplanar or noncoplanar fissures were carried out using by the rock mechanics servo testing system.Combined with CT scanning images,the influences of confining pressure and fissure geometry layout on the mechanical behavior of sandstone containing two fissures were discussed.Contrast to sandstone containing a single fissure,the failure characteristic of sandstone containing two fissures presents an remarkable triaxial compression failure mode.Besides,the influence of fissure on failure characteristic of sandtone containing two fissures is stronger than that of confining pressure.Crack classification is analysed by AE parameters and K-Means algorithm.It can be seen that rock bridge inclination has a significant effect on the failure process under uniaxial compression,whereas shear/mix cracks dominate in stress-drops under triaxial compression.(3)Using by Particle Flow Code(PFC),the numerical model for fissured sandstone was established and the deformation processes under triaxial compression was simulated based on the numerical model.Micro-parameters were calibrated by the experimental results of intact sandstone under triaxial compression.Based on the calibrated micro-parameters,the simulations for sandstone containing a single fissure or two fissures under triaxial compression were carried out.It is concluded that PFC is reliable to simulate the mechanical behavior of fissured sandstone from the perspectives of strength,deformation and failure mode.It can provide an important support for the following research on the damage fracture mechanism of fissured rock mass.(4)Using the numerical simulation method,the damage fracture progress of fissured rock mass was discussed.The evolution progresses of crack,displacement and stress field were analyzed.The influences of confining pressure and fissure geometry layout on the fracture meso-mechanism of fissured sandstone were discussed microscopicly.Six types of microcracks were defined based on the fracture mechanism of microcrack created during the deformation failure process.Based on the quantitative statistics interpretation on the six types of microcracks,the influences of confining pressure and fissure geometry layout on the microcracks were analyzed and the fracture meso-mechanism of fissured sandstone were discussed.The ratio of tensile crack is the highest while that of compression crack is the lowest.But,the ratios of the others depend on the confining pressure and the layout of fissure.The damage fracture mechanism of fissured rock mass was revealed from the view of micromechanics.