Study On Mechanical Properties Of Surrounding Rock From A Roadway Through A Fault And Stress Gradient Failure

With the development of mining and underground construction,underground rock engineering is becoming more and more popular.The deformation characteristics,mechanical characteristics and failure characteristics of the roadway surrounding rock with complex geological structure are quite different from those of the homogeneous rock mass in general.Therefore,the study on the deep roadway's brittle rock mass with naturally infilled fractures is helpful to understand the influence of tectonic fractures on various aspects of rock mass characteristics.At the same time,ultrasonic nondestructive testing method is also widely used to monitor the variation rule of ultrasonic parameters during the cracking process of the naturally filled fractured rock mass under the actual stress condition in the roadway.Combined with the analysis of deformation and failure process,it can provide certain references for on-site disaster prevention and control prediction.In this thesis,the deformation and failure mechanism of the naturally filled fractured limestone near a fault that passes through a coal mine floor has been analyzed and summarized in detail by laboratory tests and theoretical knowledge analysis.The specific content is as follows:(1)According to the X-ray diffraction experiment,optical microscope observation and X-ray CT scanning test,the approximate mineral composition of the naturally filled fracture limestone were obtained as well as the macro and micro structure.The effects of initial structural defects on the anisotropy of fault rocks have been studied by ultrasonic wave velocity anisotropy,and it was found that the penetration of natural macroscopic cracks and initial stratification cracks would lead to changes in the distribution of circumferential P-wave velocity and polarity(the Angle between the maximum direction and the minimum direction of wave velocity).According to the analysis of meso-scale section morphology and the 3D reconstruction model of macroscopic CT scan results,the reliability of the anisotropic method to distinguish internal structure has been verified.(2)Conventional uniaxial compression,triaxial compression tests were carried out on natural filled fracture limestone by using rigid hydraulic servo control experimental machine.By means of experimental phenomena and data results,and in combination with Hoek-Brown strength theory in rock mechanics,the integrity degree s and hardness degree m of the naturally infilled fractured limestone were described,extended and discussed.Based on equivalent medium theory and Hoek-Brown strength theory,parameter s of integrity degree was calculated,and the exponential relationship between s and initial damage variable D measured by the initial wave velocity was obtained.The results of the subsequent uniaxial compression experiments were used to verify the fitting.According to s-D relation curve,fracture strength indexe b were defined.and natural limestone was quantitatively classified and described.It is defined that the parameter m of limestone with different natural infilled fractures was affected by confining pressure and rock material characteristics.In the condition of low confining pressure,fractures play a dominant role in the specimens with obvious difference.However,in high confining pressure,the confining pressure makes the specimens characteristics change from hard to soft and the difference would decrease.The crack characteristic parameter B is used to describe the rock strength characteristics,moreover,the rationality of the method was verified by the experimental results.(3)Ultrasonic wave velocity was measured during the whole compression by ultrasonic monitoring system,and the dynamic mechanical parameters have been analyzed.The variation model of the whole process of rock fracture under load was established by elastic wave mechanics.The change rate of crack in specimens reflected the whole process of crack expansion.(4)According to the rock mechanics of mine,the stress gradient model of deep roadway has been proposed,and the stress state of different areas around the surrounding rock of deep roadway was also analyzed.The theoretical model of stress gradient failure in surrounding rock of deep roadway was established,and the structural gradient failure of surrounding rock affected by stress gradient was theoretically explained.The assumption of stress gradient condition in surrounding rock of roadway was simplified by laboratory test,and a homogeneous sandstone was taken as a research object for test verification.According to the test results of homogeneous sandstone,it is found that the theoretical concept map was in good agreement with the proposed one.The rationality of the existence of stress gradient in the roadway was explained from the perspective of the test,and the distribution diagram of stress gradient when the rock in the roadway was homogeneous sandstone has been obtained according to the stress gradient theoretical model.In addition,the stress gradient path test of the naturally filled fracture limestone was carried out to summarize the stress distribution in the roadway passing through the fault.According to the failure characteristics of limestone in various regions,the closer to the roadway,one or two failure modes such as along the bedding and shear gradually change to multiple mixed failure modes,and the degree of rock failure becomes more and more serious.For limestone without bedding,under the action of stress gradient,the confining pressure in different areas will be gradually relieved,which leads to the increase of shear fracture angle.The crack angle limestone with bedding will be close to that of bedding plane if shear failure exists.Based on the limestone test results and the calculation results of parameter m in Hoek-Brown criterion,it is found that the closer to the roadway,the larger the calculated value of limestone mn in this area,indicating that the unloading change of surrounding rock makes the rock change from soft to hard.Combined with Hoek-Brown criterion and Mohr-Coulomb criterion,the theoretical fracture angle expression in chapter 4 of this dissertation and the actual measurement results,it is found that the two theoretical calculated values are close to the actual measured values,which further proved that the theoretical failure model of limestone proposed in chapter 4 of this paper was reasonable to some extent.Adjacent to the roadway,the process that the fracture angle increases was the process that the limestone failure mode changed from the triaxial shear failure mode to the vertical split failure mode.The rationality of the stress gradient model was proved laterally.Finally,according to the constitutive model of jointed rock mass,the stress gradient theory suitable for the heterogeneous rock has been established.