| The study of mechanical characteristics and failure characteristics of rock under true triaxial loading and unloading conditions is of great engineering significance for scientifically and accurately predicting and evaluating the stability of underground rock engineering and preventing the occurrence of major geological disaster accidents.In this dissertation,through the combination of laboratory true triaxial tests,in-situ CT scan tests,theoretical and numerical simulation,the mechanical properties and failure characteristics of rock under different true triaxial stress evolution paths are deeply studied,and the mechanism of crack propagation and evolution within rock caused by underground engineering activities such as roadway excavation is revealed.The innovative research results obtained are as follows:(1)The conventional triaxial and true triaxial loading tests were carried out by using the self-developed true triaxial electro-hydraulic servo test system.The differences in mechanical properties such as rock deformation and strength under two stress conditions were discussed.By using acoustic emission monitoring and CT scanning technology,the great differences of rock fracture evolution law and spatial fracture morphology under the two conditions are clearly revealed.Based on experimental research,a three-dimensional mechanical model of rock with micro-fissure defects is established,and the different mechanism of rock fracture mechanism under two loading conditions is analyzed.It is comprehensively explained that the conventional triaxial conditions can not truly reflect the basic mechanical behavior and fracture characteristics of rock under general stress state.On the contrary,only the true triaxial condition can more accurately simulate the mechanical behavior and fracture characteristics of rock under real stress state caused by actual engineering activities such as roadway excavation.(2)The rock mechanics experimental research under six true triaxial loading and unloading paths was carried out,and the differences in mechanical properties such as strength and deformation of rock under different stress paths were discussed.Through the macroscopic fracture characteristics of rock and the morphology and spatial distribution of internal cracks,"the effect of intermediate principal stress induced fracture" of rock is proposed.Combined with the fracture characteristics of rock in tests and the stratified failure of surrounding rock in roadway excavation,theimportant role of intermediate principal stress on mechanics behavior and fracture law of rock is fully illustrated,and the evolution mechanism of rock fracture under true triaxial condition is revealed.Based on Mogi-Coulomb and Griffth criteria,a three-dimensional numerical model of rock with random crack defects is established.The fracture evolution law of rock under true triaxial loading and unloading conditions is calculated and analyzed,and the evolution fracture mechanism of rock under real loading and unloading stress is revealed.(3)A true triaxial loading test system for rocks compatible with X-ray computed tomography(CT)scanning systems is developed.A series of key technologies,the carbon fiber-based reaction column,a synchronous dual-indenter loading system,carbon fiber transfer beam and other core components,as well as 6 carbon fiber press plates in a staggered,interlocking pattern to form the true triaxial pressure cell.These techniques allow X-rays to effectively pass through the CT scanning area of the system.The test system was employed to perform preliminary in situ CT scans on rock along two real-time loading stress paths,namely,under conventional and true triaxial test conditions.The evolution characteristics and damage evolution of cracks in the whole process of rock deformation and failure under the influence of different triaxial stress states are studied.The influence of two stress conditions onthe spatial morphology and geometrical characteristics of cracks during rock failure is revealed.(4)The in-situ CT scanning tests of rock with a single prefabricated fissure under two true triaxial real-time loading stresses was carried out.The evolution law of crack space in the process of rock fracture from initiation,propagation to final fracture formation under two stresses,i.e.intermediate principal stress and minimum principal stress parallel to the pre-fabricated crack surface,was studied.The effect of intermediate principal stress on crack propagation in rock was revealed.The decisive role played by the direction fully illustrates the important role of "the effect of intermediate principal stress induced fracture" on rock fracture.The dissertation incudes 103 figures,18 tables and cites 216 references. |
