Triaxial Compression And Shear Deformation Characteristics And Micro-mechanical Behaviors Of Crushable Sand Under High Stress

In the process of exploiting deep-buried resources and underground space,the stress state of deep-buried sand may change dramatically,which will cause particle breakage and change the mechanical properties of sand,thererby creating serious security issues for deep underground engineering.There is still a lack of systematic and in-depth research on the evolution of deformation and breakage of sand under high stress.This paper took deep-buried sand as the research object,and comprehensively adopted laboratory test,in-situ X-ray scanning test,and discrete element method to thoroughly and systematically carry out the research of evolution of deformation characteristic and mesoscopic breakage in triaxial compression and triaxial shear tests under the conditions of high effective pressure,high water pressure,and multiple stress paths.The results revealed the effect of particle breakage on the macro and meso-mechanical behavior of sand,obtained a more precise prediction model of particle breakage,and established a more precise constitutive model of sand under high pressure condition.The main research contents and results are as follows:（1）Aiming at the high stress environment,a high pressure triaxial test apparatus was successfully developed,and the influence of back pressure on particle breakage was understood.Besides,the deformation characteristic and the evolution of particle breakage in triaxial compression and shear tests under high pressure condition were revealed.The prediction model of the relative breakage B_r in the triaxial compression test with respect to the mean effective principal stress p’and K₀ was proposed in the first time,and the prediction model of the relative breakage B_r in the triaxial shear test with respect to the confining pressure and axial strain was proposed in the first time.The linear relationship between B_r and the volumetric strainε_v,regardless of the stress path,was obtained.The constitutive models of the volumetric strain in triaxial compression and shear tests were established respectively.The hyperbolic relationship between B_r and the fractal dimension D_M,B_r and the input energy E were obtained respectively.The results showed that the back pressure had no influence on the particle breakage under drained condition.The yield stress decreased with decreasing K₀.The initial anisotropic state was more likely to cause particle breakage.（2）Based on X-ray CT,a high-stress miniature triaxial apparatus was successfully developed,which realized the visualization of the process of deformation and crushing under high stress.The changes in the particle morphology（sphericity and aspect ratio）was analyzed statistically,and the law of the influence of particle breakage on particle morphology was grasped.The three-dimensional visualization crushing process under high-stress shear test was obtained,and the mechanism of particle breakage resulted from particle interlock was revealed.The change law of fractal dimension of particle number was obtained.The results showed that the contact points between particles determined the developing direction of fractures.Due to the small particles surrounding to the quasi-spherical particles were evenly distributed,and the coordination number of the large particle was large,the particles with regular morphology and larger size did not crushing seriously.The fractal characteristic was obvious with the development of particle breakage.The particles mainly broke into flat and slender particles with increasing stress,which resulted in the decrease of sphericity and aspect ratio.（3）A real-shaped particle was established using the discrete element numerical method,and a method to quantify the particle morphology using the weighted average sphericity value S_c was proposed,and a numerical test of sand with mixed particles（composed of 6 particles with different sphericities）was established.Comparative analysis of the volume strain,coordination number,contact force distribution and fabric anisotropy of rigid samples（unbreakable samples）and breakable samples was understood.The contact force distribution model related to B_r and K₀was first proposed in high pressure triaxial compression test.The contact force distribution model related to B_r and principal stress was first proposed in high pressure triaxial shear test.The change rules of total,strong and weak normal contact force were analyzed,and the change rule of friction energy consumption and crushing energy consumption with axial strain were analyzed.The results show that the breaking strength of a single particle increased with increasing sphericity,the strong contact force distribution is affected by particle breakage and stress path,while the distribution of weak contact force was only related to particle breakage.The distribution of weak contact force was in an isotropic state,stress level and particle breakage both caused the degree of anisotropy to decrease.The research results of this paper are of great significance to further improve the mechanism of particle breakage and the theory of constitutive model under high stress,and can provide a theoretical basis for the design and construction in deep geotechnical engineerings.In this dissertation,there are 116 figures,21 tables,and 215 references.