【作者】 张涛；

【导师】 杨维好； 宋雷；

【作者基本信息】 中国矿业大学 ， 岩土工程， 2021， 博士

【摘要】 在开发深埋资源与地下空间过程中,深埋砂土的应力状态可能会产生巨大变化,引发砂颗粒发生破碎,从而改变砂土的力学特性,给工程带来严重的安全问题。关于高应力下砂土的变形与破碎规律尚缺乏系统、深入的研究。本文以深埋砂土为研究对象,综合采用室内试验、X射线原位扫描测试和离散元细观数值方法,深入、系统地开展了高应力、高水压、多种应力路径条件下砂土的三轴压缩和剪切变形特性及细观破碎规律研究,揭示了颗粒破碎对砂土宏、细观力学行为的影响规律,实现了较为精准的颗粒破碎预测,建立了更加准确的高应力下砂土本构模型,揭示了颗粒破碎和细观结构之间的关系。主要研究内容和成果如下:（1）针对砂土所处的深地高应力环境,成功研制了砂土高压三轴试验装置,并利用它获得了饱和反压对颗粒破碎的影响规律;获得了高应力下砂土的三轴压缩和剪切变形规律以及颗粒破碎规律;首次提出了三轴压缩试验中的相对破碎率B_r关于平均有效主应力p’和K₀（水平应力/竖向应力）的预测模型;首次提出了三轴剪切试验中的B_r关于围压σ’_c和轴向应变ε_a的预测模型;获得了不受应力路径影响的B_r和体应变ε_v的线性关系;分别建立了三轴压缩和剪切状态下体应变ε_v本构模型;分别获得了B_r和粒径分布分形维数D_M、B_r和输入功E的双曲线关系。试验结果表明:反压对饱和砂土在排水条件下的颗粒破碎没有影响;屈服应力随着K₀的减小而减小,初始各向异性状态更易导致颗粒发生破碎。（2）成功研制了基于X射线CT的高应力下砂土变形、破碎高压微型三轴仪,实现了高应力下砂土变形、破碎过程的可视化;根据实验过程原位CT扫描测试结果,利用三维图像重构分析技术,统计分析了砂颗粒的形态（球度和纵横比）变化,掌握了颗粒破碎对颗粒形态的影响规律;首次获得了砂颗粒在高压三轴剪切时的三维可视化破碎过程,揭示了颗粒咬合作用导致砂颗粒破碎的机理;获得了颗粒数量分形维数D_N随颗粒破碎的变化规律。研究表明:颗粒之间的接触点决定了裂隙发育的走向;形态规则、尺寸较大的颗粒未发生严重的破碎,这是因为类球形颗粒的周边颗粒分布均匀,且大尺寸颗粒的配位数较大,应力集中的可能性较低;颗粒破碎越剧烈,细颗粒越多,分形特征越显著;随着围压的增大,颗粒以断裂破碎为主,扁而细长的颗粒急剧增加,导致球度和纵横比不断减小。（3）利用离散元数值方法建立了真实颗粒离散元模型,提出了使用加权平均球度值S_c量化颗粒集群形态的方法,建立了一种混合颗粒（由6个球度不同的颗粒组成）砂土数值试样和两种单一颗粒砂土数值试样;对比分析了刚性试样（不可破碎试样）和可破碎试样的体应变、配位数、接触力分布和组构各向异性的变化规律;在砂土高压三轴压缩数值模拟研究中,首次提出了与B_r和K₀相关的接触力分布模型;在砂土高压三轴剪切数值模拟研究中,首次提出了与B_r和主应力比相关的接触力分布模型;获得了总、强、弱法向接触力的变化规律;举例分析了摩擦耗能和破碎耗能随轴向应变的变化规律。结果表明:单颗粒的破坏强度随球度的增大而增大;S_c越大,配位数分布越均匀,且强接触力的均质性越好;强接触力分布受颗粒破碎和应力路径共同影响,而弱接触力分布仅与颗粒破碎有关,与应力路径无关;弱接触力分布均呈各向同性状态;应力增长和颗粒破碎,都会导致各向异性程度下降。本文研究成果对进一步完善砂土颗粒破碎机理、高应力下砂土本构模型理论具有重要意义,可为深土工程设计和建设提供理论依据。该论文有图116幅,表21个,参考文献215篇。更多还原

【Abstract】 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.更多还原