【作者】 李波；

【导师】 房营光；

【作者基本信息】 华南理工大学 ， 岩土工程， 2019， 博士

【摘要】 土体是自然界最广泛的工程材料,合理掌握土体力学特性是保证工程安全和稳定的重要前提。然而,土体复杂的物质成分和随机易变的微细观结构使其力学特性仍未准确掌握,一些经过“精心”设计的岩土工程项目仍然经常出现严重的工程问题甚至发生工程安全事故。土体力学特性的复杂性主要源于颗粒间的相互作用特性和孔隙尺度特征(即孔隙尺度及分布特征)及其演变。颗粒间的相互作用主要是通过界面上的物理-化学效应如颗粒间摩擦和挤压、颗粒-水和气的吸附、砂粒与粘粒胶粒的胶结等来实现的,其性质与颗粒尺度密切相关,将影响作为土体力学性质而呈现“颗粒尺度效应”。土体变形通常伴随孔隙压缩和渗流,引起大孔隙湮灭和微小孔隙萌生等孔隙尺度特征变化,由此产生固结变形的“孔隙尺度演变效应”;颗粒-孔隙液的物理化学效应将显著影响土体渗流特性,该影响与孔隙尺度密切相关,随着孔隙尺度的减小而逐渐显著,由此呈现渗流的“孔隙尺度效应”,进而影响土体渗流过程,使土体力学特性更加难以掌握。本文主要针对上述问题,深入微细观尺度层次,重新考虑问题的内在机制,建立相关的模型理论并给上述问题的理论分析。主要研究工作和成果如下:(1)针对不同应力应变状态的颗粒尺度效应进行研究,展示直剪试验与三轴试验颗粒尺度效应的差异。基于剪切绕流以及基体-增强颗粒的作用特性,对直剪试验的颗粒尺度效应进行了理论分析,并对直剪试验与三轴试验之间颗粒尺度效应的差异给出合理的机理解释。(2)基于多尺度胞元模型,利用等效夹杂理论和材料的弹塑性变形特性,确定考虑基体塑性变形的应变集中张量,由此利用各尺度层次上的关联性及能量法则,导出土体本构关系;引入量纲平衡参数(即内禀尺度)对应力和偶应力进行耦合,由此获得表征土体内禀特性的特征长度参数,并分析土体颗粒尺度对其内禀特性的影响。(3)基于多尺度胞元模型,考虑细观基体-增强颗粒的作用特性,导出包含细观特性参数的Mohr-Coulomb准则,并展示屈服轨迹的颗粒尺度效应特征;引入相对转动梯度和内禀尺度,导出有限元计算的刚度矩阵,并编制有限元程序对应力局部化问题、应变局部化问题和颗粒尺度效应进行算例分析。(4)基于固结变形过程中孔隙尺度演变的试验,导出考虑孔隙尺度演化的非线性固结方程;基于文中孔隙尺度演化固结理论,给出超孔隙水压力消散、固结压缩变形和平均固结度随时间变化的一系列计算曲线,解释有关文献中报道的不同固结荷载下土体复杂的固结变形特性。(5)基于固-液界面的摩擦机制,考虑孔隙液粘结强度的影响而建立与水力梯度相关的边界滑移模型;利用Poisson方程、热力学平衡条件和Navier–Stokes方程,导出低渗透性土的渗透性模型理论,据此分析土体微-纳米尺度孔隙渗流特性的作用机理和影响机制,描述了渗透系数随水力梯度增加或减小的现象。更多还原

【Abstract】 Soil is the most extensive engineering material in nature.In order to ensure the safety and stability of engineering,it is important to master the mechanical properties of soil.However,because of the complex material composition and variable microscopic structure,it is difficlut to master the mechanical properties accurately.As a result,although some geotechnical engineering is designed "carefully",serious engineering problems or even engineering safety accidents happen very often.The complexity of soil mechanical properties derives from the interaction mechanism between particles and the pore scale characteristics and their evolution.The interaction between particles is mainly realized by the physicochemical effect on the interface,such as interparticle friction and extrusion,adsorption of particle on water and gas,cementation of clay particles and colloidal particles on sand.That is,the interaction property is closely related to the particle scale.Therefore,the mechanical property of soil presents a scale effect of particle.Soil deformation is usually accompanied with pore compression and seepage.Pore compression leads to the soil skeleton damage,aggregated particles breakage and adjustment of grain arrangement position,as well as macropore “obliteration” and micropore “initiation”,resulting in that the consolidation deformation presents a scale effect of pore.Soil seepage characteristic is significantly affected by the physicochemical effect between particle and fluid,which is closely related to the pore scale.With decreasing scale of pore,the effect gradually becomes more significant,resulting in that the seepage presents a scale effect of pore.In this paper,based on the internal mechanism of the above effect,we establish some theory and give analysis of them.The main research work and results are as follows:(1)With different stress/strain state,the effect of particle scale on the mechanical properties of soil is different,such as,the direct shear states and triaxial states.The particle scale effect under direct shear states is theoretically analyzed based on the “shear flow around” and the interaction between matrix and reinforced particles.Morever,the difference of particle scale effects between direct shear state and triaxial state is explained reasonably.(2)Based on the multiscale cell model,a multiscale constitutive relation of soil was derived by using the relationship between different scale levels and the energy equilibrium law.In addition,the stress and the couple stress was coupled by introducing a dimensional equilibrium parameter,and then a characteristic length parameter was obtained to represent the intrinsic characteristics of the soil.(3)Based on the multiscale model,a multiscale mohr-coulomb criterion including meso characteristic parameters was derived by using equivalent inclusion theory.And then,the yield curves of soil with the different reinforced particle contents were demonstrated.In addition,a material matrix is derived by introducing the relative rotational gradient and intrinsic scale,and then a finite element program is developed to analyze the stress localization,strain localization and scale effect of particles.(4)A one-dimensional(1D)nonlinear consolidation equation was reformulated based on the property relationships related to pore evolution,and the nonmonotonic change in the consolidation coefficient with an increase in the effective stress was described.In addition,a series of calculation curves for the dissipation of excess pore pressure,consolidation compression deformation and the change of average consolidation degree were given by using the present theory,and the complex consolidation characteristics of soil under different consolidation loads were explained reasonably.(5)Based on the friction mechanism of solid-liquid interface,the boundary slip model related to hydraulic gradient was established by considering the bond strength of pore liquids.And then,the permeability theory was derived by using Poisson equation,thermodynamic equilibrium condition and Navier-Stokes equation for low permeability soil.Finally,the influence mechanism of seepage characteristic at micro-nano pores was analyzed by using the permeability theory.更多还原