基于多孔介质理论的多组分非饱和土非线性多场耦合模型及其应用研究

With the development of modern geotechnical engineering such as the construction of high level radioactive waste repositories, exploitation of natural gas hydrate and geothermal resources, subgrade/slope engineering at the natural earth's surface, development and utilization of urban underground space and engineering construction in permafrost, the multi-field coupled problems in unsaturated soils including stress, seepage, temperature and chemical effect is inevitable and the related research has become an important issue internationally. In this research, soil is viewed as multi-constituent three-phase mixture. Based on the thermodynamics-based porous media theory, and by adopting the method which combining theoretical modeling, numerical simulation and experiment, the following scientific problems will be studied: how to setting up the balance equations with three levels (constituents, phases and the whole soil) and the constitutive equations, therefore a nonlinear multi-field coupled model for multi-constituent unsaturated soil can be established; based on the established theoretical framework, constitutive models and a new mathematical model that describe the coupled skeleton deformation-fluid flow-heat transfer behavior in unsaturated soils will be proposed via proper choices of free energy and dissipation function; based on the nonlinear multi-field coupled model for soils and by using existing experimental results, the temperature effect to the yield function, harden law and flow rule of the constitutive model will be studied and a new elastic-plastic constitutive model for unsaturated soils under non-isothermal conditions will be developed, besides, Fortran-based computer programs will also be used to realize the model predictive ability; a new temperature controlled triaxial test system for unsaturated soils will also be developed in this research, which can be used to conduct the SWCC test, isotropic compression test and triaxial shear test under different temperatures, so as to reveal the rules of the temperature effect to the basic properties of unsaturated soils and to verify the reasonableness of the established model.

在高放废物地下处置、地热资源开采、天然地表上的路基/边坡工程、天然气水合物的开发、地下空间开发利用、冻土区工程建设等诸多领域，都涉及到非饱和土在热、水力、力学、化学等多场作用下的耦合问题。本项目以多孔介质理论为基础，将土体视为多组分三相混合物，运用理论建模、数值模拟和试验研究相结合的方法，研究非饱和土内各层次的平衡方程和本构方程的建立方法和步骤，提出多组分非饱和土非线性多场耦合模型框架；研究特定条件下自由能函数和耗散函数的选取规则，建立非饱和土变形-渗流-传热耦合模型；研究本构模型中屈服函数、硬化规律和流动法则如何考虑温度影响，建立考虑温度效应的非饱和土弹塑性本构模型，并通过编制计算程序实现模型的预测功能；研究温控非饱和土三轴仪的研制方法，开展不同温度下的非饱和土土水特征曲线试验、各向同性压缩试验及三轴剪切试验，揭示温度对非饱和土基本性质的影响规律，并通过试验结果验证本构模型的适用性。

本项目在前期研究工作的基础上，基于多孔介质理论，开展了非饱和土多场耦合理论模型的研究工作，主要工作包括：.1. 基于多孔介质理论建立了非饱和土热-水-力耦合模型。在非饱和土多场耦合理论框架基础上，通过对自由能函数和耗散函数的选取和Taylor级数展开，从而得到耦合多种场作用的本构方程，并进一步得到非饱和土热-水-力耦合数学模型。该模型统一地描述了变形-渗流-传热耦合作用下土体的变形、流体的流动以及热传导等现象。与已有的研究土体多场耦合问题所不同的是，除了对相应的物理守恒方程进行相应的变化（增加其他场的耦合作用项以及各场之间的界面效应项）外，还对考虑弹塑性变形-渗流-传热时非饱和土中耦合考虑各种场影响的本构关系进行了推导。.2. 非饱和土热-水-力耦合模型的数值实现及应用。以非饱和土多场耦合理论为基础，基于ABAQUS 有限元分析平台，建立二维轴对称模型，对高放废物地质处置库中膨润土缓冲层的热–水–力耦合过程开展了有限元模拟，给出了处置库近场缓冲层中不同位置的温度、饱和度、竖向应力及位移、孔隙水压力及吸力的分布及变化规律，并重点分析了温度场的影响，通过与已有研究成果的对比，证实了所计算结果的合理性。.3. 温控非饱和土三轴仪的研制及试验研究。基于对既有非饱和土静三轴仪的改造，研制了一种温控非饱和土三轴试验装置。通过将所研制的温控压力室与非饱和土三轴试验系统的有机结合，实现了试验过程中对温度控制及量测的目的。利用该试验装置，对不同温度(25, 40, 60 °C)和不同净应力(40, 100, 200 kPa)下非饱和粉土土水特征曲线进行了测定，所得结果与现有理论结果相吻合。.4. 考虑变形影响的非饱和土渗流特性研究。基于饱和度随孔隙比变化的增量关系以及利用非饱和土土水特征曲线（SWCC）预测渗透系数的方法，建立了一种考虑初始孔隙比影响的非饱和土相对渗透系数间接预测方法。在非饱和土SWCC方程基础上，该方法只需增加一个参数，即可对某一种土在不同初始孔隙比条件下的相对渗透系数进行预测，通过与试验结果的对比，证实了所建立的预测模型的正确性。