本文基于物理守恒定律及非平衡态热力学理论，建立了一种针对饱和岩土体的温度场-渗流场-应力场（THM）完全耦合问题的理论模型。该模型从理论上确定了热力学体系的耗散力构成，并采用经典非平衡态热力学理论，将能量的耗散归结为一系列迁移系数模型的确定。本文从理论上统一地给出了所有物理场所应遵循的物理规律，包括一中无需屈服面、流动法则、加卸载准则和硬化/软化准则等概念的应力场本构模型。本文模型考虑了温度和变形对各相密度及土体渗透性的影响以及渗流和热传导之间的相互耦合，并推导得到了一个饱和岩土体渗流公式的一般形式。采用熵增方程作为温度场控制方程，考虑了热弹耦合和能量耗散过程（如非弹性变形发展）对温度场的影响。 本文考虑了暂态弹性和颗粒涨落这两种颗粒固体材料特有的能量耗散机制，引入了颗粒熵及颗粒熵温度的概念对颗粒涨落剧烈程度进行描述，并得到了非弹性变形发展规律与颗粒涨落和暂态弹性之间的定量关系。弹性势能密度函数给出了有效应力与弹性应变的关系，并在有效应力空间上唯一确定了一个极限应力状态面。同时，弹性势能密度函数也反映了岩土体的粘性、应力引起的各向异性和弹性模量和强度的状态相关性等重要特征。分析表明，本文得到的应力场本构模型具备统一考虑岩土体类型、密实程度、排水条件、超固结度和加载速率等因素对力学行为的影响，并且具备较好的预测能力。本文模型给出了一种与临界状态土力学中相似但又有区别的临界状态，引入等效修正应变的概念，可以合理反映岩土体的滞回特性，如滞回环的发展、残余应变的累积、有效应力的衰减和卸载刚度的退化等。 通过考虑结合水相和自由水相间随温度变化的相互转化及其激发的颗粒涨落，可较好地模拟饱和岩土体的非等温固结现象。分析表明，非等温固结过程具有显著的OCR值依赖性和不可逆性。本文模型也可反映温度荷载作用下的热破坏现象。在不排水条件下，当饱和土体存在较大剪应力时，在温度循环荷载作用下，孔压将迅速累积，热剪切应变将发生急剧增长。本文通过将温度对饱和岩土体剪切性质的影响归结于非等温固结过程引起的干密度变化和温度对某些重要参数的影响，可较好地模拟饱和岩土体排水和不排水剪切性质的温度效应。对不同土体类型、不同OCR值和不同排水条件的剪切性质温度依赖性进行了研究，并给出了统一的机理解释。
Based on the physical conservation laws and the non-equilibrium thermodynamics, a fully coupled thermo-hydro-mechanical (THM) theoretical model for saturated geomaterials is established in this paper. The dissipative forces are determined theoretically and the energy dissipations are described by a group of migration coefficients according to the linear non-equilibrium thermodynamics. From this approach, the physical laws of each field are deduced theoretically, including a generalized water flow formula and a constitutive model that does not need the concepts of yield surface, flow rule, loading-unloading criteria and hardening/softening rule. The effects of the temperature and deformation on the permeability and the densities of all phases and the coupling between the water flow and the thermal conduction are taken into account in this paper. The entropy increase equation is adopted as the govern equation of the thermal field, in which the influence of the thermoelastic coupling and the energy dissipation processes (e.g., the non-elastic deformation development) are considered. The transient elasticity and granular fluctuation are considered as two most important dissipation mechanisms for granular solid materials. The granular entropy and granular entropy temperature are introduced to describe the severe degree of the granular fluctuation. Thus, the non-elastic strain evolution is quantitatively determined by the energy dissipations induced by these two dissipation mechanisms. In this paper, the elastic potential energy density function gives a relation between the effective stress and the elastic strain and provides a unique ultimate stress state surface in the effective stress space. Furthermore, the elastic potential energy density function can represent many important features of geomaterials such as the cohesion, the stress-induced anisotropy and the state-dependency of elastic modulus and strength. Analyses show that the model is able to give a unified consideration of the effects of soil types, soil densities, drainage conditions, OCR values and loading rates on the mechanical behavior. Compared with the critical state in the critical state soil mechanics, a similar but differentiated critical state is obtained in this paper. Meanwhile, the equivalent correction strain is defined in order to avoid the exaggeration of residual strain accumulation and energy dissipation under cyclic loadings. Thus, the model captures the hysteresis features of geomaterials such as the hysteresis loops, the residual strain accumulation, the effective stress attenuation and the stiffness degradation. Through the consideration of the conversion between the free and bound water phases and the granular fluctuation simulated by this conversion process, the non-isothermal consolidation can be simulated rationally. Analyses show that the non-isothermal consolidation is heavily OCR dependent and irrecoverable. Under undrained conditions, if the existing shear stress of saturated geomaterials is large, the pore pressure and thermal shear strain will develop rapidly under cyclic thermal loadings, resulting in the so-called thermal failure of geomaterials. The effects of the temperature on the shear behavior of geomaterials is represented in the model mainly attributed to the dry density changes induced by the non-isothermal consolidation and the changes of certain model parameters induced by the temperature changes. In the last of this paper, the temperature-dependency of the shear behavior for different soil types, OCR values and drainage conditions is simulated and a unified mechanistic explanation for this temperature-dependency is given.
