Research On Multi-field Coupling Thermodynamic Constitutive Theory And Model For Saturated Geomaterials

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.