A Theoretical Framework for Modeling the Chemomechanical Behavior of Unsaturated Soils

Abstract

A theoretical framework is presented for modeling the chemomechanical behavior of multiphase porous media, in general, and unsaturated soils, in particular, which can address skeletal deformation, fluid flow, heat conduction, solute diffusion, chemical reaction, and phase transition in a consistent and systematic way. A general expression is derived for the electrochemical potential of a fluid species with explicitly accounting for the effects of osmosis, capillarity, and adsorption. The equilibrium behavior of porous media is investigated, and the composition of pore water pressure is identified. Explicit formulations are developed for the effective stress and intergranular stress, with consideration of physicochemical effects. It is shown that the negative water pressure measured by a conventional transducer can be significantly different than the true pore water pressure. It is also theoretically revealed that, other than the soil water characteristic function, a new pressure (or potential) function accounting for the physicochemical effects is generally required in analyzing the coupled chemomechanical processes in unsaturated soils. The new theory is capable of effectively explaining many salient phenomena occurring in water-saturated porous media with a degree of saturation varying from an extremely low value to 100%, including Donnan osmosis, capillary fringe, air entry value, initial hydraulic head during seepage, and pressure solution. The new theory can be used to analyze the multiple coupled physical and chemical processes in the vadose zone.Abbreviations: AEV, air entry value; PWP, pore water pressure; REV, representative volume.Chemomechanical behavior of porous media with multiphases and multispecies is of great interest in many diverse fields of science and engineering. To mention a few, this includes the industries of nuclear, hazardous, and municipal waste isolation; petroleum and gas extraction; technologies of methane gas hydrates exploitation; CO 2 sequestration; and weak soil reinforcement, landform stability assessment, structural material durability, and weathering of rock masses. Understanding, controlling, and predicting the long-term effect of physicochemical processes on the mechanical performance of geomaterials is becoming an indispensable part of environmental impact assessment and performance assessment analysis. Thus, there is a clear need to develop comprehensive chemohydromechanical mathematical modeling capacities that are able to address realistically the reactive multispecies transport, multiphase flow, chemical reaction, phase transition, chemically induced deformation, and other related physicochemical processes in deformable soils.The coupling of multiple processes in porous media, including skeletal deformation, seepage, diffusion, and heat conduction, has been extensively investigated, and the poromechanic theory that is capable of describing these coupled physical processes has been very well developed (e.g., Coussy, 2004). Thus far, however, the behavior of porous medi...

Application of the Generalized Clapeyron Equation to Freezing Point Depression and Unfrozen Water Content

“…To characterize the microscopic interactions associated with interfaces, Wei () proposed that the free energy of the pore liquid (water solution) can be decomposed into two components, that is, the free energy of the water solution free of surface forces and the surface potential accounting for the surface forces in the pores. The surface potential (denoted by Ω l ) is defined as the negative specific work done against the surface forces during the movement of an infinitesimal amount of the water solution, in a thermodynamically reversible manner, from a reservoir to the soil pores at the same state characterized by temperature, T , the mass density of the liquid solution, ρ l , and the mass fraction of species, ( l k denotes species k in the pore liquid l ).…”

“…After some manipulations, one obtains from the above equilibrium condition that where Π is the generalized osmotic pressure, accounting for the strength of physicochemical interactions, and Π D is Donnan osmotic pressure (Mitchell & Soga, ; Wei, ), given by Clearly, Π D is equal to the osmotic pressure difference between the pore solution and the reservoir solution; both Π D and Π can be viewed as part of the pore solution pressure, which is of physicochemical origin and independent of any mechanical forces applied through a reservoir solution (say ). Equation also shows that Π D results from the solvent activity difference between the pore solution and the reservoir solution, which can be intrinsically attributed to the electrically charged nature of soil mediated by the condition of electrical neutrality (Gonçalvès et al, ; Mitchell & Soga, ; Wei, ).…”

Phase Equilibrium Condition for Pore Hydrate: Theoretical Formulation and Experimental Validation

“…where V l * is the molar volume of pure water, which is assumed constant; h l * is the molar enthalpy of pure water; P l is the pressure of pore solution; a l is the chemical activity of pore water; and Ω is a surface potential, which accounts for the physicochemical interactions between the pore solution and the matrix of the hydrate-bearing soils. In general, Ω is a function of temperature, pore fluid concentration, degree of saturation, porosity, and fixed charge density as well as other physicochemical properties of the solid matrix and pore solution (Wei, 2014). Now combining equations (1)-(4), one can derive the equilibrium condition for the formation of gas hydrate in a pore solution, viz.,…”

“…In hydrate-bearing soils, the pore solution interacts with the solid matrix due to the electrically charged nature of the matrix. To account for these physicochemical interactions, an additional term must be added to the right-hand side of equation (A1) (Wei, 2014), yielding…”

Work input for unsaturated soils considering interfacial effects

“…The interfacial energy has been adopted to explain the capillary effect 4,29 and other physicochemical effects. 30 Hassanizadeh and Gray 31 discussed the capillary pressure in detail based on thermodynamic theory and pointed out that the capillary pressure must be recognized as a function of degree of saturation and interfacial area. In recent years, relationships among capillary pressure, saturation, and interfacial areas have been proposed by several investigators.…”