Constitutive Model for Thermo–Hydro–Mechanical Behaviors of Saturated Partially Frozen Cohesionless Soils: A Theoretical Pore-Scale Study

The thermo–hydro–mechanical (THM) behaviors of frozen soils are often modeled based on the thermodynamic fluxes of moisture and heat. However, existing models disregard the pore-scale granular interaction between the soil grain and ice crystal for saturated partially frozen soils. The pore-scale mechanism of pore-filling and load-bearing for the ice phase in the loaded soil skeleton has not been explored. An alternative constitutive model is therefore proposed by considering the microscopic temperature-dependent distribution of the ice phase for pore-filling and load-bearing in the soil interpore. This reflects the influence of the ice phase on the soil stress states and the associated THM behaviors as interpreted based on the critical state framework. The model was validated by published experimental results and considerably captured undrained shearing behaviors of frozen soils at various temperatures and confining pressures. A numerical parametric study was conducted to investigate the dependency of the phase relationship, stress state, undrained shear strength, and soil stiffness on temperature. The modeling suggests that the ice crystals filling in the pore are partially load-bearing to affect the soil stiffness and partially unloaded to alter the stress state. It shows that freezing turns the soil into a heavily consolidated state by increasing the specific volume and decreasing the effective granular void ratio. The undrained shear strength of frozen soils increases with a decrease in temperature because the dilatancy is enhanced due to the ice invasion in the interpore. It also demonstrates that soil stiffness is influenced by not only the stress state but the freezing history. This study highlights the temperature-dependency of mechanical behaviors and the validity of using the concept of stress states to interpret the pore-scale mechanistic soil–water–ice interactions for frozen soils.