Study on theory model of hydro-thermal-mechanical interaction process in saturated freezing silty soil

The freezing of frost susceptible soils is a dynamic hydro-thermal-mechanical (THM) interacting process. One-side freezing experiments of saturated soil in an open system with no-pressure water supplement are carried out. In these experiments, we analyzed the influence of temperature gradients, overburden pressures and cooling temperatures on moisture migration and frost heave. Based on these experiments, a mathematical model of frost heave is proposed with the variables of temperature, porosity and displacement, in which Clapeyron equation is employed as the phase equilibrium condition of water and ice in soil. Ice lens is one of the major aspects of the frost heave for frost susceptible soils. According to the mechanical and physical characteristics, a comprehensive criterion for the formation and end of new ice lens is presented. To solve the nonlinear equations, the finite element algorithm is applied to solve the general form of governing equations. Finally, numerical simulations are implemented with the assistance of COMSOL. Validation of the model is illustrated by comparisons between the simulation and experimental results. From this study, it is found that, (1) cooling temperature is the necessary condition for moisture migration and frost heave since pore water phases into ice under cooling temperature. Then negative pore water pressure occurs in soil. Pore water pressure gradient is the direct driving force for water migration in saturated soil. However, temperature gradient and overburden pressure have important influence on the pore water pressure gradient. The response of soil sample to the variation of water content lags behind the response to the change of temperature. (2) Discontinuously distributed ice lenses form near the cold front when the accumulated water phases to ice. Temperature gradient, overburden pressure and cooling temperature are key factors to determine the frost heave and moisture migration. (3) Freezing and migration of unfrozen water cause the change of porosity in soil. Ice lens will block the migration of unfrozen water. The discrete ice lenses result in the oscillation in the distributions of water content. (4) Unsaturated phenomenon occurs in the unfrozen zone under the frozen fringe which might be related to the suction effect of large negative pressure in this zone. (c) 2014 Elsevier Ltd. All rights reserved.