Thermal-Hydro-Mechanical Model of the Frost-Susceptible Soil for the Slope Stability Analysis

Abstract

For the numerical analysis of frost-susceptible soil behavior by the repeated freezing and thawing due to the seasonal temperature changes, it is essential to develop a THM(Thermal-Hydro–Mechanical) model which can consider complex mechanisms of thermal, hydraulics, and mechanics at the same time. Several numerical models have been proposed and utilized to analyze the behavior of frost-susceptible soil. The previously suggested models have the common disadvantages of not considering the isotropic tensile strength of the unfrozen-frost-susceptible soil and the change in isotropic tensile strength of the frost-susceptible soil due to the repeated freeze-thaw cycle. They also have limitations that could not be utilized for the application analysis such as slope stability analysis and ground stabilizing method in the cold region featuring permafrost or seasonally frozen soil. Therefore, this dissertation presents a new THM model which complements the shortcomings and limitations of the previous THM models. In addition, slope stability and application analysis were performed by implementing the developed THM model using internal user-subroutines(UMAT, UMATHT, and UEXPAN) in ABAUQS which is the commercial finite element analysis program. This dissertation firstly reviewed the previous literature on experimental research and numerical analysis of the frost action phenomenon of frost-susceptible soil in order to develop a new THM model. Based on the literature survey, the detailed model parts(thermal part, hydro part, and mechanical part) constituting the THM model were developed. The new THM model can couple the detailed model parts with each other and can be applied to FEM analysis. The calibration and validation analysis of the new THM model was performed based on the previous laboratory experiment literatures. Additionally, a numerical analysis model of thermosiphon, one of the ground stabilization methods, was developed to enable conjunction analysis with the developed THM model and performed the analysis to determine the performance of the buried pipeline in frost-susceptible soil. Finally, slope stability analysis was performed according to a series of strength changes following freezing, thawing and freeze-thaw repetitions due to the freeze-thaw cycles of the seasonal temperature changes. The main features of the THM model developed in this dissertation are as follows. First, the THM model developed in this dissertation considers the initial isotropic tensile strength of unfrozen frost-susceptible soil in calculating the yield surface and also considers the isotropic tensile strength that changes according to the repeated freeze-thaw cycles of frost-susceptible soil. Second, the THM model developed in this dissertation applied the CSL(Critical State Line) according to the load angle to the MCC(Modified Cam Clay) model. This application of CSL according to the load angle prevents the yield stress of the supercritical side from being overestimated by coinciding the yield surface of the MCC model with the edge of the yield surface of the MC model. Third, the THM model developed in this dissertation can perform slope stability analysis that could not be utilized in the previously suggested THM models and can derive conservative slope safety factor results. In addition, it is possible to continuously calculate the local safety factor of slopes over time due to repeated freezing and thawing of the soil, which cannot be implemented in the previous GLE(General Limit Equilibrium) and FEM slope stability analysis. Fourth, the THM model developed in this dissertation can be applied to various ground-structure interaction application analysis by interdependent conjunction with the thermosiphon model.