Study on the frost heave behavior of the freezing unsaturated silty clay

The frost heave behavior of unsaturated soils is related to the water-vapor redistribution, whereas the water-vapor migration processes cannot be directly observed due to the limitations of measurement techniques. To reveal the frost heave behavior of the unsaturated silty clay, five specimens for unsaturated silty clay were prepared. One-side freezing tests for the five specimens were carried out under uniform conditions. The test durations are 12 h, 24 h, 48 h, 96 h, and 192 h, respectively, and the water content profiles in the freezing process were studied by testing final water content profile of each specimen as the profile at that time. The heat-water-vapor migration and frost heave behavior of the freezing unsaturated silty clay were interpreted. The results showed that the temperature variations can be divided into three stages (a sharply cooling stage, a slowly cooling stage, and a steady stage) according to the change rate of temperature, and the duration times of the first and the second stage are short. A mix migration of liquid water and water vapor in unsaturated soils can be experimentally determined during the freezing, and a thin ice forms at the top of the specimens due to the vapor migration and condensation. Meanwhile, the liquid water in unfrozen zone mainly migrates to frozen fringe, and segregated ice forms in the freezing unsaturated soil once enough water accumulates at the frozen fringe. In addition, the frost heave of the specimens is discrete under the uniform test conditions. We selected the relative error and absolute error of the frost heave as evaluation indexes, and further evaluated the repeatability of the test results. Evaluation results showed that the one-side freezing test results are repeatable, and it is an useful testing method to study the heat-water-vapor migration through one-side freezing tests with different test durations under uniform testing conditions. Moreover, the confusion that the frost heave mainly occurred in the top layer of high-speed railway subgrade was explicit solved by the behavior revealed in this paper.