THE INFLUENCE OF DRY-WET CYCLES ON THE SOIL-WATER CHARACTERISTICS OF UNSATURATED SOIL IN THE HYDRO-FLUCTUATION BELT OF THREE GORGES RESERVOIR AREA

The fluctuation of water levels in the Three Gorges Reservoir causes a cyclical wet-dry environment for the soil in the hydro-fluctuation belt, leading to alterations in pore structure and soil-water characteristics. Consequently, the seepage and mechanical processes of the reservoir bank are influenced. This study conducted experimental tests on the soil-water characteristics of the hydro-fluctuation belt soil in the Bazimen landslide area of the Three Gorges Reservoir. These tests examined the effect of varying dry-wet cycles on the soil-water characteristic curve(SWCC). A novel SWCC model was proposed to analyze the changes in pore structure under dry-wet cycles and its correlation with SWCC, taking into account the influence of pore size magnitude and distribution. The alterations in microstructure were validated through a comparison of the results obtained from scanning electron microscopy(SEM). The following findings are obtained:(1)The SWCC of the soil from the hydro-fluctuation belt tends to shift towards the left as a result of dry-wet cycles. As the cycle period increases, the impact of dry-wet cycles on SWCC gradually diminishes. (2)The proposed model encompasses several parameters with clear physical meaning, and the SWCC results predicted from the proposed model matches well with the experimental data. The correspondence illustrates that the proposed model is capable of describing the SWCC behavior of unsaturated soil in relation to variations in pore size and distribution resulting from dry-wet cycles. (3)The dry-wet cycles can significantly increase the size of soil pores and simplify the complexity of pore size distribution. As the number of dry-wet cycles increases, the maximum equivalent pore radius increases while the fractal dimension decreases, leading to a decrease in water holding capacity. (4)The SEM test provides additional evidence that the soil experiences pore enlargement and an increase in micro-cracks after undergoing dry-wet cycles, resulting in a gradual loosening of the microstructure. This phenomenon is likely the primary cause for the observed increase in maximum equivalent pore radius and decrease in fractal dimension.