Exploring soil water retention hysteresis in the entire suction range and microstructure evolution of loess: The influence of sediment depths

Sediment depths can influence both soil structure and hydraulic processes, resulting in diverse soil-water retention properties. This study aimed to investigate the impact of sediment depths on soil-water retention hysteresis across the entire suction range, as well as the microstructure evolution of loess during the drying-wetting cycle. Laboratory tests were conducted to achieve this objective. The test results demonstrated that the water retention properties of the natural loess were significantly affected by sediment depths over the entire suction range. And the tested samples shrank considerably due to drying, while the swelling curves were almost linear induced by the wetting. Besides, the commonly used four water retention models were adopted to fit the test data, and Zhou's model showed the best-fitting performance. Furthermore, the local degree of hysteresis in the low suction range was generally larger than that in the high suction range for all samples, and the minimum local degree of hysteresis corresponded to the in-situ suction of tested samples. The average degree of hysteresis of JY-5 m, JY-15 m, and JY-30 m samples were 0.12, 0.14, and 0.13 respectively. Regarding microstructure change, the void ratios (which were smaller than the actual void ratios of soil samples) measured by the mercury intrusion porosimetry (MIP) tests became larger after undergoing the drying-wetting cycle, and most of the increased pores were primarily attributed to the formation of small mesopores in the vicinity of the main capillary pores. The results of this study contribute valuable insights into the dynamics of soil-water retention hysteresis and the associated microstructure evolution of loess.