Influence of Drying-Wetting Cycles on the Water Retention and Microstructure of Residual Soil

Abstract

Due to frequent changes in the humid and hot environment, the residual soil with a particle-size distribution (PSD) from gravel to clay experiences multiple drying-wetting cycles. The pressure plate test and nuclear magnetic resonance (NMR) spectroscopy were used to investigate the influence of drying-wetting cycles on the soil-water characteristic curve (SWCC) and pore-size distribution (POSD) of undisturbed residual soil. The results showed that the water-holding capacity of the residual soil decreased as the number of drying-wetting cycles increased and gradually stablilized, and then the van Genuchten (VG) model was found to perform well on the SWCC during the drying-wetting processes. The NMR results indicated a double-pore structure, and the porosity of the residual soil as well as the internal water content increased smoothly with more drying-wetting cycles. The obtained POSD curve of soil implied that drying-wetting cycles had a more obvious effect on small pores and macro-pores than on micro-pores and meso-pores. Theoretical calculations evinced that the product of the matric suction and relaxation time should be constant at a constant temperature. However, the experimental results did not effectively reflect such a relation between the matric suction and relaxation time. A modified VG model based on the cumulative pore volume was utilized to describe the POSD under drying-wetting cycles. Subsequently, the proposed Rational2D surface equation was used to accurately reflect the internal relationship between the SWCC and POSD curve under different numbers of drying-wetting cycles. Moreover, the fractal model for the SWCC derived from the capillary theory confirmed that the matric suction had a strong linear relationship with the relative volumetric water content in the log-log scale. Also, the fractal dimension can be approximated as a constant, because its attenuation is small with more drying-wetting cycles.