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Due to the formational environment and climatic variability, granite residual soils with grain-size distribution ranging from gravel to clay undergo multiple drying–wetting cycles. The influences of multiple drying–wetting cycles on the soil-water characteristic curve (SWCC) and pore-size distribution (POSD) of undisturbed granite residual soils are investigated using the pressure plate test and nuclear magnetic resonance (NMR) spin-spin relaxation time (T2) distribution measurement, respectively. Results show that the water-retention capacity and air-entry value decrease and pores become more uniform with increasing drying–wetting cycles. After four drying–wetting cycles, the soil reaches a nearly constant state. The POSD change of multiple drying–wetting cycle samples is consistent with the SWCC of the soils. Furthermore, a modified van Genuchten model in terms of cumulative pore volume is used to obtain the best-fit POSD of the drying–wetting cycle samples. The shape and changing tendency of both curves of SWCC and POSD are quite similar and achieved a better correlation. It can be concluded that the SWCC is strongly dependent on the POSD of the soil and NMR T2 relaxometry can be used as an alternative to the assessment of microstructural variation of residual soils subjected to the periodic drying and wetting process.

Abstract:In this paper, a simple and efficient fractal-based approach is presented for capturing the effects of initial void ratio on the soil-water characteristic curve (SWCC) in a deformable unsaturated soil. In terms of testing results, the SWCCs (expressed by gravimetric water content) of the unsaturated soils at different initial void ratios were found to be mainly controlled by the air-entry value (ψ a ), while the fractal dimension (D) could be assumed to be constant. As a result, in contrast to the complexity of existing models, a simple and efficient model with only two parameters (i.e., D and ψ a ) was established for predicting the SWCC considering the effects of initial void ratio. The procedure for determining the model parameters with clear physical meaning were then elaborated. The applicability and accuracy of the proposed model were well demonstrated by comparing its predictions with four sets of independent experimental data from the tests conducted in current work, as well as the literature on a wide range of soils, including Wuhan Clay, Hefei and Guangxi expansive soil, Saskatchewan silt, and loess. Good agreements were obtained between the experimental data and the model predictions in all of the cases considered.

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Free iron oxides in soils usually have distinct electrochemical properties, relatively high surface energy, a strong ability to adsorb, and provide structural cementation in the soil. Thus, soil properties are influenced by the content, characteristics and occurrence of free iron oxides. This paper reports a study on Zhanjiang clay and Longmen laterite, both of which are rich in free iron oxides, to investigate the occurrence characteristics of free iron oxides in soils. Dithionite-citrate-bicarbonate solution was adopted to remove free iron oxides from the soils, and qualitative filter paper was used to extract particles with size d \ 25 lm. X-ray diffraction, scanning electron microscopy, and energy dispersive spectrometry were used to analyze alterations in the physical and chemical properties, mineral composition, and microstructure caused by the removal of free iron oxides. The results obtained in this study show that free iron oxides act as a bridge in Zhanjiang clay and mostly in the form of cladding in Longmen laterite. Both forms enhance the cementation and structural strength of the soils and weaken the swelling-shrinkage capacity. The findings of this paper will contribute to a deeper understanding of the formation and failure of soils.