The shear strength of unsaturated granite residual soil (GRS) will decline rapidly when moisture content increases, because of loss of matric suction in the soil. This can lead to slope instability and embankment collapse during rainfall. Determining the matric suction of GRS accurately at different conditions can improve the accuracy of stability analysis. Generally, matric suction can be characterized as the soil–water characteristic curve (SWCC), and it is closely related to soil microstructure. In this study, the entire range of matric suction was measured using a combination of pressure plate method, filter paper method, and vapor equilibrium method. Microstructure change was investigated based on mercury intrusion porosimeter tests, in which the pore size distribution (PSD) curves of GRS samples with different initial densities and moisture content were measured. The PSDs of samples before and after wetting were also measured and compared to investigate the influence of water intrusion. Test results indicate that the combination of measurement methods can cover the entire matric suction range. The microstructure of GRS with different initial conditions presents a clear bimodal PSD. At the same density, the increase in initial moisture content can increase intra-aggregate pores and decrease inter-aggregate pores. When initial density increases, only the volume of inter-aggregate pores decreases. Wetting can decrease inter-aggregate pores and increase intra-aggregate pores. Finally, the Li model is applied in SWCC fitting for tested samples, the result of which performs well with a high correlation coefficient (R² > 0.95) and is recommended for GRS analysis.

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Granite residual soil is widely employed as subgrade fill material, but its tendency to undergo wetting-induced deformation under moisture infiltration poses significant challenges to pavement stability. To address this issue, this study introduces an innovative wetting device capable of precisely controlling moisture content increase, enabling multi-step wetting tests under controlled conditions. Saturated wetting tests were also conducted using both single-line and double-line methods, and the results were compared. Pore size distribution curves for granite residual soil samples with different initial states were measured using Mercury Intrusion Porosimeter (MIP) tests. Results indicate that for both the single-line method and the double-line method, the ${\epsilon }_V$-lgp curve for samples subjected to different compaction efforts remains parallel across varying initial moisture content. The increase in vertical stress will constrain the water adsorption and swelling potential. Whereas an increase in compaction effort leads to greater swelling potential, which is mitigated by an increase in initial moisture content. By integrating the test results of the soil water characteristic (SWCC) curve, the relationship between normalized wetting deformation and matric suction is primarily influenced by the initial state of the soil sample and remains unaffected by vertical stress during multi-step wetting. Based on the test results, an empirical wetting prediction model was formulated, accounting for the influence of vertical stress, initial matric suction, and matric suction after wetting. Fitting results confirmed that the established model achieved high prediction accuracy (R² > 0.9), supporting its application in practical engineering endeavors.