Experimental investigation and predictive model of entire suction range for undisturbed granite residual soil

Undisturbed granite residual soil (UGRS) in its natural state has gained attention for its broad engineering applications. Compared to remolded soils, it closely mimics natural conditions, holds unique significance in geotechnical engineering, crucial for stability analyses, environmental protection, and infrastructure development. In this study, the soil-water characteristic curve (SWCC) of UGRS were measured using indirect methods, including Pressure plate method (PPM), Filter paper method (FPM) and Vapor equilibrium method (VEM) under different wet-dry cycles. Mercury intrusion porosimetry (MIP) was conducted to obtain the pore size distribution (PSD), from which the SWCC was derived. A key novelty of this study lies in the systematic comparison and integration of experimental and pore-structure-derived SWCCs, revealing that the combination of FPM, VEM, and MIP without performing PPM can reliably cover the entire suction range while significantly reducing testing time. Test results indicate that the combination of PPM, FPM, and VEM can cover the entire range of matric suction of UGRS. However, discrepancies arise in the overlapping section where PPM records higher values than FPM. The SWCC can be derived through calculations based on the PSD of MIP. The SWCC calculated by MIP closely aligns with PPM in the low suction range of 0-100 kPa, which is the primary range of engineering concern. The combination of FPM, VEM and MIP without the PPM is the optimal method for entire suction range SWCC determination of UGRS, ensuring data accuracy while mitigating the prolonged time consumption and high data points in the low suction section of the PPM. The SWCC of UGRS can be fitted using the Fredlund&Xing model. The fitting parameters of the SWCC of UGRS can establish a strong linear correlation with the number of wet-dry cycles, enabling the establishment of a model that predicts the SWCC of UGRS across the entire suction range by incorporating pore structure evolution and cyclic effects. The results offer an efficient and accurate alternative for full-range SWCC determination and provide a basis for predicting hydraulic behavior of UGRS under cyclic environmental conditions.