A modified capillary pressure–saturation model for unsaturated flow in asphalt mixtures and hydraulic behavior analysis

The capillary pressure–saturation (P_c–S) relationship is a critical constitutive model for modeling multiphase flow in unsaturated porous media. In the context of asphalt pavements, however, existing P_c–S models predominantly rely on direct adoption of empirical formulations in unsaturated soil, lack rigorous experimental validation, and often fail to capture the unsaturated hydraulic characteristics of pavement materials. This study aims to (1) develop a continuous mathematical framework for the P_c–S relationship in asphalt mixtures by integrating the physical constraints derived from experimental data with the functional form of classical models, and (2) elucidate the impact of this relationship on unsaturated seepage dynamics. A modified van Genuchten model was formulated to provide a concise, physically consistent P_c–S function. The proposed model demonstrates improved fidelity to experimental data compared to conventional soil-based hydraulic models, while preserving mathematical tractability. By implementing this P_c–S constitutive relationship within a numerical solver (SEEP/W) for the Richards equation, the hysteretic behavior of unsaturated flow was simulated in asphalt mixtures. The simulations reveal that the unsaturated flow analysis, incorporating the proposed P_c–S relationship, predicts a more complex and realistic moisture transport regime than the saturated flow approximation, and can reflect the observed in-service behavior of pavements. This work advances understanding of moisture transport in unsaturated asphalt mixtures and provides a useful tool for evaluating the long-term hydraulic performance in pavement–subsurface water systems.