Temperature, Pore-Pressure, and Salinity Coupling Constitutive Model for Hydrate-Bearing Sediment within a Thermodynamic Framework

Marine hydrate occurs in environments that are characterized by low temperature, high pore pressure, and high salinity. However, the correlations between mechanical properties and environmental factors remain insufficient. This study initially proposed a phase transition line accounting for the salinity influence and defined the temperature, pore pressure, and salinity coupling coefficient. By integrating cementation stress with a dilatancy equation that incorporates hydrate saturation and effective void ratio, a granular thermodynamic constitutive model for hydrate-bearing sediment is established. The results indicate that the mean square error and covariance coefficient for the phase transition line are 0.24 and 0.04, respectively. The enhancement in sediment peak shear strength and dilatancy behavior under conditions of elevated pore pressure, reduced salinity, and decreased temperature is captured. The temperature–pressure–salinity coupling coefficient contributes 11.38% to volumetric strain and 5.17% to pore pressure, lowering the weight of hydrate saturation and stiffness parameter. The constructed thermodynamic model of marine hydrate provides a feasible approach for predicting mechanical behavior under temperature, salinity, and pore pressure interactions.