Study on variation characteristics of shear strength of deep-sea sediments in the South China sea under thermo-hydraulic coupling

To investigate the mechanical response mechanisms of deep-sea sediments in the South China Sea under coupled field effects, this study systematically analyzed the influence of moisture content (15%-35%), temperature (4℃-60℃), and confining pressure (100–300 kPa) on their undrained shear strength through triaxial shear tests. A thermohydraulic-coupled model was developed. Results indicate that increased moisture content significantly reduces shear strength: at low confining pressures, strength decreases by 50% across 15%-35% moisture content ranges, while high confining pressures reduce the decrease to 30% by enhancing particle confinement. Temperature elevation accelerates cement softening and particle thermal motion, causing cohesion to drop from 17.6 kPa to 10.9 kPa and internal friction angle from 23.8° to 16.7°. Although confining pressure enhances strength, it cannot reverse cement failure caused by high temperatures. The improved thermohydraulic-coupled model, incorporating moisture content sensitivity coefficient (m) and temperature sensitivity coefficient (n), achieves precise stress-strain relationship predictions under coupled field effects (correlation coefficient R²> 0.97). These findings provide quantitative theoretical support for assessing engineering stability and risk management in resource development of deep-sea areas in the South China Sea.