Thermal-hydraulic coupling effects on shear strength and particle breakage of coral sand

Coral sand is widely used in marine construction; however, its mechanical response under thermal-hydraulic coupling conditions, particularly during the artificial ground freezing process, remains insufficiently studied. This study investigates the thermal-hydraulic coupling effects on the temperature field evolution, particle breakage, and shear strength of frozen coral sand through macro- and micro-scale experiments, including unidirectional freezing, direct shear, particle size analysis, and scanning electron microscopy observations. Results show that the seepage effect has a greater influence on the temperature field of frozen coral sand and is mainly concentrated in the initial freezing stage. The greater the seepage velocity, the slower the freezing efficiency. Thermal-hydraulic coupling increases the proportion of medium-sized particles (0.125–0.5 mm) due to frost-induced breakage of large particles, accompanied by pronounced changes in particle morphology from sharpness to rounded. Under the coupled conditions, the shear strength in the phase-change region increased more significantly at other seepage velocities compared to the no-seepage case. This is attributed to intense particle crushing and enhanced interlocking induced by the coupling of freezing and seepage, whereas the strength was lowest under no seepage. Overall, both the temperature field evolution and shear strength development of coral sand are governed by particle breakage. The heat absorbed by particle breakage and the pore structure reorganization lead to changes in permeability, resulting in a slower rate of temperature decrease, while the small particles produced by active particle breakage within the phase transition and frozen zone form a denser skeleton structure through the pore filling effect, which significantly enhances the shear strength. These findings clarify the dominant role of particle breakage in controlling the thermal and mechanical behavior of frozen coral sand under thermal-hydraulic coupling and provide valuable guidance for artificial freezing applications in the South China Sea.