Study of the Particle Breakage Characteristics of Coral Sand Under the Effect of Freezing–Seepage Coupling

In the development and construction of the South China Sea (SCS), coral sand is a kind of common natural construction material. Sanya submarine tunnel is the first application of artificial ground freezing (AGF) in the SCS. Since the tunnel is located at an estuary, high-velocity seepage will have a significant influence on the particle characteristics of coral sand under freezing conditions. Therefore, taking coral sand from the SCS as the research object, the one-dimensional soil column unidirectional freezing test, particle sieving test, and scanning electron microscope (SEM, Hitachi High-Tech Corporation, Tokyo, Japan) test were carried out to investigate the particle breakage and temperature variation characteristics of coral sand under the coupling effect of freezing and seepage. The results show that under the coupling effect of freezing and seepage, coral sand particle breakage was significant. Under none-seepage and 0.5 m/d seepage velocity, the proportion of particles in the 0.5–2 mm size range in the frozen and phase transition zones decreased, while the proportion in the 0.125–0.5 mm size range increased. Through SEM analysis, the coupling effect of freezing and seepage caused serious damage to coral sand particles. Intense freezing could cause coral sand particles to break, while strong seepage could increase the roundness of particles. Seepage would affect the freezing rate and the final stabilization of the freezing temperature; when the seepage velocity was small (0–1.2 m/d), the impact of seepage was not obvious, and when seepage rate was larger (3 m/d), the impact throughout the entire freezing process both reduced the freezing rate and increased the final stabilization of the temperature. This study can provide a reference basis for the research on particle characteristics of coral sand under the coupling effect of freezing and seepage and for engineering and construction in the SCS.