Abstract
The synchronous grouting layer in subsea shield tunnels is continuously exposed to seawater during long-term service. Sustained migration and diffusion of seawater ions can induce deterioration of the pore structure and degradation of mechanical properties, thereby diminishing the support and load-transfer functions provided to the segmental lining and ultimately compromising the long-term structural safety of the tunnel. In this study, synchronous grouting mortar obtained from an actual subsea shield tunnel project was subjected to freshwater and natural seawater immersion. Compressive strength tests and X-ray computed tomography were combined to investigate the evolution of mechanical properties and three-dimensional pore structure under prolonged exposure to natural seawater. Repeated scans were performed on the same group of samples at different immersion ages to determine total porosity, connected porosity, and pore connectivity. Quantitative relationships were then established between immersion time and both connected porosity and compressive strength. The results indicate that natural seawater exerts a pronounced influence on compressive strength at different stages, shifting from an initial enhancement to progressive deterioration during prolonged immersion. At 420 d, the compressive strength was approximately 23% lower than the peak value measured at 28 d. During prolonged immersion, total porosity increased gradually, whereas connected porosity and pore connectivity exhibited substantial increases. The difference shows that the internal structural evolution was governed primarily by the propagation and interconnection of existing pores and microcracks rather than by a substantial increase in total pore volume. With increasing immersion time, the pore structure progressively evolved from a dispersed and weakly connected state into a more continuous network of pores and cracks, accompanied by the enlargement and interconnection of local defects. The evolution of connected porosity with immersion time was well described by a square root function based on the characteristic time scale of Fickian diffusion, while the variation in compressive strength followed an exponential relationship. Increased connectivity of pores and cracks provided more continuous pathways for the inward transport of seawater ions and progressively disrupted the internal load-bearing framework, thereby contributing to the degradation of macroscopic mechanical properties. The findings provide experimental support for the durability design of synchronous grouting materials and the long-term maintenance of subsea shield tunnels.