Long-term properties of grout-soil composite eroded by seawater environment in submarine tunnel

The grouting method plays a critical role in preventing seawater intrusion in submarine tunnels, particularly in regions with highly weathered rock that are susceptible to erosion and shifting seawater. The long-term properties of the strength and impermeability of the grout-soil composite are related to the durability of the tunnel lining structure, which is an important focus of the present research. This study introduces a new method for calculating material ratios to determine the optimal proportions of each component in grout-soil composites. A specialized experimental setup was designed to replicate the erosive conditions of seawater in environments characterized by significant rock weathering. The primary objective of this investigation was to analyze the weakening effects of seawater ions (e.g., Mg2+, SO42-, Cl-) on the grout-soil composite under dynamic seawater flow conditions. Therefore, the influence of water-cement (W-C) ratio, grouting pressure, and erosion duration on the compressive strength and permeability coefficient of the composite was studied. Furthermore, microscopic analyses were conducted to investigate the microstructure and composition of the weakened composite specimens. Finally, the model of damage weakening in grouted composite has been established. The experimental results indicate that the erosive ions (Cl-, SO42-) initially enhance and then weaken the strength and impermeability of the grouted composite, while Mg2+ ions continuously degrade the strength of the composite. Reducing the water-to-cement ratio and increasing the grouting pressure can improve the strength and impermeability of the grouted composite, but once a certain threshold is reached, the enhancement effect becomes negligible. Under different dynamic water environments and with various erosive ions (Cl-, SO42-, Mg2+, and seawater), the compressive strength of the specimens at the end of the erosion process decreased by 25.49%, 31.21%, 50.34%, and 39.70%, respectively, compared to static freshwater. The permeability coefficient increased by 8.5 times, 3.2 times, 5.8 times, and 8.9 times, respectively. As the W/C ratio increased from 0.8 to 1.2, the compressive strength decreased by 27.67%, 38.97%, 65.70%, and 44.58%, respectively, and the permeability coefficient increased by 55.24%, 59.70%, 134.23%, and 44.49%. As the grouting pressure increased from 1.5 MPa to 2.5 MPa, the compressive strength increased by 48.90%, 162.60%, 163.71%, and 48.35%, respectively, while the permeability coefficient decreased by 53.76%, 40.05%, 73.69%, and 32.89%. The findings of this study offer valuable insights into the erosion mechanism of grout-soil composites induced by seawater ions, thereby contributing to enhanced durability and longevity of submarine tunnel infrastructure.