Disaster mechanism analysis for segments floating of large-diameter shield tunnel construction in the water-rich strata: A case study

During the construction of large -diameter shield tunnels crossing water -rich strata near the sea, serious disasters such as segment floating, cracking, and water leakage often occur. This study, based on field tests, identified the engineering disaster factors of segment floating and adjacent misalignment, established a numerical calculation model for shield tunnel construction considering the interaction between segments, revealed the engineering disaster derivation mechanism in the water -rich environment, proposed treatment measures for engineering disaster, and the effectiveness of the treatment measures were verified in field engineering. The research results indicate that the larger grouting pressure in the lower part of the segment will cause greater compression deformation along the radial direction, leading to greater adjacent misalignment and more obvious circumferential rotation of the segment, significantly increasing the risk of local yielding of bolts and reinforcement mesh. At the beginning of grouting, the flow velocity of pore water in the strata is the highest, and as the grout around the segment gradually solidifies, the flow velocity and accumulation range of pore water around the segment gradually decrease. The optimal grout material ratio including 200 kg of cement, 380 kg of water, 680 kg of sand, 380 kg of fly ash, and 90 kg of bentonite can effectively limit the floating displacement of the segment, and improve the assembly flatness of segment along the longitudinal direction.