Shield tunneling method is widely used in subway tunnel construction because of its advantages such as not affecting ground traffic, high mechanization degree and short construction period. However, the problems of cracking and damage caused by uneven rising of segments are often encountered in the construction process of shield method, which affects the normal use of tunnel. However, the research on the mechanism and theoretical calculation of shield tunnel segment floating is relatively rare. In this paper, the floating mechanism of segments in the initial floating stage of shield tunnel is analyzed deeply, and the corresponding theoretical calculation formulas of upward buoyancy/anti-buoyancy and buoyancy are proposed. The theoretical calculation results are compared and analyzed with engineering examples. The results show that: The initial floating of the segment can be divided into two stages: rapid rising and slow rising, in which the rising amount of the rapid rising stage accounts for about 80%~90% of the total rising amount. The theoretical calculation value of the total floating amount is greater than the monitoring value, but the calculation error of the floating amount per ring is relatively stable, the theoretical calculation formula presented in this paper can provide reference for the calculation of the floating amount of the actual segment.

The case of direct cutting of obstacle piles by shield tunneling frequently appears in the project, and it is very important to study the construction impact of shield cutting piles to ensure the quality and safety of tunnels and existing structures. Relying on the Shaoxing Metro Line 2 crossing underground passage pile group project, the finite element numerical simulation method is used to analyze the influence of the double-line shield tunnel cutting pile group on the formation and existing structure, and the influence of MJS reinforcement range on the deformation of the existing structure is explored. The results show that: The lower residual pile floats up after the shield is cut, and the upper residual pile and the rest of the pile foundation have obvious settlement. The truncated pile foundation has obvious tensile stress, with a maximum tensile stress of 1.23MPa. The MJS reinforcement can reduce the channel settlement and the lateral deformation of the pile foundation caused by shield excavation. With the increase of MJS reinforcement range, the channel settlement decreases significantly. Among them, the channel settlement is reduced by more than 38% after the MJS reinforcement under the tunnel, and the channel settlement is reduced by more than 48% after the MJS reinforcement around the tunnel.