Ground Deformation Mechanism of Soft Soil Strata during Tunnel Construction by Freeze-sealing Pipe Roof Method

To investigate the ground deformation mechanism during the excavation stage of the pipe roof freezing method, this study takes the Gongbei Tunnel project as a case study. A frost-heave calculation model is established by introducing a thermal expansion coefficient, and a three-dimensional numerical model for tunnel freezing excavation is subsequently developed. Using this model, numerical analyses of ground deformation under typical working conditions are performed. The results indicate that: (1) Maximum settlement of surface deformation during the excavation of the upper layer soil at the top of the A-step arch reached 28.8mm, which occurs at the mid-span position of the excavation face and the adjacent support axis, and is basically consistent with the on-site monitoring values. (2) When the excavation face is 15 m away from the monitoring section, the surface deformation caused by the excavation of the upper layer soil, steps A and B, is significantly greater than that of the lower layer soil, steps C and D. The maximum surface settlement after the excavation of step D is 54.9 mm, and the surface settlement gradually increases. Due to the excavation of the left side guide tunnel first and the lag on the right side, the thickness of the frost heave soil on the left side is smaller than that on the right side, and the frost heave force can be released into the tunnel. Therefore, the frost heave amount on the right side of the tunnel is slightly greater than that on the left side. (3) Surface deformation was primarily attributed to the combined effects of frost heave, lining self-weight and excavation support. Frost heave (39.6%), followed by lining self-weight (38.8%), while excavation support had the least impact (21.6%).