Study on the Evolution Law of the Displacement Field during Excavation of Super-Long Connection Passage under Freezing Conditions

The artificial ground freezing method is widely used in underground engineering due to its environmental friendliness, safety, and reliability. This study is conducted based on the connection passage of a section of the Fuzhou metro. Field measurement data and numerical simulation are used to investigate the coupled evolution of temperature and displacement fields of the frozen curtain during the freezing-excavation process. The study results indicate that: A staggered freezing method on both sides was chosen to reduce the strong frost heave impact caused by large-volume freezing. Upon completion of freezing, the surface displacements on the left and right sides were 64.51 mm and 76.72 mm, respectively. Upon completion of the freezing process, the surface displacements on the left and right sides were 64.12 mm and 73.84 mm, respectively. During the excavation, the maximum convergence displacement of the frozen curtain was 1.52 mm, and the maximum displacement convergence rate was 0.082 mm/d, both well below control values. Under varying support timing conditions, deformation and displacement of the frozen curtain increased with extended support timing. The average vertical displacement change at each monitoring point is less than 0.03 mm/d. Increasing the excavation step length from 1.5 m to 2.5 m resulted in the frozen curtain's bottom heave and top settlement increasing by 1.16 times and 5.56 times, respectively. Vertical displacement changes were greater when the excavation step length was between 1.5 m and 2.0 m compared to when it was between 2.0 m and 2.5 m. These findings can be a reference for future freezing-excavation projects of ultra-long subway connecting passages.