Impact of Evaluation of Freeze–Thaw Cycles on Collapse Zone at Entrance and Exit of Loess Tunnel

The entrance and exit of loess tunnel are easily affected by freezing and thawing, which leads to collapse in cold regions. Based on the slip line network method, this paper proposed a graphical method and analyzed the impact of evaluation of freeze–thaw cycles on the collapse zone at the entrance and exit of a loess tunnel. Firstly, the slip line network method was improved for a graphical calculation program, and the program was validated by monitoring the data of collapse evolution in an unlined loess tunnel in situ. Then, some triaxial tests of freeze–thaw cycles were carried out on loess, and the attenuation law of loess strength was summarized after freeze–thaw cycles in cold regions. Finally, the graphical calculation program was applied to evaluate the impact of freeze–thaw cycles on collapse range in the unlined loess tunnel. The results show that introducing the classic slip line field into the analysis process of a tunnel collapse zone can provide a feasible and efficient analysis method for a tunnel surrounding rock collapse arch for tunnel engineering, and the collapse zone increases with the growing number of freeze–thaw cycles. Moreover, the attenuation of loess strength which suffered from the freeze–thaw cycles would cause the collapse zone area to increase by a maximum of 16.13%.