Tunnel portal lining behavior in freeze–thaw conditions and innovative anti–freezing insulation structures

Freezing can greatly impact the capacity of tunnel linings in soil, making it essential to investigate the changes in stress state caused by frost heave. Therefore, a finite element model for frost-heave analysis is established for a cold-region tunnel portal. The research investigates the impact of lining thickness and tunnel burial depth on the distribution and trends of axial force (N) and bending moment (M) in the tunnel lining before freezing, after freezing, and after a freeze-thaw cycle. The results show that frost-heave-induced internal forces concentrate at the side wall and arch haunch, identifying these as key vulnerable sections of the portal lining. Generally, the axial force and bending moment of the lining increase after freezing and continue to increase after a freeze-thaw cycle. In addition, a composite anti-freezing insulation structure with both thermal-insulation and anti-freezing buffer functions is proposed, and the influence of its thickness on insulation performance is analyzed. Compared with the case without insulation, the composite layer reduces the maximum frozen-zone thickness in the surrounding rock by up to about 75% and decreases the transverse and longitudinal displacements/convergences of the lining by approximately 80-90%. With increasing thickness, the composite layer also lowers the frost-heave force (F_f-h) on the secondary lining and makes its distribution along the lining more uniform.