Numerical simulation of freeze-thaw in short period of secondary lining at tunnel transition section in seasonal frozen area

Repeated freeze-thaw in the lining and surrounding rock is always accompanied by weathering process such as cracking and spalling. The conventional frost-preventing theory will underestimate the freeze-thaw frequency because of the assumption that the ambient yearly temperature changes as sinusoidal function. A kind of automatic temperature recorder adapting to the low temperature environment is developed, through which the long-term and high frequency monitoring of lining surface temperature at tunnel transition section in seasonal frozen area is carried out. The difference equation for transient heat conduction of multilayer cylinder in one-dimension is also derived, whose calculation accuracy is verified by an example. The freeze-thaw process of lining surface temperature at tunnel transition section in seasonal frozen area is simulated after the monitored data are chosen as the boundary condition in the difference equation, and the impact of position of insulation layer on the freeze-thaw is discussed. The result shows that the temperature of the region fluctuates acutely, where it is shorter than 5 cm from the lining surface. A seasonal freezing and several freeze-thaw cycles in a short period appear in secondary lining inside every year. When the position of insulation layer is located between the secondary lining and the initial lining, the frequency of freeze-thaw cycle in a short period and the start and stop time of seasonal freezing change little, but the influence depth of freeze-thaw cycle in a short period is extended. When the position of insulation layer is located at the secondary lining surface, the inner temperature is always greater than 0℃, which is beneficial to the for frost-prevention of tunnels.