Investigation into the Early Cracking Behavior of High-Geothermal Tunnel Lining Concrete Based on Thermal–Mechanical Coupling Model

As a typical extreme environment, a high-geothermal environment poses severe challenges to tunnel construction in western China. In this paper, a thermal–mechanical coupling model was formulated to evaluate the cracking behavior of lining under high-geothermal conditions, considering the early property evolution of concrete. This was further validated by field monitoring and analyzed by adjusting the relevant parameters. Results indicate that the higher cracking risk occurred on the external surface of the lining sidewall after 24 h of casting. With the increase in surrounding rock temperature, the duration of cracking risk in the lining was extended. When the surrounding rock temperature exceeded 68.7 °C, thermal insulation measures were required for the lining structure. Notably, superior thermal insulation was achieved by applying a sandwich structure of rigid polyurethane materials with a thickness of 20–60 mm. In terms of curing conditions, adopting formwork with a larger heat convection coefficient was conducive to reducing the cracking risk of the tunnel lining, with an appropriate removal time of 48 h. This work provides insights into the thermal–mechanical behavior of lining concrete, thereby mitigating its early cracking in a high-geothermal environment.

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