Abstract: Lining structures of high-altitude railway tunnels are prone to early circumferential cracking. To address this problem, a case study is conducted on a deep railway tunnel located in a plateau region. The spatial distribution and propagation characteristics of lining cracks are identified through defect surveying. Based on the "long wall on foundation" temperature stress theory, an extended finite element method is employed to establish a refined reinforced concrete lining model to simulate the temperature field and shrinkage stress field of the arch wall under the constraint of the low sidewall. The simulation results show that the maximum temperature difference between the inner and outer sides of the concrete at the lining wall foot reaches 19.95 ℃, and the shrinkage displacement at the midspan of the lining arch wall is only 0.12% of the maximum shrinkage displacement at both ends. The restraint effect of the low sidewall induces stress concentration at the junction between the low sidewall and the arch wall, where the maximum shrinkage restraint stress reaches 2.35 MPa, exceeding the tensile strength of C35 concrete. A "shrinkage displacement suppressionconstraint stress concentration" effect is thus revealed at the junction of the low sidewall and the arch wall. The simulated crack location, initiation time, propagation path, and crack length are in good agreement with field observations, confirming that the coupled effect of arch wall shrinkage and low sidewall restraint is the dominant mechanism governing lining cracking. Accordingly, anticracking optimization measures based on the concepts of "three low and one high" and a "crack-resistant" concrete mix proportion are proposed. Test results show that: (1) during the temperature drop stage, the shrinkage strain of the "crack-resistant" specimen is reduced by 31.1% and 28.7% compared with that of the conventional "three low and one high" specimen; (2) field application demonstrates that the "crack-resistant" mix proportion reduces the maximum temperature difference between the inner and outer sides of the wall foot by 50.3% during the temperature rise stage, and decreases the shrinkage strain by 65% during the temperature drop stage. These results indicate that a coordinated control mechanism combining temperature regulation and shrinkage compensation can effectively alleviate stress concentration at the interface between the arch wall and the low sidewall, thereby improving the crack resistance of the lining joint.

Key words: high-altitude tunnel, deep railway tunnel, circumferential cracking, shrinkage constraint, optimization of mixing ratios, crack-resistant concrete, numerical simulation