Investigation of passive failure mechanism of shallow-buried inclined shield tunnels

Inclined shallow tunnels can achieve the designed depth within a shorter distance of advancement, but their excavation faces often experience a high potential for passive failure. This work proposes a novel kinematic method to investigate the passive instability of inclined shallowburied tunnels. The proposed mechanism, consisting of three rigid blocks, includes both translational and rotational failure modes. The limit support pressure is obtained by the upper bound approach. Validation is conducted by comparing the predicted results with existing analytical and numerical references. The influence of governing parameters, including soil strength, tunnel geometry, and inclination angle, is systematically examined. The findings reveal that the internal friction angle mainly affects the magnitude of support pressure, whereas cohesion more prominently governs the deformation pattern, particularly the extent of the rotational zone. Similar trends are observed for variations in surface surcharge and depth ratio, which mainly alter the overall size rather than the configuration of the failure region. Although the impact of the inclination angle is relatively minor, it becomes notable in soils exhibiting higher friction. Finally, the applicability and reliability of the developed mechanism are demonstrated through an Ultra Rapid Under Pass (URUP) project case.