Large-Deformation Mechanisms and Optimization of Excavation and Support for Layered Carbonaceous Slate Tunnels

Résumé fourni par la source

Large deformation is one of the most critical hazards in tunnels excavated under complex geological conditions. It often causes significant economic losses and threatens construction safety. For layered soft rock tunnels subjected to high in situ stress, the deformation and failure mechanisms are largely governed by the bedding dip angle. To clarify these mechanisms and optimize the corresponding construction control measures, this study investigates a carbonaceous slate section of a railway tunnel in the Western Sichuan Plateau. Field monitoring and FLAC3D numerical modelling are coupled. The influence of the bedding dip angle on the plastic-zone evolution and the failure modes of the surrounding rock is analysed. The micro-bench, three-bench, and reserved core soil methods, together with the rock bolt length, are comparatively evaluated. On this basis, a differentiated reinforcement strategy is proposed for bedding-induced asymmetric loading. The results indicate that: (1) The bedding dip angle governs the failure mode of the surrounding rock. Under the micro-bench method, the plastic zone in subvertically bedded rock masses exhibits a quasi-symmetrical distribution along the normal direction of the bedding planes. The sidewalls predominantly undergo flexural failure. In contrast, under bedding-induced asymmetric loading, the plastic zone concentrates at the left springline and right shoulder. An asymmetric composite failure mode is formed, characterized by shallow flexural–tensile cracking and deep-seated interlayer shear. (2) Under the subvertical bedding condition (89°), the reserved core soil method mitigates the excavation-induced unloading disturbance most effectively. It achieves the lowest peak stress and the smallest tunnel convergence, which is 15.7% and 33.0% lower than those of the micro-bench and three-bench methods, respectively. Its plastic zone reaches full numerical convergence. The reserved core soil method is therefore identified as the optimal excavation Scenario under this condition. (3) The rock bolt length exhibits a threshold effect on deformation control. The most substantial improvement occurs when the bolt length is increased from 4 m to 6 m, beyond which the benefit tends to plateau. A bolt length of 6 m is therefore recommended as the best-performing Scenario among the tested values (4, 6, 8, and 10 m) for the investigated geological and support conditions. For surrounding rock subjected to bedding-induced asymmetric loading, a differentiated reinforcement strategy targeting the vulnerable zones reduces the maximum deformation by 18.8% and 28.3% compared with the uniform reinforcement Scenario and the baseline Scenario, respectively. These findings provide practical insights into excavation-method selection and support optimization for layered soft rock tunnels under similar conditions.

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