Load-Bearing Characteristics of a Double-Layer Primary Support in Asymmetrically-Pressurized Entrance Section of a Tunnel With Shallow Cover and Super-Large Span
In the construction of the Xiabeishan No.2 tunnel featuring a shallow cover depth of 10.6 m and an impressively wide span of 26.3 m,a novel approach is adopted to enhance its structural integrity.Specifically,a double-layer primary support system is implemented in the entrance section where the tunnel experiences asymmetric pressure.The effectiveness of single-versus double-layer primary supports is investigated through numerical simulations.Furthermore,the performance of the double-layer primary support is examined in real-world conditions via field measurements.The following results are obtained in this study:(1)In the area stretching from the right arch foot to right arch shoulder on the side facing asymmetric pressure,the support structure undergoes substantial stress,with remarkable local stress concentrations.Here,the double-layer primary support system offers enhanced load-bearing safety and has more effective control over crown deformation than its single-layer counterpart.(2)Field measurements indicate that the maximum horizontal convergence of the asymmetrically-pressurized entrance reaches 18.5 mm.Additionally,the settlement of the surrounding rock at the crown on the pressurized side measures 49.5 mm.These findings suggest that secondary support plays a critical role in stabilizing the deformation of the surrounding rock during the dismantling stage of the primary support.(3)The contact pressure transmitted to the secondary support accounts for 16%-33%of the total at the respective locations.Moreover,the axial force exerted on the secondary support at the right arch foot and shoulder on the asymmetrically-pressurized side is 15%of that on the primary support.This underscores the vital role of secondary support in enhancing the load-bearing capacity of the primary support under asymmetrically-pressurized conditions.In conclusion,the adoption of double-layer primary support technology substantially improves the structural safety of the entrance section in the Xiabeishan No.2 tunnel,which faces asymmetric pressure owing to its shallow cover and wide span.
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