Abstract: 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.

Key words:  , shallow superlargespan tunnel, asymmetricallypressurized entrance, doublelayer primary support, loadbearing characteristic, numerical simulation, field monitoring