Optimization Construction Scheme of Single-Sided Shallow Tunnel Enlarging

At present, the construction schemes of tunnel enlarging are analogized by creating a new tunnel through a traditional method. However, the traditional excavation method ignores the advantage of long-term support and stability of the original tunnel lining to the surrounding rock, and the construction efficiency is also reduced. The optimized construction plan was theoretically analyzed using the case of the Chongqing Yuzhou tunnel enlarging. The optimization construction schemes of single-sided shallow tunnel enlarging were extracted in accordance with the foundation of the study on the construction mechanics of a shallow tunnel, that is, the way of the rational arch axis enlarging in the crosswise direction and the jump-driving method in the lengthwise direction. The construction analysis was compared with the field monitoring measurement results and ANSYS simulation of the 3D finite element calculation of the entire tunnel enlarging process. By optimizing the construction scheme, the calculation value of the compressive stress of the original tunnel lining was increased by excavation face advancing. The original tunnel lining that had not been dismantled can play the role of the column and bear the surrounding rock pressure caused by the jet-hopping excavation. This lining could effectively improve the safety and stability for tunnel enlarging. The adopted rational arch axis excavation in the transverse direction could reasonably and significantly transfer the surrounding rock pressure. This process could reduce the tensile stress and increase the compressive stress of the initial support structure. Results of the numerical calculation and field monitoring data showed that the excavation scheme of the shallow tunnel is optimized after unilateral expansion. The optimization construction schemes can change the original rock stress path and play a role in the surrounding rock stability. The displacement of the tunnel vault can be reduced by 16%-20%, and the tunnel vault does not appear in the tension zone. The optimization of construction scheme can reduce the arch subsidence and tensile stress of the lining, ensure the safety of construction, and shorten the construction period. These obtained beneficial conclusions can be supplied to the next similar tunnel enlarging projects.