Abstract: When applying ultra-high-performance concrete (UHPC) to shield tunnel reinforcement, varying UHPC layer thickness at different positions creates a variable cross-section reinforcement, thereby improving UHPC utilization efficiency. To investigate the mechanical performance of tunnel linings reinforced with variable cross-section UHPC, a full-scale test is conducted on a shield tunnel lining. In the test structure, the UHPC layer thickness is 70 mm at the tunnel crown and mid-sides and 40 mm at the shoulders, reducing the amount of UHPC used at the shoulders while increasing the clearance between the train and structure. Moreover, a calculation method for UHPC-reinforced shield lining is established based on curved beam theory to reveal the internal force distribution mechanism. The findings are as follows: (1) The failure mode of the variable cross-section UHPC-reinforced structure is characterized by UHPC cracking and bond interface failure, encompassing three phases—elastic, elastoplastic, and plastic—demonstrating good ductility. (2) Compared with the uniform cross-section UHPC reinforcement method, the load-bearing capacity of the variable cross-section UHPC-reinforced structure increases by 25.0%, allowing for an additional 4.1 m of overburden load. (3) The uniform cross-section UHPC-reinforced structure exhibits a distinct "over-reinforcement" failure mode, where the segments fail first, followed by the reinforcement body. In contrast, in the variable cross-section structure, the UHPC and interface fail first, followed by the segments, thereby improving UHPC utilization. (4) The calculation method for UHPC-strengthened tunnel lining based on curved beam theory is thus validated as accurate and effective through experimental results, demonstrating that shear failure primarily governs the failure of the strengthened interface.

Key words: shield tunnel, variable-section ultra-high-performance concrete reinforcement, secondary loading, optimization design, full-scale test, reinforcement effect