Enhancement mechanism of UHPC secondary lining on the mechanical performance at joints of shield tunnels: Full-scale experiment and simulation

• Proposes an innovative double-layer lining using UHPC as the secondary lining. • UHPC secondary lining enhances ultimate bearing capacity. • UHPC secondary lining improves initial characterization of bending stiffness . • UHPC secondary lining reduces debonding and slipping between linings. The use of conventional concrete as the inner lining in double-layer lining shield tunnels presents challenges, including cracking and poor durability. This study proposes a novel double-layer lining structure with Ultra-High Performance Concrete (UHPC) as the secondary lining. The enhancement mechanism of this structure is analyzed through full-scale experiments and refined numerical simulations. The experimental results reveal that, compared to the joints using C30 as the inner lining with a thickness of 20 cm, those with UHPC as the inner lining exhibited an increase in ultimate bearing capacity by 27.23 % and 39.47 % under sagging and hogging moments. Additionally, the initial characterization of the bending stiffness of these joints improved by 21.00 % and 30.06 %. The joints with UHPC as the inner lining exhibited a 29.61 % increase in initial cracking load under sagging moments and a 22.61 % increase in crushing load under hogging moments compared to those with C30 as the inner lining. Notably, the joints with UHPC as the inner lining show lower debonding and slipping between the inner and outer linings. Finally, refined numerical simulations reveal that a 10 cm thick inner lining with UHPC offers a bearing capacity equivalent to that of a 30 cm thick inner lining made of C30, while a 20 cm thick UHPC inner lining exhibits bending stiffness matching that of a 30 cm thick C30 lining. Comparative analyses of different concrete grades and axial forces further highlight UHPC’s significant advantages in enhancing overall structural performance.