Experimental and Numerical Investigation on Mechanical Performance of Shield Tunnel Segments Strengthened by Novel Prefabricated Basalt-Fiber-Reinforced Composite Profiles

To address the challenge of deformation control in operating tunnel structures, this study investigates a novel reinforcement method for operating shield tunnels using a basalt-fiber-reinforced polymer-wrapped concrete-filled steel tube (BFRP-CFST) composite profile. Two primary study variables were considered. At the segment level, the reinforcement condition comprised two levels: unreinforced and BFRP-CFST-reinforced, with three replicate specimens at each level (US-1 to US-3 and RS-1 to RS-3, respectively). At the full-ring level, the number of installed composite profile frames comprised five levels (n = 0, 1, 2, 3, and 4), where n = 0 represented the unreinforced reference condition. A combined experimental and numerical framework was established, including full-scale four-point bending tests on individual tunnel segments and finite element simulations of full-ring linings. Experimental results demonstrate that the ultimate bearing capacity of reinforced segments increased from 534.4 kN to 921.3 kN, corresponding to a 72.4% improvement. The load level before visible cracking increased by 84.2%. At maximum crack widths of 0.2 mm and 2.0 mm, the mid-span displacement of the reinforced segments was reduced by 30.0% and 21.4%, respectively. The test observations indicate that the prefabricated composite profiles effectively delayed crack development and improved the post-cracking stiffness of the segment. Full-ring numerical simulations further showed that installing one to four composite profile frames increased the external load corresponding to a convergence displacement of approximately 10.5 cm by 12.4%, 21.1%, 28.5%, and 37.4%, respectively. Scientifically, the results reveal a staged load-transfer process in which adhesive bonding provides distributed load transfer during the initial response, while mechanical anchors maintain residual load transfer after local interface debonding; they also establish a quantitative relationship between the number of profile frames and full-ring convergence resistance. From an applied engineering perspective, the proposed profile increased the ultimate load and crack-initiation load of the segments by 72.4% and 84.2%, respectively, while its lightweight and prefabricated configuration provides a potentially rapid rehabilitation option for operating shield tunnels.