Experimental and analytical study on the mechanical response of shield tunnel segmental linings strengthened by a steel-UHPC composite

Various structural diseases for existing shield tunnel segmental linings are often observed caused by the variation of surrounding condition in long-term operation, such as excessive deformation, cracks, and leakage. This paper proposes a novel strengthening method for segmental linings using steel-UHPC composites. Four-point bending tests were conducted to investigate the mechanical characteristics of the strengthened segment linings. A three-layer analytical model for steel-UHPC strengthening was firstly proposed and validated against the test results. Strengthening mechanism was revealed for the first time through calculating stress response of each material. Additionally, sensitivity analyses were conducted to explore the influence of the strengthening parameters. Test results demonstrate that the ultimate bearing capacity of the strengthened specimens under sagging moments can reach 2.27 to 3.15 times that of the un-strengthened specimen, higher than the 1.81 times under hogging moments, confirming the efficiency of the proposed strengthening method. The steel plate thickness is more crucial for enhancing the bearing capacity under sagging moments. Analytical solutions of interface stress can be used to better explain the observed failure pattern at the concrete-UHPC interface in the test. These results can improve our understanding of the strengthening mechanism using steel-UHPC composites and provide a theoretical basis for actual engineering design.