Experimental study on the bearing characteristics of UHPC prefabricated lining structures for drill-and-blast tunnel

Abstract

Insufficient vault lining thickness and back cavities in the cast-in-place secondary linings of drill-and-blast tunnels cause cracking, spalling, threatening safety. To resolve these issues, the waffle-slab ultra-high performance concrete (UHPC) prefabricated lining was developed; four-point bending tests were conducted to investigate segments bearing characteristics with varied web and rib reinforcement ratios, focusing on bearing capacity, deformation, failure mechanisms and crack propagation. The results indicate that: (1) UHPC prefabricated lining reinforced segments undergo four failure stages through the slope change of the load–deflection curve: the elastic stage, the strain-hardening stage, the ultimate bearing stage, and the strain-softening stage; (2) The load-bearing performance of the web and rib in the UHPC waffle-slab prefabricated lining structure varies significantly. An increase in the reinforcement ratio of the rib directly enhances the load-bearing capacity of the segments. The cracking load for samples S6-S9 increases by more than 260%, while the peak load rises by over 80%. Additionally, an increase in the web reinforcement ratio leads to the formation of more micro-cracks in the concrete, which helps dissipate load and improves the energy absorption capacity of the segments, thereby influencing the peak load. However, this increase has a minimal effect on the cracking load. For samples S2-S7, the cracking load increases by only 0.37%, whereas the peak load rises between 5.48% and 24.20%.; (3) Based on the load–deflection curve, the deformation behavior of concrete and steel bars under load, and the failure characteristics of concrete segments, the following reinforcement scheme for UHPC prefabricated linings is recommended: For rib plates, the total reinforcement ratio must exceed 1.75% and the tensile zone reinforcement ratio must be over 1.3%, with double-layer reinforcement allowed. For web plates, the reinforcement ratio should exceed 1%; (4) Analysis of crack propagation characteristics in the specimens, conducted using Digital Image Correlation (DIC) equipment, reveals that the crack propagation in unreinforced specimens occurs in two distinct stages: microcrack initiation and crack development. At loads ranging from 25.67 kN to 45.92 kN, microcracks initiate and develop, eventually evolving into macroscopic through cracks, which leads to a loss of bearing capacity in the specimens. In contrast, the crack propagation in reinforced specimens is categorized into three stages: microcrack initiation, microcrack development, and macroscopic crack development. During this process, the localization of the strain field gradually intensifies, deformation damage becomes increasingly concentrated, and crack propagation stabilizes as it approaches the post-peak stage. These findings are provided as reference for the design and bearing performance analysis of UHPC prefabricated linings for drill-and-blast tunnels.