Seismic performance of straight-bolt joints in prefabricated groove-shaped assembled utility tunnels: Experimental, numerical, and engineering study

Prefabricated utility tunnels are increasingly adopted in modular and industrialized underground construction. However, while previous studies have mainly focused on the seismic behavior of individual joints, the structural response and failure mechanism of assembled utility tunnel systems remain insufficiently understood. This study investigates the seismic performance of straight-bolt-connected groove-shaped prefabricated assembled utility tunnels through experimental and numerical analyses. Four half-scale joint specimens with different bolt diameters, bolt numbers, and concrete types were tested under cyclic horizontal loading. A validated finite element model was subsequently extended to establish an overall utility tunnel model for structural response evaluation and parametric analysis. The experimental results indicate that increasing bolt diameter and bolt number can effectively improve the seismic performance of straight-bolt joints, while lightweight concrete exhibits comparable bearing capacity and enhanced deformation capacity. Based on the validated model, the seismic behavior of the overall utility tunnel structure was further investigated. The results reveal that the global failure of the assembled utility tunnel is governed primarily by the variable cross-section regions of the sidewall haunches rather than by the central joint locations. Plastic hinges develop progressively in these critical regions and ultimately control the structural failure mechanism. Parametric analyses further demonstrate that the improvement in structural capacity becomes limited when the bolt diameter exceeds 24 mm, the bolt number exceeds six, or the concrete strength exceeds C40. Based on these findings, engineering recommendations are proposed regarding bolt configuration, concrete strength selection, and critical strengthening regions, providing guidance for the seismic design and practical application of prefabricated assembled utility tunnels.