# Experimental Study on Shear Performance of Longitudinal Joints in Prefabricated Invert Arch for Mountain Mining Method Tunnels

## TL;DR

This study examines how different bolt-tightening torques affect the shear performance of joints in prefabricated tunnel invert arches used in mountain mining method tunnels.

## Contribution

The paper introduces a method to evaluate shear capacity and identifies the optimal bolt-tightening torque range for structural safety.

## Key findings

- Bolts sheared off or threads were damaged at 50% and 70% of standard torque.
- Optimal torque range for shear capacity is 70% to 85% of standard.
- Proposed calculation method shows conservative results for structural safety.

## Abstract

In order to improve the efficiency of highway tunnel construction and ensure the construction quality, the design concept of a prefabricated inverted arch and partial cast-in-place lining of highway tunnels by a mining method is put forward. During the assembly of prefabricated tunnel invert arches, the longitudinal joints between adjacent invert sections were subjected to shear forces due to the combined effects of the invert’s self-weight and construction equipment loads. This study investigated the shear performance of these longitudinal joints under construction loads, with a particular focus on the influence of bolt-tightening torque. Three longitudinal joint specimens were designed and fabricated, varying the bolt-tightening torque as a key parameter, and subjected to shear tests. The failure modes, load–slip behavior, and shear capacity of the joints were analyzed in relation to the tightening torque of high-strength bolts. The results indicate that when the bolt-tightening torque was set to 50% and 70% of the standard torque, the upper bolts of the joint sheared off, while the threads of the lower bolts were damaged. When the torque reached the standard value, all bolts were sheared off. The ultimate shear capacity of the longitudinal joints increased with higher bolt-tightening torque, with the optimal torque range identified as 70% to 85% of the specified standard. Ultimately, a method of calculation for evaluating the shear-bearing capacity of inverted arch longitudinal joints was proposed, with computational outcomes demonstrating a conservative bias that aligns with structural safety requirements.