Instability mechanism and reinforcement measures for segments of Ultra-Large diameter shield tunnels when constructing cross passages by mechanical methods

Construction of cross passages by mechanical methods represents an emerging construction technique with numerous advantages. However, cutting through segments of ultra-large diameter (ULD) shield tunnels poses significant safety risks, and the instability mechanism of those structures with lateral openings remains unclear. Based on China's first ULD shield tunnel project that adopted the mechanical method for construction of cross passages, refined three-dimensional finite element models were created with ABAQUS. The models considered material and geometric non-linearity and were employed for comprehensive analyses of construction disturbance responses, instability mechanisms of segments, and reinforcement measures. The findings reveal that (1) mechanical construction of cross passages leads to a 24.84 % to 58.57 % increase in structural convergence deformation and a significant stress concentration; (2) The C-shaped ring and the semi-split rings no longer behave consistently in terms of force and deformation after the opening is formed. Additionally, there are significant shifts in the instability characteristics under localized surcharge loads of the segmental lining, resulting in a 30 % decrease in instability start load; (3) After adopting the reinforcement measure of steel ring girders (SRG), the instability process of the structure with opening is similar to that of the initial structure. The loss in instability start load, caused by mechanical construction, has been reduced by 34.4 % similar to 84.4 %. However, increasing SRG thickness beyond 40 mm has a limited impact on reinforcement efficacy. This study offers theoretical and practical insights for similar engineering projects, positively contributing to project safety.