Elaborate numerical investigation on surcharge-induced mechanical responses and strengthening effects of single-line shield tunnels with typical cross-sections

Investigating the mechanical responses and strengthening effects of surcharge-disturbed shield tunnels with varying cross-sections is crucial for ensuring the effectiveness of structural safety evaluations and treatments. This study introduces an elaborate modeling approach for segmental rings of three different sizes. Using calibrated finite element (FE) models and a proposed numerical simulation method for strengthening deformed segmental rings, the variations in surcharge-induced mechanical behaviors and strengthening effects in shield tunnels with typical cross-sections were analyzed. Based on these findings, safety classification standards and recommendations for selecting strengthening measures for shield tunnels with typical cross-sections are established. The results indicate that surcharge-disturbed shield tunnels with typical cross-sections exhibit certain variations in structural internal force levels and transmission characteristics, as well as in the timing of plastic hinge formation and joint mechanical response. Additionally, there are also commonalities in the overall deformation and damage mechanisms of the structures. For instance, the stable and accelerated development stages of segmental deformation are primarily driven by segment cracking, while the rapid development stage is triggered by both the surrounding stratum reaching passive earth pressure and the formation of structural plastic hinges. Accordingly, a safety classification standard for single-line shield tunnels with typical cross-sections is established. For a 6.2 m diameter tunnel, a steel plate-UHPC composite structure (SUCS) is recommended if spatial conditions permit, owing to its excellent interface properties. For a 6.7 m diameter tunnel, both SUCS and prefabricated UHPC slab (PUS) are preferred, provided that the layer thickness and interface measures are appropriately designed. For an 8.8 m diameter tunnel, EBSP, SUCS, and PUS are all viable options, although modifications to the conventional strengthening measures are necessary to enhance mechanical performance.