Mechanical response of broken surrounding rock by in-situ expansion of highway tunnel

Many early highway tunnels are no longer adequate for current traffic demands and require renovation. For short tunnels, in-situ expansion is a proposed solution. However, most research on tunnel construction mechanics and load calculations has focused on new tunnels, with limited studies on in-situ expansion, particularly in tunnels with fractured surrounding rock. This paper presents research using similar material models, field pressure and deformation monitoring, and surrounding rock pressure theory. Key findings indicate that removing the upper bench lining and excavating the surrounding rock on the expansion side reduces surrounding rock stress, with a more significant pressure decrease on the expansion side. The shallow surrounding rock experiences greater stress reduction than deeper strata. Field monitoring shows similar three-stage patterns (rapid change, slow change, and stability) in anchor shaft force, steel arch stress, and contact stress between the initial support and surrounding rock, stabilizing after 15 h. Stress concentrations occurred at the left and right arch shoulders, with higher stress at the left. The removal of upper bench lining and surrounding rock expansion had a significant impact on stress changes, while lower bench excavation had minimal effect on arch shoulder stress. Physical and field data showed severe damage and stress reduction at the right arch shoulder on the expansion side, causing asymmetric stress and damage distribution. The rates of perimeter rock pressure change during tunnel expansion were significantly higher than during sub-conductor excavation, indicating the substantial influence of construction steps and expansion width.