Dynamic Response and Safety Control of the First-Excavated Tunnel Lining Under Adjacent-Tunnel Blasting in Closely Spaced Tunnels

Dynamic disturbances induced by blasting excavation of the later-excavated tunnel in closely spaced tunnels may propagate through the inter-tunnel rock pillar and induce significant vibration and stress responses in the lining of the first-excavated tunnel. To investigate the spatial response characteristics and governing factors of the first-excavated tunnel lining, a three-dimensional blast-dynamics model was established based on the Zhongliangshan Tunnel project. The axial and circumferential distributions of peak particle velocity (PPV) and maximum principal tensile stress were analyzed, and the effects of rock-mass grade, charge weight per blasthole, and tunnel clear spacing were evaluated. The relationship between PPV and maximum principal tensile stress was further examined, and the relative variability of the lining responses under different governing factors was evaluated using the coefficient of variation (CV). The results show that the lining response is concentrated mainly near the face of the later-excavated tunnel, with the blast-facing haunch and shoulder exhibiting the strongest vibration and tensile-stress responses. Both PPV and maximum principal tensile stress decrease with increasing axial distance from the face, with generally faster attenuation behind the face than ahead of it. As the rock-mass condition deteriorates from Grade III to Grade V, both response indices increase overall. Increasing tunnel clear spacing significantly reduces both responses, with the most pronounced reduction occurring when the spacing increases from 1 to 3 m, whereas increasing the charge weight per blasthole generally intensifies the lining response. PPV is strongly correlated with maximum principal tensile stress. The CV analysis shows that tunnel clear spacing produces the greatest relative variability in both response indices within the investigated parameter ranges. For PPV, the response variability follows the order of tunnel clear spacing > charge weight per blasthole > rock-mass grade, whereas for maximum principal tensile stress, the order is tunnel clear spacing > rock-mass grade > charge weight per blasthole. These findings provide a basis for blasting-parameter optimization and safety control of existing linings in closely spaced tunnel construction.