Response Characteristics of Micro Difference Blasting Technology for Ultra-Close Range over Existing Tunnels

A micro difference blasting construction technique was employed on-site to investigate the changes in the first principal stress, vibration velocity, and displacement of the initial support structure of the lower main tunnel after the blasting construction of the upper smoke exhaust channel, in response to the construction of the main tunnel structure at a close range (4.44 m). The finite element numerical calculation method was used to quantitatively assess the safety of the adjacent tunnel. The results show that: After the full section blasting of the longitudinal exhaust duct, the most dangerous point of the initial support structure of the main tunnel is located at the arch position directly below the blasting face of the longitudinal exhaust duct. The peak vibration velocities in the X, Y, and Z directions of the control nodes are all less than the standard value of 20 cm/s, and the displacement value is within 3.63 mm; The stress and deformation curves of the lining structure peak within 200 ~ 300 ms and subsequently exhibit oscillatory attenuation over time. The local tensile stress concentration state is formed at the arch crown, and the overall risk of the structure is controllable. After using the micro difference blasting technology for construction on site, the smoke exhaust channel smoothly crossed through the main tunnel structure. At the intersection of the initial support arch crown, there was a local phenomenon of initial support falling off due to individual anchor rod bending and prying, and the overall stress was relatively safe. The on-site monitoring results are consistent with the numerical calculations, proving the reasonable reliability of the micro difference blasting construction technology. The research results can provide reference for similar projects.