Blast vibration characteristics and its isolation scheme selection of existing tunnels in multi-arch tunnels without middle wall

In the blasting excavation of the following excavating tunnel, it is extremely important to ensure the blasting vibration safety of the existing excavated tunnel, as the traditional middle wall is cancelled in the new multi-arch tunnel and the neighborhood spacing between the following and existing tunnels is very small. In this study, the blasting vibration characteristics of the secondary linings of the excavated tunnel at the side wall are analyzed based on the on-site blasting vibration monitoring at a highway tunnel. Then, based on the inheriting of the explosion pressure simulated by the two-dimensional (2D) fine model into the three-dimensional (3D) real-scale model, a coupling simulation method for blasting dynamic response is established. The distribution laws of the peak particle velocity (PPV) at the longitudinal and cross sections of the excavated tunnel are studied respectively using the aforementioned simulation method. Moreover, the safety control standard of the blasting vibration is calculated according to the fitting relationship between the PPV and the maximum tensile stress of the secondary linings of the excavated tunnel. Finally, the vibration control effects of the excavated tunnel under different vibration isolation schemes are analyzed, and the suitable vibration isolation scheme is recommended considering of the construction feasibility, efficiency and cost, etc. The results indicate that the vibration velocity near the arch-waist and arch-shoulder at the blasting side of the existing tunnel is the largest during the excavation of the following excavating tunnel. Besides, the PPV is governed by the horizontal radial vibration, and it decays faster at the excavated direction than that at the opposite direction. The blasting vibration control standard of the secondary linings of the excavated tunnel is calculated as 24.54 cm/s. The vibration isolation layer can effectively ensure the blasting vibration safety of the excavated tunnel, and the vibration isolation material is recommended as the plate-type L-600 polyethylene closed-cell foam, whose appropriate thickness and height are respectively 5 cm and 5 m. By using the above vibration isolation scheme, the vibration reduction ratios of the arch-waist and arch-shoulder of the exiting tunnel can reach 54.8% and 63.5%. Furthermore, the measured PPVs all do not exceed the safety threshold, and no cracks are found at the secondary linings of the excavated tunnel.