Effect of blast-induced vibration on existing tunnels in soft rocks

In urban areas, it is common to excavate rocks adjacent to existing tunnels. Excavation of rock ground is commonly carried out using the drill and blast method which may cause blast-induced damages to the existing tunnels. In this case securing the safety of the existing tunnels is one of the main issues, and the possible effects need to be evaluated. Empirical approaches using the velocity formulae are frequently adopted to evaluate the influence of the blast-induced vibration. This method cannot, however, appropriately consider various influencing factors on vibration and basically needs to be validated using the trial blast in the field. In this paper, attempts to identify the effect of blast-induced vibration on the immediately adjacent tunnels are made, and a preliminary guideline for evaluating the protection zone for the blast vibration is proposed. A numerical method is adopted for the dynamic modeling of a tunnel in soft rock. A two-dimensional blast load is evaluated by modifying the detonation pressure formula based on the results of field tests. Tunnel behavior due to the blast-induced vibration is investigated in terms of particle velocity, displacement, and stress of the linings. A guideline for the blast protection zone is proposed based on a parametric study on blast location, tunnel depth, and the amount of explosives.