Vibration Energy Comparison Helps Identify Formation Time of New Free Surface in Urban Tunnel Blasting

Cut blasting creates new free surfaces that facilitate rock breaking and reduces blast vibration. However, the time when the new free surface is formed is not clear in on-site blasting practices. This paper proposes a method to identify this formation time through a vibration energy comparison. Firstly, a variational mode decomposition method identified the initiation time sequence of high-precision detonators from the observed blast-induced vibration wave. Based on the superposition of the single-hole waves extracted from the single-free-surface blasting test, we constructed a predicted wave that shared the initiation time sequence with the observed one. The Hilbert transform found the accumulated energy curves of the two waves separately. By comparing the linear correlation of the two curves, we identified the new free surface’s formation time to improve the blast design. The tunnel-blasting case showed that 64.5 ms was required to form the new free surface. In the actual blasting, each cut hole used 1.0 kg of explosives. The maximum vibration velocity was 0.90 cm·s−1, which met the control target of less than 1.0 cm·s−1.