We provide an analysis of a tunnel opening cut in a tunnel face. In particular, we focus on the effect of a 'burn cut,' which is a typical parallel hole cut. As the evolution of the opening consists of an increase in the volume of the cut prism and the consequent reduction of confinement of the holes along the initiation sequence, we analyze such an evolution using photographic documentation, as well as evaluating the reduction of confinement by measuring the energy transferred from the explosive charge to the rock mass. The energy was estimated by monitoring and analyzing the vibrations induced by the blasting at three different points. We adopted low-cost shock sensors for monitoring the effect of the detonation times at short distance from the blast (about 10 m). The results show an evident reduction of the shock transfer to the rock mass (i.e., a reduction of confinement) with the increase of the cavity opening. Nevertheless, when only the response of the piezometric sensors was considered, a disparity in behavior among the sensors was noted. On the other hand, when the sensor response was integrated over time, the behavior of all sensors normalized, showing the need to focus both on the oscillation and the duration to properly address the shock effect.
- Burn cut