Acoustic emissions from flat-jack test for rock-burst prediction

Rock-burst is a local instability problem affecting compact rock-masses in deep tunnels: it can generate important over-breaks, but even more, safety issues for workers in the area where it occurs. The experimental study, aimed at predicting rock-burst occurrence during the excavation of the Brenner Base Tunnel, was carried out in massive granite below an overburden of 1000-1200 m. The experiment consisted in compressing a portion of the rock-mass at the tun-nel side-wall by means of a couple of flat-jacks, measuring simultaneously the generated acoustic emissions with velocimeters and accelerometers. Recorded data clearly show a peak in the acoustic emission energy few instants before the failure of the rock-mass. This value, after careful site validation based on the continuous monitoring of acoustic emissions during the excavation, has been implemented as an alarm threshold for the interpretation of measurements and for the adoption of appropriate countermeasures.