A new methodology for the rock-burst assessment during tunnel construction

Risk associated to rock-burst occurrence in tunnels is nowadays managed by a continuous monitoring of acoustic emissions recorded during the excavation, whose characteristics depend on the failure processes ongoing in the rock-mass. A new in situ test was specifically designed with the aim of calibrating the alarm threshold for rock-burst occurrence. The test consists in compressing to failure a portion of the rock-mass at the tunnel side-wall by means of a couple of flat-jacks, measuring simultaneously the generated acoustic emissions with accelerometers. The test was for the first time carried out at the Brenner Base Tunnel in massive granite below an overburden of about 1200 m, nearby tunnel locations affected in the past by few mild rock-burst phenomena. Recorded data clearly indicated that a correlation exists between the energy and frequency content of the acoustic emissions and the rock-mass failure. Following the experimental investigation, and consistently with the available monitoring evidence, a new higher alarm threshold was implemented at the construction site. Further tests are currently in progress: These are aimed to validate the results and to produce a testing standard that may be applied to the construction of tunnels affected by rock-burst phenomena.