A standard procedure for rock-burst prediction during tunnel construction. The “Mules method”

Rock-burst phenomena occur frequently during the excavation of deep tunnels in good-quality rock-mass. This paper describes the procedure developed and successfully applied during the excavation of the Brenner Base Tunnel in the Brixner granite formation for the early detection of rock-burst related risk. It consists in the continuous monitoring of acoustic emissions and in their comparison with an alarm threshold calibrated on specifically designed in situ tests. During the in situ test the rock-mass is brought to failure by the compression applied by two flat jacks at the tunnel wall while the related acoustic emissions are simultaneously measured by a system of accelerometric sensors, whose records are interpreted in terms of power spectrum density. Two in situ tests were carried out providing consistent results despite varying geomechanical conditions, also highlighted by the results of uniaxial compression tests performed in the laboratory. The procedure has been validated over the last three years and has proved its reliability in the reduction of risk.