Rock-burst risk assessment by acoustic emission measurement and interpretation at the Brenner Base Tunnel: the "Mules method"

The paper describes the experience gained during the excavation of some stretches of the Brenner Base Tunnel system, a complex underground network for the connection of the city of Fortezza, in Italy, to the city of Innsbruck, in Austria, in the framework of the Scandinavian Mediterranean corridor 5 of the Trans European Network-Transport. The occurrence of some rock-burst phenomena, although of mild intensity, during drill-and-blast tunnelling in the Brixner granite formation at depths over 1000 m has promoted the implementation of efficient and accurate strategies for the early assessment of risk and for its management. A comprehensive procedure, named the "Mules method", has been developed and successfully applied long all the tunnel segments at risk. This approach, based on the combination of the continuous monitoring of acoustic emissions near the tunnel face and a newly developed in situ test for the experimental validation of the alarm threshold, can be considered of general validity and potentially exportable to other construction sites.