Gotthard Base Tunnel. Rock Burst Phenomenon in the Gotthard Base Tunnel during Construction of a Multifunction Station Faido in a Fault Zone

TUNNEL SYSTEM OF THE MULTIFUNCTION STATION AND GEOLOGY The Gotthard Base Tunnel (GBT) in Switzerland, a double tube single track railway tunnel with a length of 57 km is currently under construction. In addition to the northern and southern portals, the tunnel is being excavated from 3 intermediate attacks. Two of these intermediate attacks are constructed as so-called multifunction stations (MFS) for the stage of utilization containing facilities for operation control and emergency stations. The tunnels’ crossovers and the galleries for ventilation and rescue form a complex tunnel system (Fig. 1). The southern intermediate attack in Faido, serving as multifunction station had to be accessed by means of a 2.7 km long adit inclining with 13%. In the area of the multifunction station the overburden is between 1400 and 1600 m. The excavation works for the MFS Faido have been completed recently. During design the MFS Faido’s location was determined based on a geological forecast, predicting favorable Leventina-gneiss and optimal tunnelling conditions in the area considered. However, during construction a huge, unknown fault zone was encountered. The fault’s kernel consisting of intensively jointed gneiss crossed the center of the MFS. The fault strikes at a small angle to the axis of the MFS and dipped sub vertically NE (Fig. 1). This fault led to considerable geotechnical risks such as squeezing, loosening and rock bursts. This article describes the esults of seismic model simulations and seismic measurements helping to better understand the rock burst’s origination and its impacts on the tunnels. The investigation’s results enabled to apply effective safety measures to avoid accidents during constructions. In the critical tunnel sections flexible support elements were installed. Furthermore, tunnel areas with increased rock burst potential were closed during excavation of adjacent tunnels. During construction of the MFS Faido no accident occurred due to rock bursts.