Analysis of stress conditions at Deep Seated Tunnels-A Case Study at Brenner Base Tunnel

Deep seated tunnels like the Brenner Base Tunnel suffer from high in situ or primary stress conditions. This has significant effects on the deformation pattern and failure mode and therefore on the construction of the tunnel. High in situ stress conditions are a result of the rock cover which reaches approximately 1,700 m at the Brenner Base Tunnel. In collaboration with the Brenner Base Tunnel Society BBT SE, in situ stress conditions were modeled in a multitube tunnel system in different rock conditions. A rather ductile rock type, the Innsbruck quartzphyllite, and a rather brittle rock type, the Tux Central Gneiss, were used to calculate several models with the 2D-finite element code Phase2 and the 3D-finite element code RS3 (rocscience). Due to residual tectonic stresses caused by the collision of the African and Eurasian plates horizontal stresses are often increased. For the Innsbruck quartzphyllite section a 70° dip angle was used, within the section of the Tux Central Gneiss no deviation from the vertical was assumed due to geological and geophysical results. Apart from the rock cover and the direction of the main principle stress, further parameters were needed for modeling: young´ s modulus, uniaxial compressive strength, poisson ratio, friction angle and cohesion of the rock types have been used from site investigation reports. The performed numerical modeling showed that the secondary stresses have the same orientation as the primary stresses, hence the maximum stress appeared in the tunnel walls and minimum stress is concentrated in invert and crown.