Scoping calculations of TBM advancement in flysch and breccias of Strait-of-Gibraltar Tunnel.

The planned railway tunnel of the Strait of Gibraltar is destined to be one of the most challenging underground structures ever built, given the depth, exceptionally high pore pressures and very poor rock quality at several stretches of the tunnel alignment. The more critical issues of the on-going study are large water in-flows when intercepting fractured arenaceous layers within the flysch formation and excessive deformations in the breccias of the two paleo-channels. The countermeasures designed for the first risk are grout injections and drainage in advancement. For the second risk a consolidation by drainage before the advancement is executed is proposed. Given the limited knowledge of the distribution of rock units and related mechanical parameters, different scenarios have been investigated with respect to undesirable effects as huge water venues, large plastic zones and convergences. FLAC 2D and 3D in the framework of hydro-mechanical coupled and uncoupled analyses have been utilized and the results are illustrated herein.