Drainage and relative increase of short term strength of low permeability rock mass

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. In fact the tunnel profile crosses mostly clayey tectonized flysch and two paleo-valleys filled with weak clayey breccias for 3-4km. Specifically, serious problems might arise with the advancement in the latter formation, given the low short-term strength of the material. A series of numerical analyses have been performed to investigate the risks associated to the advancement by TBM in this problematic geological context. As far as the breccias are concerned, a consolidation before the advancement takes place seems necessary. The conceptual solution may consist of several long drains which produce a dissipation of the pore-pressure in the ground ahead of the face. A plane strain FLAC numerical model has been utilized to simulate the evolution of the consolidation and the increase of short-term strength in a tunnel crosssection. Even if breccias possess rather low values of permeability, the dissipation occurring in the model brings to a benefic increase of the short-term strength and consequently to more favorable excavation response. In fact, as clearly shown by a 2D axial-symmetrical numerical analysis, the convergence when the excavation takes place in the consolidated rock results hampered.