The influence of shear and fracture zones when tunneling in deep-seated landslide areas

Tunnel excavation in mountainous or sloping regions can reactivate or accelerate landslide phenomena. On its turn, tunnel excavation can be affected if it crosses weak rocks in landslide-prone areas. In the present study the interaction between tunnelling and slope stability is analysed in presence of Deep-Seated Gravitational Slope Deformation (DSGSD). At this aim a case study is considered, located in northern Italy (Central Alps), where a landslide has been reactivated along the border of a wide DSGSD, close to a tunnel recently excavated. The conceptual model of the slope dynamic was first reconstructed. Afterwards, the stress-strain behaviour of the system was numerically simulated both in two-dimensional and three-dimensional fields and considering different scenarios: geological reference model (presence of shear zones and faults), hydrogeological conditions (groundwater table level), and excavation phases of the tunnel. Numerical results showed that the presence of shear zones connected to DSGSD brings about a significant worsening of the stability, for both the tunnel and the slope: actually, when the tunnel excavation intersects the shear zones of the DSGSD severe convergence takes place, as well as a decrease in the local slope stability both in term of safety factor and surface settlements. Moreover, the presence within the slope of fault zones brings about a further worsening of the stability, as they act as high permeability flow paths, draining groundwater towards the tunnel and increasing the surface settlements in a wider zone of the slope.