Tunnelling in landslide areas connected to deep seated gravitational deformations: An example in Central Alps (northern Italy)

Tunnel excavation in mountainous regions often interacts with landslide phenomena. This paper deals with potential hazards arising from tunnelling in presence of a Deep-Seated Gravitational Slope Deformation (DSGSD), with specific reference to a case study located in Valtellina (northern Italy, Central Alps). In the study area a landslide has shown evidences of activation along the border of a wide DSGSD, where a tunnel has recently been excavated. The conceptual model of both the slope dynamics and the tunnel construction was first reconstructed, by integrating surface and subsurface surveys (from drillings and geophysical investigations to convergence tunnelling measurements, from topographic and inclinometric monitoring to radar interferometry and dendrogeomorphology). Data interpretation showed the presence within the slope of a thick shear zone, characterised by very weak rocks and significant water flows. When the tunnel intercepted this zone, convergences and face instabilities occurred, as well as tunnel inflow; in the meantime, slope deformation developed along the slope. These latter were probably the result of the superimposition of different effects, arising also from the long and heavy rainfall of that period. The investigation of the landslide activity based on dendrochronological analysis confirmed its correlation with rainfall, as well as the contribution of tunnelling. A similar behaviour is also confirmed by the results of stress-strain numerical simulations. Numerical results showed that groundwater rising due to long and heavy rainfall can contribute to slope instability much more than tunnelling itself. Moreover, they showed that the existence of shear and fracture zones connected to the DSGSD determines both an increase in tunnel inflow as well as in tunnel convergences. Actually, fracture and shear zones act as high permeability flow paths within the slope, draining groundwater towards the tunnel and increasing the surface settlements in a wide zone of the slope, worsening the slope stability conditions without triggering the collapse.