Prediction of the displacement field induced by the excavation of very shallow and large tunnels: a case study

Numerical simulation of mechanized tunnelling in urban areas, both in 2D and 3D conditions, is often carried out by simply applying a displacement pattern at the excavation boundary, especially when the objective of the analysis is the prediction of the displacements away from the tunnel. The principal assumption of this method is that the deformed tunnel shape is similar to the original shape, i.e. circular, while its centre is translated downward. This paper proposes to apply a different, more general, displacement pattern, based on further optimization of previous approaches, considering an elliptical contraction shape. Specifically, the upper and lower parts of the tunnel are allowed to contract assuming two independent elliptical shapes. The method is validated through three-dimensional finite element simulations, with special reference to the deformation pattern of a real case-history of a very shallow and large-diameter tunnel. The results show that: (1) the approach is applicable to shallow-buried and large-diameter tunnels; (2) the deformation pattern of very shallow and large-diameter tunnels is characterized by an elliptical shape in the upper portion of the section and by a circular one in the lower part; (3) the new approach can produce a more accurate prediction of the soil displacement field, if compared with the homothetic contraction of the tunnel section.