Numerical prediction of tunnelling-induced displacement field in large and very shallow tunnels

The application of a displacement field at the excavation boundary is a widely used and effective numerical technique to compute tunnelling-induced displacements. The principal assumption of this method is that the deformed tunnel shape is similar to the original one, i.e., circular, while its bottom remains fixed in its original position. In this paper, three-dimensional numerical analyses are described to propose a more suitable criterion for very shallow and large-diameter tunnels, characterized by two separated elliptical contraction shapes for the upper and lower parts of the tunnel section. Furthermore, quantitative description of the deformed pattern is provided, and the relationship of contraction and translation parameters with tunnel depth-to-diameter ratio is summarized and validated against monitoring data. The results show that the most appropriate deformation pattern for very shallow and large tunnels is characterized by an elliptical shape in the upper section and a circular shape in the lower part. The new approach provides a more realistic prediction of the surface displacement field if compared to that predicted by using the homothetic contraction of the tunnel section.