Three-dimensional numerical simulation of mechanized twin stacked tunnels in soft ground

The increase in transportation in large cities has made it necessary to construct twin tunnels at shallow depths. As far as the parallel excavation of mechanized twin tunnels is concerned, most of the cases reported in previous studies have focused on the interactions between two horizontally driven tunnels. However, less work has been devoted to the interactions between tunnels stacked over each other. The numerical investigation performed in this study has made it possible to evaluate the influence of the construction process on two stacked tunnels, using the FLAC3D finite difference element software. The structural forces induced in each of the stacked tunnels and the displacements in the surrounding ground have been highlighted. The results of the numerical analysis indicate that new tunnel construction can have a great impact on an existing tunnel. The greatest impacts are observed when the upper tunnel is excavated first. The excavation of the upper tunnel generally leads to greater surface settlements than when the lower tunnel is excavated first. This study also shows that the normal forces induced in the lower tunnel are always greater than those developed in the upper tunnel. The normal displacements and the bending moments induced in the lower tunnel are usually smaller than those in the upper tunnel