Three-Dimensional Numerical Analyses of Perpendicular Tunnel Intersections

This paper investigates the effect of constructing a junction tunnel, intersecting an existing main tunnel at a normal angle, via parametric 3D Finite Element (3D-FE) analyses. The 3D interaction between the tunnels significantly modifies the stress state of the primary support and the surrounding rockmass at the intersection area, compared to that of the single tunnel (quasi plane strain problem), thus making 3D-FE analyses necessary for the realistic design of the primary support at the junction area. The numerical investigation includes deep, circular intersecting tunnels with the junction tunnel excavated, after the main tunnel, via a conventional (non-TBM) method and supported with shotcrete lining. The parametric analyses are performed for a wide range of junction tunnel’s diameter, overburden height, in-situ horizontal stress ratio, strength and deformability of the surrounding rockmass. They focus on calculating the axial forces acting on the primary support at the intersection area before, during and after the construction of the junction tunnel. In addition, they identify the extent of the zone influenced by the tunnel interaction. The results of the analyses indicate that the construction of the junction tunnel causes significant additional compressive loading at the springline of the main tunnel. In contrast, the crown/invert of the opening of the main tunnel is subjected to either compressive loading or unloading (often reaching tensile loading). The results of the analyses are presented in normalized design charts of the axial forces, versus key geomaterial and geometry parameters to facilitate preliminary estimations of primary support requirements at tunnel junctions.