Three-Dimensional Numerical Investigation of the Interaction Between Twin Tunnels

The construction of twin tunnels is a mandatory guideline and a prevailing practice in either conventional or mechanized tunneling. Nevertheless, most of the design methods for calculating the tunnel loads focus on single tunnels, neglecting thus the potential interaction between neighboring tunnels. The effect of such interaction can be significant, especially for closely-spaced twin tunnels. In this context, this paper investigates via parametric 3D finite element analyses the interaction between deep, parallel-twin, circular and non-circular tunnels excavated with a conventional (i.e., non-TBM) method and supported with shotcrete lining. The numerical investigation focuses on the axial forces acting on the primary support of the tunnels by examining the effect of a wide range of geometrical (pillar width, overburden height, tunnel diameter and section (shape), lagging distance), geotechnical (strength and deformability of the surrounding rockmass, horizontal stress ratio), structural (thickness and deformability of the shotcrete lining) and construction parameters (full- or partial- face excavation and support of the tunnels). The results of the analyses indicate that the construction of the subsequent tunnel influences the loads of the precedent. The stress state of the single tunnel is used as the reference for the quantification of the interaction effect. The output is presented in normalized design charts of the quantified interaction effect on the axial forces versus key geomaterial and geometry parameters to facilitate preliminary estimations of primary support requirements for twin tunnels. Furthermore, nomographs are provided for preliminary assessments of the optimum pillar width (spacing) between twin tunnels, which practically eliminates the interaction effect.