The stability of dual circle tunnels buried in layered soils with different shear strengths was investigated by using finite element limit analysis (FELA). The emphasis of this study is in quantitating that the existing tunnels affects the newly-built one, and design suggestions have been provided especially in the optimum location of construction. By imposing a FELA modelling, the variation trends of undrained bearing capacity with different influential factors, including the horizontal distance, vertical distance, the thickness of the top layer, the shear strength ratio of the layered soil, were further investigated. It is concluded that there exists an inclination-fixed worst-band, in which there would be a worst undrained stability once the bottom tunnel was constructed in the band. It is interested that the inclination seems constant by varying several factors but the horizontal distance would be changed with the soil properties. In addition, three patterns of collapse were summarized.

In this study, a self-developed adaptive finite element limit analysis (AFELA) code was adopted to explore the stability of dual tunnels in cohesive–frictional soil subjected to surcharge loading and seismic action. Parametric studies of different influential factors, including the depth of tunnels, horizontal distance between tunnels, seismic acceleration coefficient, unit weight, cohesion and internal friction angle of soils, were conducted using the AFELA code. An adaptive meshing technique was adopted for optimal accuracy and efficiency, and a pseudostatic method was used to simulate the seismic action. Strict upper bound (UB) and lower bound (LB) results with relative errors of less than 7% were acquired. Detailed design tables were presented to facilitate the engineering design, and three typical failure patterns, including single side-wall failure, half-cross-shaped failure and cross-shaped failure, corresponding to different stable levels, were summarized for a deeper insight into how the failure mechanism evolved under different conditions. The results indicated that the variations in soil unit weight and void depth affected the seismic bearing capacity almost linearly. Furthermore, the dual tunnel system is vulnerable to seismic actions, and the stability of tunnels was further undermined by the adverse effects of additional seismic-caused interactions between two adjacent tunnels.