Stability of two circular tunnels at different depths in cohesive-frictional soils subjected to surcharge loading

Adaptive finite element limit analysis (AFELA) is employed to investigate the stability of two circular tunnels at different depths in cohesive-frictional soils subjected to surcharge loading. The upper and lower bounds of the tunnel stability numbers are obtained, with the relative errors being found within +/- 5%. Five parametric studies are conducted to evaluate the effects of the horizontal and vertical spacing ratio, overburden stress factor, cover depth ratio and soil internal friction angle on the tunnel stability numbers. Design tables and charts are provided to facilitate engineers' use. In addition, typical failure mechanisms are discussed and examined.