Stability of dual circular tunnels in cohesive-frictional soil subjected to surcharge loading

The stability of dual circular tunnels in cohesive-frictional soils subjected to surcharge loading has been investigated theoretically and numerically assuming plane strain conditions. Despite the importance of this problem, previous research on the subject is very limited compared to that on single tunnels. At present, no generally accepted design or analysis method is available to evaluate the stability of dual circular tunnels/openings in cohesive-frictional soils. In the design stage, it is important to consider the interaction effects of dual circular tunnels. Unlike the case of a single tunnel, the centre-to-centre distance appears as a new problem parameter and plays a key role in tunnel stability. In this study, continuous loading is applied to the ground surface, and a smooth interface condition is modelled. For a series of tunnel diameter-to-depth ratios and material properties, rigorous lower- and upper-bound solutions for the ultimate surcharge loading are obtained by applying finite element limit analysis techniques. For practical suitability, the results are presented in the form of dimensionless stability charts and a table with the actual tunnel stability numbers closely bracketed from above and below. As an additional check on the solutions, upper-bound rigid-block mechanisms have been developed, and the predicted collapse loads from these are compared with those from finite element limit analysis. Finally, a discussion is presented regarding the location of the critical tunnel spacing between dual circular tunnels where interaction no longer occurs