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

The stability of dual square tunnels in cohesive-frictional soils subjected to surcharge loading has been investigated theoretically and numerically assuming plane strain conditions. From the viewpoint of the efficient utilization of underground space for human activities, noncircular openings and tunnels should be preferred in the design stage. Despite the importance of this issue, previous research on the subject is very limited. At present, no generally accepted design or analysis method is available to evaluate the stability of multiple tunnels-openings in cohesive-frictional soils. In the design stage, it is important to consider the interaction effects of dual tunnels. Unlike the case of a single tunnel, the centre-to-centre distance appears as a new parameter that must be considered 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 size-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 verification of the solutions, upper-bound rigid-block mechanisms have been developed, and the predicted collapse loads from these mechanisms are compared with those from finite element limit analysis. Finally, a discussion is presented regarding the location of the critical tunnel spacing between dual square tunnels where interaction no longer occurs.