Stability of dual square tunnels in rock masses subjected to surcharge loading

The stability of dual unlined square tunnels subjected to surcharge loading in rock masses, obeying Generalized Hoek-Brown (GHB) failure criterion, is investigated by adaptive finite element limit analysis (AFELA) under plane strain conditions. Both upper bound (UB) and lower bound (LB) of the dimensionless stability number N are obtained, with the relative errors being found within 7%. Extensive parametric studies are conducted to investigate the effects of the center-to-center distance ratio, cover depth ratio, geological strength index (GSI) and material constant for intact rock (m(i)). The results are presented in the form of dimensionless stability tables and charts for engineers to use, and typical failure mechanisms are discussed. The results obtained in this study compare reasonably well with the solutions reported in the literature.