Upper-bound stability analysis of dual unlined elliptical tunnels in cohesive-frictional soils

This study investigates the stability of dual unlined elliptical tunnels in cohesive-frictional soils using upper-bound finite element methods with rigid translatory moving elements (UBFEM-RTME). Results are determined in terms of dimensionless stability numbers, which increase with ϕ and decrease with increasing B/D and C/D, and collapse mechanisms, which mainly include two wedge-shaped zones and a complete non-plastic wedge. The center-to-center distance S significantly affects the stability of dual tunnels, and the interaction between the elliptical tunnels tends to disappear when Sc lies within (i) 2.5D-4D for C/D = 1, and (ii) 4D-11D for C/D = 5. The results agree well with those in literature.