Stability of an Unsupported Elliptical Tunnel Subjected to Surcharge Loading in Cohesive-Frictional Soil: Fid-Bau Portal

Stability of an Unsupported Elliptical Tunnel Subjected to Surcharge Loading in Cohesive-Frictional Soil

Dutta, Puja / Bhattacharya, Paramita

This study investigates the stability of an unsupported elliptical tunnel subjected to surcharge loading in cohesive-frictional soil. Non-circular tunnels, like an elliptical tunnel, provide more space and structural stability due to their smooth tunnel profile. The analysis has been performed assuming a plane strain condition by using lower bound limit analysis in conjunction with finite elements and second-order conic programming (SOCP). Limit analysis determines the collapse load at which the soil mass undergoes an unrestricted plastic deformation. The soil medium around the tunnel has been modelled as a homogenous and isotropic Mohr–Coulomb material obeying an associative flow rule. SOCP enables the usage of the non-linear Mohr–Coulomb yield criteria, which is expressed as a set of second-order cones and overcomes the difficulty of singular apex point of the Mohr–Coulomb yield function and is robust and efficient in solving a huge number of variables. In this study, a uniform surcharge loading is applied to the ground surface, and the stability is determined for smooth interface condition between the loading surface and the underlying soil. The results are presented in the form of dimensionless stability charts. The stability number in this analysis is indicated as σ_s/c obtained for different combinations of soil cover ratio H/B, aspect ratio of tunnel D/B, γB/c and ϕ where H is soil cover depth of tunnel, B and D are width and height of tunnel, respectively, and c, γ and ϕ are cohesion, unit weight and peak friction angle of soil. It has been noted that the stability number increases with increase in the peak friction angle of soil and soil cover depth ratio and decreases with an increase in D/B and γB/c. The size of the plastic zone increases as H/B keeps increasing and extends downwards enclosing the entire tunnel.