Seismic Performance of Circular Tunnels in Liquefiable Soil

Contrary to the popular belief that underground structures such as tunnels are less vulnerable to seismic damage, post-earthquake observations show that tunnels have collapsed under strong ground motions. Liquefaction of soil is identified as one of the leading causes of tunnel failure. For instance, during the 1989 Loma Prieta earthquake, a submarine tunnel was heavily damaged due to the liquefaction of the surrounding marine soil. The study focuses on evaluating the behavior of circular tunnels in liquefiable soil. To achieve this, two-dimensional models are created and analyzed using the finite element software OpenSees. Two different ground conditions are considered—a two-layered soil profile with loose sand overlying dense sand and a three-layer soil profile with loose sand sandwiched between dense sandy layers. The tunnel is assumed to be located in the top loose layer in the two-layered profile, whereas, for the three-layered soil profile, the tunnel is considered in the middle loose soil layer. The tunnel is modeled with displacement-based beam-column elements, and the liquefaction behavior of the soil is captured using the advanced PDMY02 constitutive model implemented in OpenSees. The responses in terms of tunnel deformation, tunnel moment, and vertical displacement of the ground surface are measured and compared to identify the most critical soil profile. The study indicates that the presence of dense sand at the top of liquefiable soil limits the tunnel uplifting and reduces the bending moment developed in the tunnel lining.