Undrained seismic response of tunnels

The seismic response of tunnels has been well-studied for structures placed in linear-elastic ground, but the results may be only applicable to tunnels placed in stiff soil or for moderate earthquakes. There is little information regarding the behavior of buried structures in soft soils, since they do not show a linear behavior, and for undrained conditions, where excess pore pressures accumulate during the earthquake. The paper presents the results of two-dimensional dynamic numerical analyses, under drained or undrained loading, to assess the seismic response of deep circular tunnels located far from the seismic source. It is assumed that the liner remains elastic and that the ground response can be approximated with a nonlinear elastoplastic constitutive model that includes excess pore pressures accumulation with cycles of loading. The liner deformations and load demand strongly depend on the flexibility ratio, F, which is a measure of the relative stiffness between the ground and the tunnel. Undrained conditions, compared with drained conditions, tend to reduce deformations for flexible liners and increase them for stiff tunnels early during the earthquake. With further cycles of loading, as the excess pore pressures increase, the differences in tunnel distortions between drained and undrained loading are reduced, i.e., they increase for flexible and decrease for stiff tunnels. Undrained loading, for stiff tunnels with F < 2, produces larger thrust in the liner than drained loading. For more flexible tunnels, with F > 2, the behavior is the opposite: smaller axial forces for undrained loading. For tunnels with F > 2, the bending moments in the liner are not affected by the type of loading, drained or undrained, or by the magnitude of the excess pore pressures. For tunnels with F < 2, the bending moments increase from drained to undrained loading and with the magnitude of excess pore pressures.