Seismic response of underground structures under undrained loading with excess pore pressures accumulation

Abstract

The seismic response of tunnels under undrained loading with excess pore pressures accumulation is poorly understood. Two-dimensional dynamic numerical analyses are conducted to assess the seismic response of deep circular tunnels located far from the seismic source under undrained loading conditions. For comparison purposes, analyses under drained loading are also performed. It is assumed that the liner remains elastic and that plane strain conditions apply. A cyclic nonlinear elastoplastic constitutive model is used to simulate the nonlinear behavior of the ground and excess pore pressures accumulation with cycles of loading.It is found that the tunnel’s response is determined by the loading of the liner, or by the distortions of the cross section, depending on the flexibility ratio (F). For stiff structures, with F⩽2, important thrust (axial) forces and bending moments are produced in the liner, with larger magnitudes for the undrained case, while the distortions of the cross section are small. When the tunnel becomes more flexible, i.e., F increases, the loading on the liner decreases, but the distortions of the cross section become important. For very flexible structures, with F⩾10, performance is largely determined by the distortions of the cross section, while the thrust forces and bending moments are almost negligible. Such distortions are larger for drained loading than for undrained loading early during the earthquake; however, the distortions drastically increase with the increase of excess pore pressures and can be even larger for undrained than for drained loading, late during the earthquake.