Seismic Response of an Immersed Tunnel on Liquefiable Sand Under Bidirectional Excitations Using Centrifuge Model Tests

This study examines the seismic behavior of an immersed tunnel in a liquefiable sand deposit under horizontal shaking and combined horizontal and vertical shaking (i.e., bidirectional excitations). A 1:100 centrifuge model was designed to satisfy the relevant similitude requirements for geometry, material properties, structural stiffness, and input motion. Fujian fine sand was used for the liquefiable foundation, and Fujian coarse sand represented the gravel bed and backfill. A recorded offshore earthquake motion was imposed in an alternating sequence of horizontal excitation and bidirectional excitation with increasing intensity. The measurements show that liquefaction developed most readily in the middle part of the sand deposit. Once excess pore pressure accumulated, the associated loss of stiffness and increase in damping strongly reduced the upward transmission of horizontal acceleration to the tunnel. The additional vertical component promoted a faster build‐up of excess pore pressure, advanced the onset of liquefaction, and further weakened the upward propagation of horizontal motion; these effects intensified as the intensity of seismic excitations increased. Consequently, the tunnel experienced smaller peak horizontal accelerations during bidirectional excitations than during horizontal excitation at the same horizontal peak acceleration. The greatest bending demand occurred at the corner between the roof and side wall and at the middle of the bottom slab, whereas the largest incremental dynamic lateral earth pressure was measured near the base of the side wall. Under stronger motions, the vertical component increased both the peak bending moment and the incremental dynamic lateral earth pressure acting on the immersed tunnel.