Sand liquefaction and seepage pore pressure around shield tunnels in multilayered seabed under action of waves and currents

At present, the existing theoretical studies on the seepage field around subsea shield tunnels under wave action generally consider the linings as an impermeable medium, and seldom consider the permeability of the tunnel linings, especially the influences of the coupling action of waves and currents on the tunnel. In addition, the existing theories generally regard the seabed as being homogeneous and isotropic, ignoring the influences of multilayered seabed. Firstly, based on the dynamic boundary conditions of seabed surface under wave-current interaction, the pore water pressure response of pure seabed under wave-current interaction is obtained by the transmission and reflection matrix method. Secondly, the mirror image method is introduced to establish the governing equation for the excess pore water pressure caused by the existence of tunnel, and the analytical solution of the equation is obtained by the Fourier series expansion under the continuous seepage between sand and linings. Then, based on the superposition principle, the seepage pressure of the sand around the tunnel in multilayered seabed under the action of waves and currents is obtained. Finally, the theoretical analytical solution is compared with the numerical results and the existing experimental results, and a good agreement is obtained. In addition, the influencing factors for the permeabilites of seabed and tunnel linings are analyzed. The results show that the following currents will increase the pore pressure in the seabed and the liquefaction degree of the seabed, while the opposing currents will reduce the pore pressure in the seabed and the liquefaction of the seabed. The relative difference of the seabed response to the opposing currents at the same velocity is generally greater than that of the following currents. When the permeability coefficient of the upper seabed is large (k_s > 1×10^-2 m/s), the overall pore pressure of the seabed is large, and the pore pressure at the first stratification changes significantly. When the permeability coefficient of tunnel linings is small (k_l < 1×10^-6 m/s), the tunnel has an obvious "block" effects on the propagation of the excess pore pressure in the seabed.