title: Liquefaction around a Submarine Tunnel under Natural Dynamic Loading
Creators: name: Han, Shuang
Publisher: name: Griffith University
Year: 2020
Subjects: subject: Seabed instability
subject: immersed tunnel
subject: wave-induced liquefaction

description: Seabed instability surrounding an immersed tunnel is a vital engineering issue regarding the design and maintenance for submarine tunnel projects. It has been recognised that the pore water pressures and stresses in seabeds are affected by the water pressures generated by the natural dynamic loading. If the pore water pressure reaches the initial mean stress, the liquefaction could occur with the effective stress in seabed vanishing. To avoid seabed instability around the immersed tunnel, the study of seabed dynamic behaviour is necessary under the real hydrodynamic loading. Two mechanisms of wave-induced liquefaction has been reported in the literature, based on a mass of laboratory tests and field exploration, which are transient liquefaction and residual liquefaction. The transient liquefaction is motivated by the oscillatory excess pore water pressures under wave pressure vibration which usually happens with amplitude reduction and phase lag of pore pressure in seabed soil. While the residual liquefaction is on the consequence of the excess pore water pressure build-up under cyclic wave loading. The liquefied seabed soil will behave like a heavy fluid without any shear resistance to supported structures on it, thus leading to catastrophic failure of the immersed tunnel. In the present study, the main objective is to investigate the mechanism of soil response and liquefaction caused by waves and currents in the seabed foundation around the immersed tunnel. An integrated numerical model is established to analysis the seabed behaviour under natural dynamic loading, including ocean waves and currents. In the integrated model, the fluid sub-model is responsible for simulating the two-phase incompressible flow motion inside and outside the porous media, which is governed by the VARANS (Volume-Averages Reynolds Averaged Navier-Stokes) equation, while the seabed model is established adopting the LRBFCM with Biot’s "u− p" approximation which considered the inertial term of soil skeleton. The new conceptual meshfree model for residual mechanism considers the coupling effects between the development of the pore pressure build-up and the evolution of the seabed stresses by adding a source term associated with the shear stresses in the seabed. Good agreements with analytical solution and laboratory experiments validates this newly proposed numerical model. The LRBFCM is examined to be reliable in simulation of wave-induced oscillatory and residual liquefaction behaviour of a seabed. The wave-induced dynamic response of the oscillatory and residual seabed response is investigated adopting the developed integrated model. A series of results, including the seabed stresses, the pore pressure accumulation and the liquefaction potential in the seabed foundation are obtained. The existence of the immersed tunnel affects surrounding seabed dynamic behaviours significantly, including the seabed stresses and the pore water pressures, leading to the local redistribution in the adjacent region of the immersed tunnel. Both the maximum oscillatory and residual liquefied depth on the right-hand side of the tunnel is smaller than that on the left-hand side (the ocean wave is set as propagating along the x-direction from the left-hand side to the right-hand side). From the numerical results, the seabed oscillatory liquefaction is more likely to occur under a shallow water area with the waves of large wave height and long period, moreover, a seabed with lower permeability and degree of saturation is more likely to be liquefied. For the residual seabed response, the residual liquefaction is more likely to occur in the seabed foundations with low relative density and poor drainage condition. The seabed response around the immersed tunnel under combined nonlinear Stocks waves and currents loading is investigated both in oscillatory and residual mechanisms. The simulation results show that the risks of both oscillatory and residual liquefaction are much higher for the seabed under wave combined following currents, and the appearance of opposing current could decreased the probability of the liquefaction occurrence.