Tsunami-induced pore water pressure response of unsaturated soil beds: Numerical formulation and experiments

Tsunamis attack coastal areas and cause significant damage to coastal buildings and infrastructure. Many coastal areas are dominated by fine, poorly-graded sands, and in many cases, the upper portion of the nearshore sand bed is unsaturated before tsunami attack. During tsunami runup, inundation of water into the unsaturated sand bed occurs, and the sand bed can eventually become pressurised under the correct conditions. During tsunami drawdown, the total stress is relieved from the sand bed surface, and excess pore water pressure dissipation process can cause sediment instability via fluidisation. The foregoing physical description is complicated by the large bed shear stress caused by tsunami loading, which can motivate significant sediment transport and scour around coastal structures. Herein, we present a coupled seepage-deformation model to estimate the temporal and spatial pore water pressure changes in sand beds during tsunami loading. The numerical formulation is described in detail, and then numerical experimentation is performed to demonstrate modelling capabilities. The results of the numerical experimentation show that tsunami drawdown can induce large pore water pressure head gradients and the speed of the infiltration front is important for examining the potential for tsunami-induced sediment instability.