Recent tsunami field surveys from the 2011 Great East Japan Earthquake and Tsunami have recorded numerous examples of tsunami-induced soil instability: significant scour around foundations, foundation failure of piles, and other damage caused by liquefaction. From the observations of soil instability leading to the failure of critical coastal structures, it is apparent that the behavior of tsunami waves has a unique effect on the soil bed. Previous research in the area of soil instability has predominantly focused on storm-generated waves, and a complete understanding of soil hazards from tsunami events remains rudimentary. In an effort to expand understanding of tsunami-induced soil instability, the primary objective of this thesis is to evaluate the mechanisms of sediment response and provide an analytical framework for evaluating soil hazards related to tsunamis.
Evaluating tsunami-induced soil instability is particularly important in the Pacific Northwest, due to the presence of the Cascadia Subduction Zone. Soil instability and sediment transport were estimated at Seaside, Oregon for a hypothetical "500-year" tsunami, using the predicted flow height and velocity time series taken from the Park et al (2014) ComMIT/MOST inundation model. Two onshore locations of interest were selected from the model for evaluation of soil instability and sediment transport potential. Additionally, sand was sampled from the beach at Seaside to characterize the sand's fabric. Significant sediment transport and enhanced scour caused by the large bed shear stress during tsunami drawdown is expected based on Shields parameter and Rouse number estimates. A modified Shields parameter from Yeh and Mason (2014) was applied to account for the gradient of excess pore water pressure head that develops in the sand during tsunami inundation and drawdown. Based on initial estimates of drawdown, it is expected that soil instability caused by momentary liquefaction would be minor at Seaside, but soil instability caused by enhanced scour could be significant.
To reach an improved understanding of the mechanisms behind tsunami-induced soil instability, a complete analytical model was developed to examine the influence of a tsunami wave on soils from runup to drawdown. The Carrier et al. (2003) shallow water wave solution for tsunami runup on a plane beach was combined with analytical solutions for pore pressure gradients and sediment transport. Within a probabilistic model of tsunami-induced soil instability, a sensitivity analysis was conducted on both the wave and soil parameters using a Monte Carlo approach. Threshold behaviors of wave forces and resulting soil instability were discussed under general site conditions. The height and the velocity of the inundating wave have a predominant influence on the response of soils. The beach slope is found to affect the response of pore pressure gradients, and ultimately, the momentary liquefaction risk. The entire range of the coefficient of consolidation produced cases of enhanced scour and momentary liquefaction, indicating potential tsunami-induced soil instability for the majority of fine sands. Analysis of soil behavior at critical thresholds remains important for developing structures that are resistant to wave forces and soil instability failure.