Liquefaction potential of coastal slopes induced by solitary waves

Abstract

Tsunami runup and drawdown can cause liquefaction failure of coastal fine sand slopes due to the generation of high excess pore pressure and the reduction of the effective over burden pressure during the drawdown. The region immediately seaward of the initial shoreline is the most susceptible to tsunami-induced liquefaction failure because the water level drops significantly below the still water level during the set down phase of the drawdown. The objective of this work is to develop and validate a numerical model to assess the potential for tsunamiinduced liquefaction failure of coastal sandy slopes. The transient pressure distribution acting on the slope due to wave runup and drawdown is computed by solving for the hybrid Boussinesq-nonlinear shallow water equations using a finite volume method. The subsurface pore water pressure and deformation fields are solved simultaneously using a finite element method. Two different soil constitutive models have been examined: a linear elastic model and a non-associative Mohr-Coulomb model. The numerical methods are validated by comparing the results with analytical models, and with experimental measurements from a large-scale laboratory study of breaking solitary waves over a planar fine sand beach. Good comparisons were observed from both the analytical and experimental validation studies. Numerical case studies are shown for a full-scale simulation of a 10-m solitary wave over a 1:15 and 1:5 sloped fine sand beach. The results show that the soil near the bed surface, particularly along the seepage face, is at risk to liquefaction failure. The depth of the seepage face increases and the width of the seepage face decreases with increasing bed slope. The rate of bed surface loading and unloading due to wave runup and drawdown, respectively, also increases with increasing bed slope. Consequently, the case with the steeper slope is more susceptible to liquefaction failure due to the higher hydraulic gradient. The analysis also suggests that the results are strongly influenced by the soil permeability and relative compressibility between the pore fluid and solid skeleton, and that a coupled solid/fluid formulation is needed for the soil solver. Finally, the results show the drawdown pore pressure response is strongly influenced by nonlinear material behavior for the full-scale simulation.

Process-based modeling of tsunami inundation and sediment transport

“…If the tsunami waves broke before inundating, breaking‐wave‐generated turbulence, which is not accounted for here, could have affected the patterns of both turbulence generation and sediment suspension. Furthermore, pore pressure gradient‐induced sediment liquefaction, which can be important during drawdown [ Tonkin et al , 2003; Young et al , 2009], and infiltration of water into the soil onshore are not accounted for. There may also be other processes not yet identified that play an important role during tsunamis owing to the large flow velocities generated.…”

Hydro- and morpho-dynamic modeling of breaking solitary waves over a fine sand beach. Part II: Numerical simulation

“…Interested reader should refer to (Young et al, 2009a) on this topic. In the present study, the wave loads on the bed are assumed to be small enough and the duration of the loading is short so that the soil behaves linear elastically and no failure model is necessary.…”

“…Hence, the responses and failure of coastal slopes due to long wave runup and drawdown were not considered. Young et al (2009a) investigated the influence of solitary wave runup and drawdown on the soil responses using an uncoupled wave-soil model. The wave model was the same as the one presented here, but the sediment transport processes were not considered.…”

Continuum deformation and stability analyses of a steep hillside slope under rainfall infiltration

“…It would thus be reasonable to characterize the constitutive response of the soil skeleton with a Mohr-Coulomb plasticity or similar models, since the compression cap is unlikely to be activated particularly when the effective-mean-normal stress decreases further as a result of the loss of suction with increased saturation. A simpler plasticity model such as the Mohr-Coulomb model also requires fewer material parameters than any of the more sophisticated constitutive models available in the literature (see [76]). For the soil at CB1, which is mostly colluvium, laboratory-determined values of friction angle are available [53,54].…”