2D modelling of initiation of failure of a clay slope under wave overtopping load

Abstract

A growing risk of flooding in coastal areas and the corresponding development of legislation drive continuous development in the field of hydraulic engineering. Failure due to
wave overtopping is not fully understood from a physical point of view. Existing knowledge gaps, processes that are not yet explained by physics or not even discovered, are
explained by empirical relations. The fact that knowledge on the relevant processes for overtopping load and soil resistance is fragmented and limited, causes limited application of it in practice.
To improve the understanding of failure caused by overtopping, a model that computes the soil stresses (soil stress is related to failure) in a dike cover, during overtopping load, is developed. It is possible that this model approach leads to additional knowledge on wave overtopping, e.g. new failure mechanisms or shift in normative mechanisms.

It is shown that overtopping has a large variety of appearances and so does failure due to overtopping. Literature study shows the large number of processes and characteristics, all with variable magnitude, that is relevant in the wave load and the soil strength during wave overtopping. Main wave load processes are shear stress (gradients), turbulence and impact. The considered soil strength process is the ratio between the stress (relative to the strength), for which the Young’s Modulus, Poisson’s ratio, soil weight, hydraulic conductivity and the subsoil stiffness are shown to be the main soil characteristics.

The developed model, a numerical 2D model with the model domain oriented parallel to the flow direction, computes the stress distribution and development in a loaded soil. For this case it is focused on dike land side slopes loaded by overtopping waves in particular, that is on dealing with clay, saturated conditions, fast varying load and a sloped surface. Stress equations in the model are derived, based on equilibrium of forces (horizontal and vertical) and motionless soil. The model is able to indicate initiation of failure for failure mechanisms that are associated with soil stresses, e.g. lifting of soil and head cut erosion. Soil in the model is loaded by normal and shear stress. The latter is, deviant from conventional methods, modelled as a shear stress gradient.

Test runs for verification and validation show that the model gives good results for cases with constant loads. Comparison of load cases with non-continuous overtopping wave loads shows both similarities and differences in the resulting soil stress. The most outstanding difference is the magnitude of the stress at depth. The differences indicate
starting points for further research.
Model employment demonstrates the future possibilities. A single wave load, computed by the model itself, is modelled. This run gives, at first view, plausible results.

The current model gives a reasonable representation of soil stress development. Further development is recommended to make the model useful for dike assessment and design. To do so, the model should be improved by probabilistic parameter definition, addition of soil stress damping and considering the impulsive nature and turbulent oscillations of wave load. Furthermore, it can be extended by the comparison between soil stress and strength, addition of spatial variability of the soil, enabling computations of non-planar parts of the slope and enabling the connection with other hydrodynamical models.