Insights into the erosion process following a slope instability of the dike and guidelines to estimate the residual dike resistance.
Sequence of slope instabilities that lead to flooding (source: Photos by für Grundbau und Umwelttechnik (GGU) and schemes adapted from Calle, E.O.F., Dijk doorbraak processen (2002), translated here to English).
Motivation and practical challenge
The challenge of this project is to predict for which dikes slope instability can be allowed. The failure process of slope instability starts with a crack in the crest or inner slope (photo 1). After the crack developed, settlements start to occur as the slope slides (photo 2). For dikes with residual resistance these settlements may stop early, while for others large deformations occur (photo 3) leading to secondary deformations. For some dikes flooding is unlikely to occur even after very large deformations. For others, the deformation will lead to flood due to a dike breach (photo 4). Allowing an initial slope instability for dikes with residual dike resistance may be possible when flooding is unlikely to occur due to a dike breach. Considering this residual resistance can lead to more efficient designs, especially for dikes with a large width so that dike reinforcement takes place where it is most necessary. Modelling and understanding the failure process helps to predict in which cases the failure process stops before flooding due to a dike breach. Thereby, we can help engineering expertise to evaluate and expand the existing guidelines.
The implementation of the new safety standards requires more realistic estimates of the probability of failures. Therefore, my interest on how to determine the effect of residual dike resistance onto the probability of flooding due to a dike breach?
To predict if a dike may breach after the initial slope instability, I develop and use the (Random) Material Point Method or (R)MPM. Thereby, I determine the residual dike resistance, which is the difference between the probability of slope instability and the probability of a dike breach (see figure). MPM is a new modelling approach similar to the widely used Finite Element Method that allow us to model the start of the initial failure and the dike deformations that may follow up.
The model is set up for a given dike section and subsoil properties which are often variable. Due to the variability, the failure process is highly variable so that the model is expanded into a fully probabilistic tool (Random MPM). Thereby, I extend the current probability framework for dike design to include residual dike resistance. The tool generates many versions of the same dike based on the variable subsoil properties and computes for each version if initial instability and flooding occurs. From all the versions of the dike the probability of initial instability and flooding can be estimated. Thereby, the residual dike resistance is estimated.
Finally, the existing guidelines for slope instability probability estimations will be assessed and extended if necessary to provide outcomes for the user practice.
Relevant for whom and where?
The research is relevant for anyone designing or assessing dikes who is considering to take residual dike strength into account for dikes with a large width or over height.
So far the research components are developed for typical dike sections in the Netherlands without a specific case study or location in the map.
Progress and practical application
Residual dike resistance is definitely present in almost any dike with a reasonable width, and can reduce the probability of flooding significantly compared to the probability of initial failure. However, the reduction is highly dependent on the geometry, material properties, soil variability and river/sea water level. Referring to current guidelines, these guidelines often assume a ‘safe’ remaining dike geometry, in other words a remaining dike geometry which will never result in flooding. However, such a geometry has not been found in the examples tested. On the other hand a lower river/sea water level compared to the dike height does increase residual dike resistance considerably. Click on related outputs below for additional details about these findings.
Status for day-to-day practice
Currently, this research mainly shows that using residual dike resistance is promising but depends of the dike and subsoil characteristics as well as the river/sea water level. To test the model capabilities, the research focused on simple dike geometries and further investigation is necessary to determine how large residual dike resistance is in more realistic cases. So far, we can already conclude that dikes with a lower river/sea water level compared to the dike height have a considerable reduction in the overall probability of flooding.
3D modelling will be required to further analyze the slope instability failure and make better predictions. The model should be further applied to more complicated dike geometries by, for example, including a berm or ditch. Contributing organizations can help the research progress by providing more realistic dike geometries (at the moment preferably made up mostly from a single material).
Last modified: 21/06/2020
Delft University of Technology
- Remmerswaal, G., Hicks, M., & Vardon, P. (2018). Ultimate limit state assessment of dyke reliability using the random material point method. 89-90. Abstract from ComGeo IV: 4th Internation Symposium on Computational Geomechanics, Assisi, Italy.
- Remmerswaal, G., Hicks, M., & Vardon, P. (2019). Influence of Residual Dyke Strength on Dyke Reliability Using the Random Material Point Method. In J. Ching, D-Q. Li, & J. Zhang (Eds.), Proceedings of the 7th International Symposium on Geotechnical Safety and Risk (ISGSR 2019): State-of-the-Practice in Geotechnical Safety and Risk (pp. 775-780). [IS4-4] Taipei, Taiwan. https://doi.org/10.3850/978-981-11-2725-0 IS4-4-cd
- Remmerswaal, G., Bolognin, M., Vardon, P., Hicks, M., & Rohe, A. (2019). Implementation of non-trivial boundary conditions in MPM for geotechnical applications. In Proceedings of the Second International Conference on the Material Point Method for Modelling Soil–Water–Structure Interaction: 8 – 10 January 2019, University of Cambridge, United Kingdom
- Remmerswaal, G., Hicks, M.A., Vardon, P.J. (2019) Slope Reliability and Failure Analysis using Random Fields, in The Material Point Method for Geotechnical Engineering: A Practical Guide (ed.) J. Fern, A. Rohe, K. Soga, Eduardo Alonso, New York, pp. 287-296.
- González Acosta, J. L., Vardon, P. J., Remmerswaal, G., & Hicks, M. A. (2019). An investigation of stress inaccuracies and proposed solution in the material point method. Computational Mechanics, 1-27. https://doi.org/10.1007/s00466-019-01783-3