In the Netherlands dikes protect land from flooding by the numerous rivers flowing through. The stability of the Dutch dikes and the failure mechanisms that might occur are well understood, thanks to the improved knowledge of soil mechanics in recent decades. However, one process for which we would like to know more is the so-called backward erosion piping (BEP) (Beek, 2015). BEP is an erosion process driven by a hydraulic gradient which occurs at high water levels on the riverside of a dike. When a crack or well is present in the hinterland this hydraulic gradient can cause transport of sand particles and pipe formation (see Figure 1a). The pipe formation erosion process works its way to the riverside, and if the erosion process continues the dike-structure may collapse.
This BEP process is well understood for uniform layers with a narrow gradation, with all soil particles having approximately the same size. However, for soils with a wide gradation the current analytical methods for assessing piping (Sellmeijer and Shields-Darcy) are not suitable. This wide gradation in particle size is typically present in the province Limburg along the river Meuse, where many dike-sections need to be strengthened in the coming decade. A fluid-coupled, micro-geo-mechanical investigation is needed to extend existing analytical models such as the well-known Sellmeijer equation.
Fig. 1: On the left a schematic is shown of the BEP process at dike level where the pipe progresses towards the river due to the hydraulic gradient caused by the river. On the right we have zoomed in on the erosion process right at the interface between clay and sand.
Assignment
Recent advances in grain-scale modeling and particle-fluid coupling have made such a detailed investigation possible. The idea is to quantify the physics accurately at the grain and pore scale and then obtain upscaled quantities to examine the Sellmeijer model. In this assignment we want you to investigate how a wide gradation in soil particles influences the dynamics of the BEP process, using the coupled Discrete Element-lattice Boltzmann method (LBM-DEM). This will include getting a thorough understanding of the physics of the problem such as the groundwater flow conditions and the mobility and interaction between particles, as well as how the flow rate and the pore pressure differ as the content of the gravel and pebbles varies. In addition, a link between the Sellmeijer and Shields-darcy parameters and the DEM-LBM output should be made to give indications on how to improve the Sellmeijer model for soils with a wide gradation.
Learning goals:
- Understanding the physics of the erosion process for soils with a wide gradation
- Linking the physical insight to theoretical/analytical models
- Assessing the capability of the model for various content of gravel-pebble vs sand-silt
Supervision
Are you interested in this assignment? Contact the Master thesis coordinator:
