Graduation of Steven Coevert

Deeper piping: finding how to model it and what contributes to the emergence of a deeper pipe

• Professor of graduation: Dr. J.P. Aguilar-López (TU Delft) (Chair)

• Supervisors: Dr.ir. W. Kanning (TU Delft, Deltares), Dr.ir. A.P. van den Eijnden (TU Delft), Ir. E.M. van der Linde (TU Delft), Ir. S. Engels (RPS)

The Netherlands is a country that is being threatened by water, both from the rivers and from the sea. The Dutch have built dikes to keep their lands from inundation. However, dikes can fail in all sorts of ways, one of these being Backward Erosion Piping, or piping for short.

Piping is a well-known failure mechanism in dike safety assessment. Tests on piping in tidal subsoil were conducted in the summer of 2021, where a pipe was found to have grown at greater depth than normal, but the occurrence of this deeper piping has never been seen before, let alone described. This lack of knowledge poses a potential safety risk, as it may underestimate the vulnerability of certain subsoil configurations. Therefore, the objective of this thesis is to develop a comprehensive understanding of deeper piping and identify the key parameters influencing its formation.

To achieve this objective, a definition of deeper piping and its differentiation from conventional piping is established. Sub-mechanisms governing deeper piping are examined by analyzing the forces responsible for grain movement and the forces that maintain grain stability. A Finite Element Model is constructed to quantify the moving forces within the subsoil, which, when combined with resisting forces, enables the determination of whether deeper piping can occur in a given subsoil configuration.

To investigate the factors contributing to deeper piping, a series of simulations were conducted using a Finite Element Model of the subsoil. By varying the parameter values while keeping other factors constant, the influence of each parameter on the occurrence of deeper piping was examined. The analysis revealed that several key parameters significantly affect deeper piping formation, including cohesion force, cohesion anisotropy, permeability and thickness of the top layer, permeability of underlying layers, and permeability anisotropy. These findings provide valuable insights into the mechanisms underlying deeper piping and enhance our ability to identify subsoil configurations that are prone to this phenomenon. These findings enhance the identification of subsoil configurations prone to deeper piping, thereby improving risk assessment and mitigation strategies associated with this failure mechanism.