• ## Agenda

The Netherlands is a low-lying country which is protected against floods by a flood protection system consisting of dikes, dunes and other hydraulic structures. The quality of the flood defense system should be of a sufficiently high level to guarantee protection against a flood. If a flood defense is unsafe, measurements are required to improve the safety of the flood defense. The Dutch governmental agency Rijkswaterstaat'' stimulates companies and research institutes to come up with innovative measures to strengthen dikes. Therefore, the Dutch companies Heijmans, de Vries \& van de Wiel and Movares have developed an innovative measure named the Waterontspanner'' (in English: \emph{water relaxation well}). The Waterontspanner is comparable to a passive vertical drain which is designed to reduce the failure mechanism macro instability of the inner slope. The Waterontspanner has been implemented in project Schoonhoven-Langerak (SLA). The project was successful and it is desired to use the Waterontspanner more often. To do so, more information has to be obtained on the extent to which the Waterontspanner influences the failure probability of a dike due to macro instability of the inner slope. In this thesis, research is carried out to provide insight in the latter. First, an analytical model was set up to understand the behavior of pore pressures in the subsoil and to find out if it makes sense to use a Waterontspanner. Herafter, a safety assessment was set up to investigate how the failure probability of a dike due to macro instability of the inner slope is influenced by Waterontspanners.