Graduation of Hilco Bruins Slot

Enhancing reliability of dikes: An approach for assessing benefits of pore pressure monitoring and pressure relief wells in spatially variable soils

• Professor of graduation: Prof.dr.ir. S.N. Jonkman

• Supervisors of graduation: Dr. J.P. Aguilar-López (TU Delft), dr. T.A. Bogaard (TU Delft), ir. W.J. Klerk (TU Delft), ir. J. Steenbergen-Kajabová (Sweco)

The Netherlands is a country prone to flooding. Recent assessments led to the insight that protection levels of many flood defences should be increased. Integrating reinforcement measures is a difficult task as many dikes are situated in densely populated areas. Conventional reinforcement measures include berm construction or the implementation of sheet pile walls. The first can become very expensive in case houses are situated close to a dike, the latter is rather expensive and irreversibly changes dike composition.

Geotechnical failure modes piping and slope instability are considered most important failure modes for Dutch fluvial dikes. In this thesis a case study is carried out on such a dike that is disqualified for those failure modes. The dike is situated in an urban area with limited space available for reinforcement works. It is studied whether measuring pore pressure behind the dike can reduce uncertainty on the subsoil. Subsequently the possibility of implementing relief wells is researched as a way to mitigate the risk for the considered failure modes.

An optimal pore pressure monitoring strategy was defined by finite difference modelling groundwater flow through random fields. First, a principal component analysis was conducted to define the least amount of pore pressure sensors needed for measuring hydraulic potential behind the dike. It was found that for a dike section of 100 m at least two sensors were required and for a section of 2000 m at least five sensors. This result was implemented in the model. Measurement errors were included and the amount of sensors was increased until enough variation in pore pressure was observed. Finally four sensors were implemented on the 100 m section and six sensors on the 2000 m section. Pre-posterior analysis was applied to improve reliability for piping. Results showed that probability of failure decreased significantly, however it did not meet required target reliability.

Pressure relief well implementation was considered as a measure to mitigate risk for slope instability and further mitigate piping risk. For the case study implementation of relief wells behind the dike was considered. The system was designed based on mitigating the risk for slope instability. Again finite difference modelling in combination with random fields were used to derive results. A script was written that, for each random field, assessed the critical area beneath the dike and determined most critical uplift pressures occurring in the field. This was repeated multiple times and eventually mean values were used as input for stability checks in stability software. It was shown that for the 100 m trajectory a well spacing of 50 m would sufficiently increase the reliability estimate for slope instability. For the 2000 m trajectory a well spacing of 45 m was found. The difference can be attributed to the length-effect: the chance for weak spots increases with increasing dike length. Again pre-posterior analysis was applied to assess the risk for piping. It was shown that for both trajectories the required target reliability was amply achieved.

It was shown that pressure relief wells provide a good design alternative for dike reinforcement in urban areas. Implementation of relief wells is economically attractive compared to berms as for the latter relocation of houses is an important cost driver. Construction costs for a pressure relief well system are low and it was shown that pore pressure monitoring prior to installation of the wells is economically beneficial. Looking at the lifetime costs of relief well systems it was concluded that maintenance costs outweigh benefits of low construction costs and for the considered case study sheet pile walls are most cost-effective.

Invitation link upon request: h.bruinsslot@student.tudelft.nl