Enhancing reliability of dikes

Abstract

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 most important failure modes for Dutch river 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 pore pressure measurements behind the dike can be used to improve the reliability estimate for piping. Subsequently it is analyzed whether implementing pressure relief wells can be used to increase dike reliability for both considered failure modes. For the case study an advanced modelling framework was used consisting of groundwater modelling software and a random field generator. The case study was divided into two parts. The first part consisted of a dike section of 100 m based on a dike section at Wijk bij Duurstede. The second part consisted of the same dike section, only now extrapolated over a length of 2 km for which variations in soil conditions become more important. First, an analysis was conducted to define the optimal amount of pore pressure sensors behind the dike. It was found that for a dike section of 100 m a total of four sensors could be used to perform reliability updates, for a dike section of 2000 m it was found that a total of six sensors could be used. For the 100 m section piping failure probability improved from 5.21E-3 per year to 1.62E-4 per year. For the 2000 m section piping failure probability improved from 5.21E-3 per year to 1.89E-4 per year. Pressure relief well implementation behind the dike was considered as a measure to increase dike reliability for both failure modes. The system was designed based on a target reliability level for slope instability. The same modelling framework was applied. An analysis was conducted and it was shown that for the 100 m section a well spacing of 50 m would sufficiently increase dike reliability for slope instability. For the 2000 m section a well spacing of 45 m was found. For the 100 m section failure probability for slope instability increased from 8.81E-5 per year to 1.22E-6 per year, failure probability for piping increased from 1.62E-4 per year to 2.03E-7 per year. For the 2000 m section failure probability for slope instability increased from 8.81E-5 per year to 1.30E-6 per year, failure probability for piping increased from 1.89E-4 per year to 1.02E-7 per year. It was shown that for both trajectories target reliability levels for all failure modes were met. For this case study it was shown that pressure relief wells provide a good design alternative for dike reinforcement in urban areas. A life cycle cost analysis was applied and it was shown that implementation of relief wells is economically attractive compared to traditional design alternatives berms and sheet pile walls. For the first relocation of houses forms an important cost driver, for the latter initial construction cost are high. Furthermore it was shown that implementation of pore pressure monitoring prior to the design of a relief well system yields a positive value of information.