Impact of Damages on the Stability and Reliability of Pattern-Placed Revetments

An analysis with use of a finite element model into the different types of damages for pattern-placed revetments under wave loading

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Over time, degradation processes might cause damage to pattern-placed revetments. Examples of such damages are missing elements, deformation, and the loss of joint filling. However, little is known about the exact consequences of those damages. Therefore, when damage is observed during an inspection, it is estimated based upon experience what the possible consequences are. If the consequences are misjudged, this will lead to inefficient maintenance.
Better insight into the exact consequences of damages will expand the possibilities of how risk-based maintenance can be used to maintain revetments. Consequentially, maintenance interventions can be planned more efficiently, reducing the societal costs incurred due to inefficient maintenance. This study contributes to this topic by investigating whether it is possible to estimate the impact of damages on the stability and reliability of a revetment using a model.
Within the study, first, damages are analyzed based upon a literature review. Next, data of old flume experiments with Basalton and basalt revetments are analyzed to study and quantify damages. This analysis focuses on the uplift of elements, deformation around the wave impact zone (S-profile), and washed-out joint filling. Then, a finite element model (FE-model) is created to simulate the wave impact on pattern-placed revetments. The main focus of the FE-model is to study the uncertainty due to structural changes, which are the damages. The damages quantified during the analysis of the flume experiments are also included in order to be able to assess damaged revetments. Finally, the study uses the FE-model within a sensitivity analysis to study the most important uncertain parameters. Based on the samples used in the sensitivity analysis, response surfaces are fitted to obtain a model that can predict the damage for any set of parameters. Finally, these models are demonstrated in a case study of a coastal dike near Den Helder. An increase of 10 – 100 times of the failure probability has been observed for small deformations, while for medium to large deformations, the failure probability increased by 1000 – 10000 times. For no joint filling or a missing element, the failure probability increased by 10 - 100 times.
This study showed that it is possible to create a finite element model that can estimate the impact damage has on the stability and reliability of a pattern-placed revetment. The obtained results can be used within the daily practise as part of risk-based maintenance as the study provides a way to obtain a first indication of the impact of missing elements, deformation, and washed-out joint filling. Additionally, the developed methodology can be used to obtain the impact of other types of damage. Although Basalton is the primarily investigated type of top layer, analysis of the flume experiments showed that basalt revetments are subject to identical types of damage. Therefore, it is expected that the findings within this study can be applied to a broader range of top layer elements with similar characteristics to Basalton (e.g. basalt, C-Star, and Hydroblocks).