A clay layer as a revetment for sea dikes

Abstract

Environmental awareness and sustainable development have been popular themes over the last few years. To improve dikes along the estuaries in Zeeland (The Netherlands) where a high foreland with natural areas of salt marshes is present, alternatives to stone revetments are examined. From a natural and aesthetic point of view, a slope covered with vegetation is preferred, because in this way a more gradual transition between the salt marshes and the dike with the hinterland is obtained. Revetments which obtain their strength from the grass cover or the clay layer are possible alternatives. This research project looks at the possibility of using a thick clay layer to form a sustainable and feasible revetment for the outer slope of sea dikes in the presence of a high foreland, with sufficient strength and stability to withstand a normative load, a design storm. This research concentrates on the behaviour of clay under wave loading and the influencing factors on this behaviour, because there is still little knowledge on this behaviour of cohesive material. All large scale experiments which involve clay under wave loading are investigated and compared in this research. This resulted into a complete overview, which is analysed to obtain new valuable conclusions and to validate existing conclusions and assumptions. The behaviour of clay is researched with respect to the development of erosion, the location of erosion and the shape of erosion profile. The influence of the clay condition, the soil structure development, on the strength of the clay under a wave load appears to be significant. Within the subdivision in clay conditions, the clay type (the sand percentage of the clay) and the wave conditions are of influence. Comparisons with the erosion behaviour of sand and with observations in actual storms give additional conclusions on the behaviour of clay and a validation of the conclusions from the experiments. Clay appears to have a significantly lower erosion rate than sand; the experiments however show less erosion development over time than in actual storms was observed. A semi quantitative model to determine the erosion depth of a clay layer over time under wave loading was developed in this research. Also a formula to the soil structure development over time and in depth was developed for this. The model is developed in a way that it can be extended if more influencing factors on the erosion process are quantified. A case study to Het Verdronken Land van Saeftinghe was performed with the semi quantitative model. On the basis of this research it can be concluded that a thick clay layer can possibly form an environmentally acceptable alternative to stone revetments, however research is not yet sufficient enough.