Graduation of Rik Wegman

10 February 2020 14:00 - Location: TBA - By: Webredactie

"Strength of grass covers on dikes" | Professor of graduation: Prof. dr. ir. S.G.J. Aarninkhof (TU Delft), supervisors: Dr. ir. M. van Damme (TU Delft), dr. ir. G. Keetels (TU Delft), ir. G.J. Steendam (Infram Hydren), ir. R.J.C. Mom (Infram Hydren).

Wave overtopping is responsible for many dike failures, dike breaches and severe flooding. The erosion resistance of dikes to overtopping waves is determined by the strength of the grass cover.   Therefore, a thorough understanding of the erosion resistance of grass covers is desired to enable a good and effective dike management. In the existing approach to assess the erosion resistance of grass covers, the erosion resistance is empirically quantified in terms of local hydraulic loads. However, despite the cyclical nature of overtopping waves, no insights into the dependence of the erosion resistance on successive loads are available.

To improve the understanding of the resistance of grass covers to wave overtopping induced loads, understanding of the development of the mechanical behaviour of grass sods is essential. In this study the mechanical behaviour of grass covers under tensile loads have been examined by means of direct pull tests executed in the field. Especially the influence of repetitive wave loading and the conditions during testing were of interest.

The results of this study show that the deformation of the grass cover due to tensile load is time dependent. The failure mode of the grass cover is influenced by the degree of saturation of the grass cover. The maximum strength and mechanical behaviour of the grass cover is influenced by accumulation of damage. The findings presented in this study provide a better understanding of the processes that influence the mechanical behaviour of the grass cover. Moreover, insights in the development of the process of damage accumulation are revealed.

Based on the findings of this study, a method is proposed to determine the resistance of grass covers against repetitive loads by means of direct pull tests. In this method the resistance of grass covers against repetitive loads is described by an estimation of the fatigue limit. Relating the fatigue limit of a grass cover to hydraulic loads, enables an assessment of the erosion resistance of a grass cover at a specific location.