Graduation of Casper Boon

Strength of grass covers on dikes

• Professor of graduation: Dr. Ir. A. Antonini (TU Delft)

• Supervisors: Prof. Dr. Ir. M.R.A. van Gent (TU Delft), Dr. Ir. G.H. Keetels (TU Delft), Ir. G.J. Steendam (Infram Hydren), Ir. R.J. Wegman (Infram Hydren)

Wave overtopping, in which a wave washes over the crest of a dike and damages its inner slope, is one of the many mechanisms that is found to be responsible for dike failures. As these dikes are generally covered with grass, insight into the erosion resistance of these grass covers against wave overtopping is desired. To assess the performance of dikes against wave overtopping, various concepts exist, such as the widely used cumulative overload method. However, these concepts lack a thorough understanding of the physical mechanisms at play during grass cover failure as a result of wave overtopping. Furthermore, the methods are generally not time-efficient and are labor intensive, leading to high costs. Therefore, a time-efficient and predictive method is desired that is based on the physical mechanisms of grass cover failure.

The grass pull device, which is used extensively in this thesis, may serve as an alternative for the existing assessment methods. The device, which is reminiscent of a tensile test used in mechanical sciences, is able to exert various load mechanisms on the grass cover. In this thesis, the grass pull device is used to study various aspects of grass cover failure. Special attention is given to the influence of cyclic loading on the grass cover. Additionally, material properties have been derived that may serve as input for numerical grass erosion models. Furthermore, the influence of grass roots, subsoil type and pore saturation on the failure mode of grass covers was investigated.

The results of this study showed a continuous growth of deformation and a decrease in stiffness when the grass cover is loaded cyclically. The behavior of grass during cyclic loading was found to be comparable to other composite materials, such as fiber-reinforced plastics. The material properties Young's and shear modulus were derived. The Young's moduli were found to be slightly overestimated, while the shear moduli were found to be comparable to what may be expected from literature. Differences in grass cover properties on different subsoils were identified, showing that grass covers on clay are generally better at resisting deformation, while having a brittle failure mode. For grass covers on sand, a large spread was observed and the material was found to deform easily, while still providing resistance at large deformations.

Based on the findings of this study, recommendations were made to improve the grass pull device. It was found that the grass pull device was successful in providing insight into various physical processes. Whether the grass pull device could entirely replace existing assessment methods remains questionable, but it has proven to be a successful addition to existing methods.