Dr.-Ing. Marko Nehrig

Layers of dumped rocks are used as mechanical protection or as a foundation for constructions in the field of coastal engineering. For economic and technical reasons, rocks of different sizes are used in this process. Through the use of fall-pipes during the dumping process it is possible to place the rocks very accurately and also control the thickness of the dumped layer. This dissertation focuses on the risk of grain-size segregation during the process of falling through the fall-pipe in water. The grain-size segregation is caused by the higher settling velocity of larger grains compared to smaller grains. Constructing the dumped layer using segregated grain sizes will result in danger of erosion. This could lead to a lack of usability or stability of the entire construction. Conducting extensive numbers of physical model tests with falling objects of different shapes, densities and surface conditions, the settling velocity of objects in a fall-pipe was made predictable. The parameters influencing the settling process where identified. Conducting further physical model tests with bulks of rocks in the fall-pipe confirmed the assumption that bulks of rocks behave like a larger number of single rocks. The grain-size segregation of bulks of rocks depends on the length and the diameter of the fall-pipe and on the grain size distribution. It could be shown that every change in the flow of the rock mass within the fall-pipe, such as beginning or end of the dumping process, inevitably causes grain-size segregation. Recommendations for the design and construction process are given which lead to a minimization of grain-size segregation when a fall-pipe is used. Using the recommendations it is possible to determine the amount of the inevitably segregated rock material. This amount of segregated material must be taken into account during the design process in order to construct permanently stable and usable structures in the field of coastal engineering.