Stability analysis of XblocPlus crest element

Abstract

In the past humans used to protect their shores mainly with rocks. In the past decades the shore protections did gently shift to concrete element protections. An example of an often applied concrete armour unit is the Xbloc. This element is quite strong and well investigated. BAM Infraconsult has developed a uniformly placed armor unit, the XblocPlus. The new XblocPlus is placed in a regular pattern, which is easier for contractors to construct. A design detail of a XblocPlus armor layer which still needs some attention is the transition from the slope to the crest. The elements are placed in horizontal rows, locked in place by two elements in the row above and two elements in the row below. However, the upper element is not supported by any element above it, leading to less interlocking. This study focusses on the physical processes which lead to (in)stability of the upper element and tries to increase this stability.

First, physical laboratory tests were performed with the main aim to visually observe the failure methods of the upper XblocPlus element. After the initial tests, a computational fluid dynamic model was built with the aim to get a better insight in the load distribution on a single XblocPlus element under wave impact.

To increase the stability of the element as much as possible, several increments to the stability of the upper element are proposed. The first method which increases the stability of the element is to cover the back of the element with rock, this changes the rotation point of the element backwards which increases the wave load required to initiate rocking. A second possibility is to face the top of the upper element downward, this does decrease the drag on the element. A third possibility is to face the top of the upper element upward, this leads to a higher drag on the upper element. However, this orientation makes it possible to bury the element in a rock backfill, increasing the weight of the element.

This thesis did slightly research the applicability of CFD models to concrete armor design. The main conclusion for this trial is that it is possible to apply numerical models for the design of breakwaters, however physical model tests are still required to validate the obtained data from the numerical model, since the flow around a coastal structure is that complex, it only estimates the loads in the right order of magnitude when not validated. The validity of the model can be increased by calibrating the soil parameters using measured pressures in the different layers of the breakwater. To increase the accuracy of a numerical model, one could construct a three dimensional structure, which requires much computational grid cells, making the computational demand of the system quite high.