Stresses in tetrapod armour units exposed to wave action

Abstract

In the late seventies, begin eighties a number of large breakwaters was severely damaged. The armour layer of these breakwaters consisted of slender concrete armour units, like dolosse or tetrapods. It appeared that one of the main reasons of failure of these breakwaters was breakage of the armour units. Obviously, the mechanical strength of the armour units had been exceeded. In this study an analysis concerning the static and quasi-static portion of the tensile stresses inside tetrapod armour units is presented. The data has been obtained from a series of small scale model tests. Stresses have been measured using a load-cell technique developed by CERC (Coastal Engineering Research Centre) in association with AUC. (Aalborg University Center) In general, the stress signal can be divided into three parts. Firstly, a static part, i.e., stresses caused by the weight of the armour units. Secondly, a quasi-static part can be distinguished. Quasi static stresses originates from the motion of the water around the armour units. Thirdly, a dynamic part can be identified caused by the concrete to concrete collisions. The obtained stress signal has been processed using a preliminary analysis. This analysis was similar to a simple surface water wave analysis, resulting in the maximum value of the quasi-static stress within each stress wave. These maximum values were used in a statistical analysis.The stress distributions can be described using a Log Normal distribution. The average of these Log Normal distributions increases with increasing wave height. The standard deviation of the distribution decreases with increasing wave height. However, because large differences between subsequent test runs have been observed under identical conditions, the randomness of the process involved must have large influence on the variation in stress level. As the number of repetitions for each of the combinations of the parameters involved, i.e. Hs , ht o e , sop, location and orientation, was rather small, it was not possible to derive trends between all individual variables and the accompanying stress distributions.