Design Caisson breakwater

Abstract

The growing need for breakwaters in deep water due to the increasing draught of large vessels draws the attention to caisson breakwaters. These monolithic structures are more economical compared to rubble mound breakwaters. Especially in deep water lower construction and maintenance costs and considerable savings in construction time can be realised. A caisson is built on shore and towed out to the actual offshore site. Unfortunately, damage at a caisson is often progressive. This causes an abrupt collapse of the structure. By understanding the dynamic processes involved, the design of the structure can be soundly based. The formula of Goda (1985) is a worldwide used design method for vertical breakwaters based on the quasi-static approach. His design method is very useful as a first indication for the dimensions of the caisson. In order to be able to analyse Goda's method, the design of a caisson breakwater is roughly divided in three phases. First the crest elevation of the caisson, the design wave and the design water depth, are determined with probabilistic considerations about the economy of the harbour. Subsequently the wave load follows from the wave pressure formulae. Thirdly, the width of the structure sets the weight of the structure which defines the safety against failure. Goda sets the design parameters on definite values regardless the cost-benefit analysis of the harbour. His design wave is the highest wave in the design sea state, which is based on the principle that a breakwater should be designed to be safe against the single wave with the largest pressure among storm waves. From the comparison of the measured wave forces of the hydraulic model study and the values calculated with the wave pressure formulae of Goda and of the linear wave theory no conclusions can be drawn. This is partly due to the close resemblance of the results of the linear wave theory and Goda's formula for the conditions at Europoort Rotterdam and partly caused by the scatter in the measurements. An experiment about the failure mechanisms of the caisson confirms the introducing of uncertainties concerning the placing of the caisson on the rubble mound foundation. Goda's wave pressure formulae turned out to be in fact design formulae. Not only his design parameters but the formulae themselves include safety considerations. Evaluation of Goda's formula is therefore only valid when the whole design process is taken into account. It is noted that the accuracy of the calculated wave pressure on the wall is very good with respect to the uncertainties introduced in the foundation forces and the determination of the design parameters.