The breakwaters of IJmuiden are of a unique design; a riprap core is covered with a thick impermeable asphalt slab. During construction and after completion, slope instability caused extensive damage. Placement of a concrete cube armour layer prevented further damage to the asphalt, but proved to be unstable and required a significant amount of maintenance. Rijkswaterstaat (RWS), which is responsible for the maintenance, contracted a number of companies to investigate the strength and loading of the breakwaters. Lifting of the asphalt slab as a result of overpressure in the breakwater core was found to be the decisive failure mechanism. To determine the amount of overpressure, measurements were performed in both breakwater heads. A bigger favourable damping of wind waves was measured in the southern breakwater. Based on these measurements and other research outcomes RWS decided to change the maintenance strategy; armour units above the NPA – 2 m line will not be maintained in the future. The new strategy is based on the reasoning that the damping of wind waves reduces lifting forces and makes the weight of the armour layer redundant to prevent lifting. The mechanism(s) causing the larger damping in the southern breakwater are however unknown, this makes it hard to predict the amount of damping and therefore the magnitude of the loading of the asphalt during storm conditions. Aim of this thesis is to get insight in the stability of the asphalt slab during design storm conditions, and the necessity of an armour layer. Therefore the damping mechanism and the amount of damping during storm conditions need to be determined. Numerical modelling is performed to describe wave transmission through the breakwater and to evaluate the influence of different damping mechanisms. Most important mechanism causing additional damping is siltation of the toe structure of the southern breakwater. Along the Dutch coast the net longshore sediment transport is directed northwards. Therefore sediment passes the southern breakwater, part of the sand might settle in the toe and core of the breakwater. A sand layer with a height of 3.3 m reduces the flow of water enough to cause the measured damping. The stability of the sand during storm conditions is checked using open filter sediment transport formula. Erosion of the sand layer is expected, however the erosion is expected to be in the order of centimetres which is insignificant. The damping mechanism causing the measured damping in the southern breakwater is determined; hence loading of the asphalt slab during design storm conditions can be determined. The thickness and quality of the asphalt slab is uncertain and might vary significantly over the length of the breakwaters. In order to get insight in the quality of the asphalt two cores were drilled in 2004. One showed high quality cohesive asphalt, the other showed low quality with low cohesion. Lifting of the asphalt cannot be ruled out. The dead weight of the designed asphalt slab in combination with a partial armour layer is not sufficient to resist the upward pressures during a design storm. The additional resistance needed against lifting can be provided by the weight of a complete armour layer or bending strength of the asphalt slab. The bending strength of the asphalt slab depends on the quantity and quality of the asphalt present. In order for the asphalt slab to have sufficient bending strength a top layer of high quality cohesive asphalt is required. Concluding, without additional information concerning the asphalt quality and thickness lifting of the asphalt slab and thereby failure of the breakwaters cannot be ruled out in case the armour layer erodes above a level of NAP -2 m.