STABILITY OF VERY WIDE GRADED MATERIAL, DESIGNED AS BREAKWATER CORE, UNDER WAVE ATTACK

A breakwater core typically consists of quarry run with a wide gradation up to 500kg or 1000kg. The use of this material can however lead to significant logistical and environmental problems during construction, not in the least associated with the intensive use of road going (construction) vehicles. As an alternative so-called Wide Grade material can be considered. This material, with a typical grading in the range of 0-200mm (~ 0/20kg), is readily available and can be produced, loaded and discharged quickly with large marine-borne bulk carriers and, given the right conditions, placed directly into the breakwater core. The main question however is whether this relatively small material is stable under typical wave conditions that may be expected during construction, before it is covered by subsequent filter and armour layers. This paper presents the results of extensive physical scale model testing carried out on this material. The tests show that the damage is relatively small for submerged berms, up to fairly extreme wave events. Comparison with existing theory is partly inconclusive for low-crested and near-bed structures because of the limited validity range of existing design methods. For emergent structures a relatively good match was found with the BREAKWAT model predictions. Finally, a method is suggested to use Wide Grade in the construction of a breakwater core, making use of the test results.