A floating factory for the Masvlakte 2 caisson breakwater

Abstract

It is foreseen that the future expansion of the Port of Rotterdam will face the problem of lack of space. A typical Dutch solution to create the necessary area for harbour and industrial terrain, is reclamation of land from the sea. This can be achieved by expanding Maasvlakte I with Maasvlakte 2. The exact amount of required terrain is subject to many elements which are continuously changing such as political, economical, environmental and social views, and therefore is very difficult to predict accurately. A solution to meet this uncertainty, is to create the Maasvlakte 2 in several phases. By maintaining a flexible planning of the reclamation, the expansion works can be adapted to changed views if necessary. The construction works of the Maasv lakte 2 are very extensive, and consist of 3 main elements, the terrain itself which is to be created, sea-defence works which must protect the terrain against inundation and erosion, and a breakwater, which must reduce the height of the incoming waves from the North sea to an acceptable level in order to give access for shipping to the harbour under storm conditions and limit downtime of the harbour activities. For the phased execution of Maasvlakte 2, sections of the newly gained terrain must be protected against the sea by a breakwater during each phase. As breakwaters are very expensive structures, a flexible breakwater, a breakwater which can be reused several times, might be economical. Caissons are an ideal alternative for such a breakwater as these can be brought afloat again and repositioned at a new location. The caisson dimensions have been determined using the Goda design formulas to calculate the wave forces on vertical walls and the wave transmission over the caisson, and the formulas of rinchHanssen to calculate the bearing capacity of the soil layers. These formulas have been implemented in the computer program 'Outer Caisson Dimensions', (O.e.D. ) written in this study. This program indicates that for the conditions of the future Maasvlakte 2 breakwater site, slip of the subsoil is the decisive failure mode. Construction of the caissons will be on a floating construction yard moored in the Europe Harbour of the Maasvlakte I equipped with slip formwork. The main advantage of the floating caisson construction method is that there is no need of an expensive construction dock or specialised lifting equipment. When the caisson is completed, it is moored at a temporary location where it is trimmed for stability and prepared for transport to the breakwater site. The caissons are towed to the breakwater construction site by tugs, and are lowered onto the foundation by flooding the cells. Finally the cells are filled with sand, concrete capping plates are placed, and the rocks of the rubble mound bottom protection are placed. The caissons can be considered as building blocks, easily reusable components of the Maasvlakte 2 breakwater. For caisson reuse, the capping plates must be removed and the sand content of the cells replaced with ballast water. When the caisson is ready to be transported, the water is pumped from the cells and the caissons become buoyant. They can now be transported to their new destination. The construction method of caissons on a floating construction yard is technically feasible and financially competitive with other construction methods, such as the Dutch traditional construction method in a dock or the Japanese construction method on a yard located above the ground water level, with use of heavy lifting equipment. By designing the caisson in such a manner that it is capable to float on its own buoyancy, it is an ideal solution to form the components of a reusable breakwater.