Experimental study on Xbloc placements for breakwater curvature sections

The use of concrete armour layer units is becoming more common nowadays as a replacement for the rock armour in the breakwater rubble mound design. The development of concrete armour units in time led to various individual types and forms with specific individual characteristics. One of the latest developments of concrete armour layer units is the Xbloc® unit by Delta Marine Consultants (DMC). The primary objective of this thesis is to investigate and validate the study of the placement of the Xbloc unit specifically on a curved section of the breakwater using a large scale model test. Xbloc unit are placed on a breakwater slope row by row starting from bottom. Then the placement will be continued to another new row above until the units are completely covering the whole slope of the breakwater. Placement of the Xbloc unit on a straight section is quite straight forward with a constant relation between the surrounding units. The relations are based on the horizontal distances to the other unit in the same row and the distance to the unit upslope on the next row above. With a correct placement distances a required packing density and the interlocking effect between the units can be achieved. However, the relations of the distances are not applicable for a curved section of a breakwater. A curved section of a breakwater has a difference in radius length further upslope. The differences in radius lengths create improper placement distances between the units in the higher upslope. The horizontal distance between the two units becoming smaller and very close to each other. Eventually one unit has to be left out. The calculation of the packing density are also differs from the method used to calculate for the unit on a straight section. Hence, a new method was used to calculate the packing density for the breakwater curved section. The relations of the unit placement distances on a breakwater curve have been studied by ten Oever in his MSc thesis in 2006. A theoretically study are made and was validated by a placement tests on a model of a breakwater straight section. From the study, a computer model was developed to generate an optimal design grid position to place the Xbloc unit on a breakwater slope. ten Oever suggested a new unit are place off centre with an angle when the horizontal distances on the row below are becoming closer and closer. The off centre distance will be based on the horizontal distances between the two units. The maximum off centre distance are 0.4D and the unit will be left out. The angle is called the unit adjust angle and to be 450 from the horizontal line. Hence, this thesis focuses on validating the design placement grid generated by the computer model. Two placement tests have been performed using a model of a quarter of a breakwater head. The first test was performed following the generated design grid from the computer model. The output result of the final Xbloc position from the first placement tests was generally can be achieved according to the design grid with some distortion. The distortions of the units are mostly on the upper part of the breakwater slope especially in the row above where there is a left out unit. Furthermore, the packing density was 3% lower than the required theoretical packing density. In accordance to the first placement test, the second placement test was made to further improve the placement of the Xbloc unit. The second test is performed by correcting the design grid by using a coherent judgement before placing a new unit. Judgments are made by visually looking to the overview of the units in surrounding to ensure the new unit are placed in a suitable position for better fit between the two base units. The packing density resulted in higher values. Most of the units after the row where there is a left out unit are placed with a lower upslope distances than the design grid. The gap resulted from the left out unit are also being reduced when the unit a placed with a lower upslope distances. In conclusion, the placement quality can be improved by lowering the upslope distances in the computer algorithm for generating the optimal design grid. This could possibly be done by changing the parameters of maximal off centre distance to 0.3D and unit adjust angle in the order of 300 to 400. A comparison of the placement made in the laboratory and the placement of the prototype unit from the breakwater project in Port Oriel are made. Although a direct comparison could not be made in accordance to the relations between the placement grids, nevertheless there are several conclusions that can be drawn from the placement made in Port Oriel. The placement of the units in the Port Oriel was denser than the placement of the model units in the laboratory. This is mainly due to the larger forces on the edges of the units and due to the use of the excavator which actively pushed the units into a tight placement pattern. This is not done in the laboratory as the placements made are based on the use of a crane. Denser placement is acceptable for the stability of the armour layer but it leads to higher concrete use on the breakwater head.Hydraulic EngineeringCivil Engineering and Geoscience