Stability of the first row of an XblocPlus armour layer

Abstract

XblocPlus is a new uniformly placed single layer armour unit, developed by Delta Marine Consultants (BAM Infraconsult). This report focuses on the stability of the first row of armour units, as this row is interlocked less than other rows. When the first row starts to slide, this is a big threat to the stability of the entire armour layer. Furthermore, not much is known what exactly influences the stability of the first row. In this study physical modelling is used to determine the stability the first row of an XblocPlus armour layer and investigate what influences this stability. The results of the tests were analysed with a method called Digital Displacement Analysis. Two simplistic theoretical models were devised. The first model considers the destabilizing forces to consist only of the drag and uplift forces caused by the wave on the armour units of the first row itself, this is called direct hydraulic loading. The second model also considers destabilizing forces transferred by the armour layer to the first row, this is called indirect hydraulic loading. From the model tests, it was found that indirect hydraulic loading plays an important role, as the movement clearly occurred during down-rush. When waves started to plunge, the waves failed to destroy the breakwater. Furthermore, loose armour units placed in front of the first row moved at higher heights than the first row, indicating that the armour layer pushes out the first row. Water depth influences stability, as lower water depth gave a lower stability. It is feasible that this effect is caused by the down rush being closer to the first row. The second model is most appropriate when taking these findings into account. From the tests it was found that a toe berm increases stability as it adds weight to the first row and thus increases friction resistance. A foundation layer also increases stability as friction between the foundation layer and the first row is higher than between the first row and the concrete slab that was used in other tests. Both measures also increase the damage threshold. The damage threshold is the amount of damage that a first row can take before brittle failure occurs. Brittle failure is an increase in damage of 0.2Dn (Dn is the nominal armour unit diameter). When both measures are used together, the foundation layer makes the toe berm more stable and this is why the stability of the first row is also increased. Based on the tests in this study the stability of the first row can be stated as Ns =3.87 (Ns=Hs/(delta*Dn), the stability number is defined as the significant wave height divided by the nominal diameter of the armour units and the relative density of the armour units) as at the end of the tests the damage was only 0.13Dn. The test results were used to determine a working hypothesis for a design formula. More tests are necessary to validate this formula and determine its accuracy. A remarkable finding of this study is that two test series showed contradictory influences of wave steepness. The first test series showed that a lower wave steepness is detrimental for stability of the first row, while the second test series showed the opposite. More research should be conducted to investigate on this curiosity.