Stability Optimisation of the top armour row of a breakwater with Xbloc^Plus units

Abstract

The XblocPlus (referred to as Xbloc+) is a new, uniformly placed single layer armour unit, developed by Delta Marine Consultants (BAM Infraconsult). Although a breakwater armour layer with XblocPlus combines material saving and easy placement with increased stability, the transition between the armour layer and the crest is not adequately stable. The XblocPlus units of the top armour row (crest units) become easily displaced. In this research, several solutions to this problem are investigated through physical modelling, in order to determine the best one. Firstly, tests are performed on a breakwater with single, XblocPlus crest elements without rear support, in order to determine the most important parameters and mechanisms contributing to failure. The criterion for initiation of failure for the top armour row is 10%, so when 1 of the 10 crest units fails. Failure of a unit is defined as the condition where contact with the units of the row underneath is lost under at least one of the two wings. During wave run-up, under the forces resulting from the wave velocities, the XblocPlus crest units initially rotate and, subsequently, make a combined motion consisting of rotation, vertical and horizontal translation. The main parameters of influence to this movement are the crest freeboard (Rc/Dn) and wave steepness (sop): for Rc/Dn≥1.7, stability increases for increasing Rc/Dn and for increasing sop (from 2% to 4%). For zero freeboard, wave impact forces at breaking proved to be an important mechanism in the case of sop=4%, where collapsing breakers occurred. Failure and partial displacement were initiated at stability numbers (Ns=Hs/(ΔDn)) of 1.39 and 0.74, respectively, for Rc/Dn≤1.7, which are much lower than the stability number for displacement of the armour layer (Ns>3.88). A second set of tests was conducted, in order to investigate a way of optimising the stability of the transition area between the armour layer and the crest. Under the most critical conditions of the first tests, 7 crest configurations, based on their potential ability to resist the failure mechanisms, were investigated: 2 orientations of the Xbloc+ crest units (with the tail or nose tilted upwards), 2 different ways of placement of Xbloc at the crest, placement of Xbloc with a concrete crown wall element, underlayer rock material at the crest and underlayer rock material with a concrete crown wall element. From the above “trial and error” approach, it was concluded that the most effective way of increasing stability (optimised configuration) is the provision of a backwards support, which fulfils the criteria of no erosion and no uplift and resists rotation by bringing the rotation point of the Xbloc+ crest elements further to the back. The form of this concept tested was the placement of underlayer material between the Xbloc+ crest units and under their tails and a concrete element behind, which functions as backwards support to prevent erosion of the rock fill. No displacement of the Xbloc+ crest elements occurred for Rc/Dn≤1.7, 2%≤sop≤4% and no failure happened for Rc/Dn=0, sop=4% for a stability number up to 3.55. Rocking was decreased to zero. Failure and rocking of 10% happened only at the case of Rc/Dn=1.7, sop=2% for Ns≥2.78, due to the erosion of the fill, resulting from the uplift of the supporting crest element, with the final damage (at Ns=3.49) being repairable and limited at the 4 upper rows of the breakwater.