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 nearbed 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. Introduction Background A breakwater core typically consists of quarry run with a wide gradation up to 500kg or 1000kg. The principle idea being that the quarry run forms the left-over from the quarry after under layers and rock armour have been extracted. Hence it would be economical to use this in the breakwater design. In practice however, there may be several drawbacks to use core material from local quarries. First of all, the quarry run is handled using equipment such as front loaders and excavators and often has to be transported by (dump) trucks to either project site or loading quay. This means that the production rates are limited. Secondly, many quarries are land locked and using these for marine projects often means intensive use of the local road network, sometimes through large cities, congestion and significant environmental impacts. The use of dump trucks and end tipping requires a wide enough crest that may be dominated by construction requirements (passing of trucks and cranes, etc.), and may become a limitation in terms of work progress. The above challenges become even more significant, considering that the core material often constitutes a significant part of the overall breakwater volume. As an alternative, the use of 0-200mm Wide Grade material is proposed. This material is produced in a few specific quarries as a base material for a range of aggregates. Distinct (intrinsic) qualities of the material are depending on the source, but can typically be characterised as: high quality, constant grading, crushed and angular material, free of organic matter. Distinct logistical advantages are that the material can be produced quickly, loaded quickly on large bulk carriers and given the right vessel type, unloaded quickly as well by using conveyor belts or vessel gantry cranes. As a result, the material is Van der Plas, Van der Meer, Dominguez and Bijl BW17:71:2 very economical to utilize even in projects thousands of kilometres from the quarry, in particular when considering large deep water breakwater projects. Because of the appealing prospects, Wide Grade is currently applied in large breakwater projects and is under consideration for other breakwater projects as well. The main question posed by clients, contractors and designers is whether the material is sufficiently stable during construction, before protective filter and/or under layers and armour layers can be placed on top. Existing design guidance for this type of structure and material during construction is very limited, or non-existing, because of the following unique characteristics, see also Figure 1: • A D85/D15-ratio in the range 10–20, i.e. a very wide gradation; • A D50 in the range of 40-70 mm; • A long fine tail with D10 in the range of 1-10mm and including fines. For the above reasons it was decided to undertake 2D physical modelling of this material, on a large scale, to represent as much of the original grading and minimize model scale effects. Various construction stages with submerged as well as emerged crest levels have been considered. Figure 1: Typical Wide Grade grading curve Physical model tests and results This section provides a summary of the physical model tests that were conducted at INHA (Barcelona) in the summer of 2016. The primary objective of the model tests was to measure the stability of the Wide Grade material under wave attack, for a range of conditions and construction stages. Some further tests were carried out simulating continuous placement of the wide grade as well as looking into the performance of wide grade as filter layer under an armour layer.