1. Why is a single-layer armour facing system preferable to a double-layer system?
The single-layer system consumes far less concrete than a double-layer system, significantly reducing construction times and costs while at the same time enabling the construction of more reliable and more robust structures.
One of the most significant risks in designing a structure is armour unit motion initiated by wave action. Typically, this motion will cause an armour unit to become displaced, potentially resulting in damage during storms. In the case of large units, motion or rocking can cause stresses that lead to cracking in the unit. A cracked block is then likely to break, and the broken piece can act as a projectile that in turn breaks other units, leading to gradual deterioration. This type of deterioration has been documented in double-layer concrete armour layers, and is typical form of deterioration. Paradoxically, the “reserve stability” often referred to in double-layer systems (or rockfill systems) does not always exist, especially in larger double-layer systems. By contrast, single-layer systems keep this advantage on account of the fact that each unit’s freedom to move reduces over time, provided that it was initially placed according to CLI’s standards.
The main advantage of single-layer armour facings is the substantial economic savings achieved; this allows the units to be sized more conservatively, reducing the risk of units moving and thus becoming damaged. Units are always sized such that extraction is not acceptable at the design stage. Furthermore, placing the units in a single layer on a controlled grid provides a high degree of keying for every unit, which is not the case with a double-layer system, where a unit may be less securely keyed, leading to a risk of increasing motion and breakage.
To conclude, a properly placed single-layer system provides greater protection against wave-induced settlement or motion, at less cost to the Owner.
Overall, the single-layer system remains much more cost-effective than any equivalent double-layer system, even through the crest elevation or width may need to be adjusted.
Typical savings of armour concrete:
40% less than with Tetrapod units.
50% less than with Antifer cubes.
On top of these savings, little maintenance is required on a single-layer armour facing that has been properly designed and built.
2. What are the stability coefficients used at the design stage?
The recommended design Kd values, obtained through extensive physical modelling, are shown in our brochures.
Note that these Kd values are generally cautious, and may be improved through physical scale modelling studies. A conservative approach is recommended for the reasons outlined in question 1 above. Physical scale model tests have demonstrated high Kd values before damage begins to occur. However, as a practical guideline, the values stated above are recommended for the preliminary design and for simple project cases.
Alternatively, the Hs/ΔDn values defined by J. Van Der Meer can be used as a basis for the preliminary design.
These values must be reduced if the seabed slope in front of the structure is greater than 1%.
Physical modelling is usually advised for breakwater projects that are particularly exposed to wave action or where overtopping is a decisive factor, on slopes with a large number of armour rows and when the seabed slope exceeds 1%.
3. Are model units available?
Yes, model units can be made available to laboratories to perform stability testing on specific projects. The model units are rented from CLI at a price of €2 per kg per week.
The model units required can be sent by air freight or road depending on their availability and their storage location. The outward shipping costs are borne by CLI, and the return shipping costs are borne by the laboratory.
If the required model units are not available, new model units can be fabricated on request at a cost depending on the time allowed and our workload at the time of ordering.
This rental fee does not include the visit by a CLI specialist to provide technical assistance for initial placing in the laboratory where the tests are to be performed. A standard rental agreement can be obtained from: firstname.lastname@example.org.
4. Don’t single-layer armours usually yield a higher overtopping discharge?
They do yield slightly more backwash and overtopping than more absorbent double-layer facings. However, thanks to its higher percentage of air voids, a single-layer armour has a greater energy dissipation capability than any other armouring system.
Overtopping coefficients were given in the paper by Pearson following the CLASH tests performed in Edinburgh.
5. What is the best armour slope?
3H/2V or 4H/3V armour slopes are optimum, and both are suitable for single-layer facings.
Adopting a milder armour slope will not necessarily yield a greater hydraulic stability. The slope must be selected on the basis of construction considerations as well as other site-specific factors such as wave exposure during the construction works.
6. What about the robustness of armour units?
A balance must be struck between the slenderness (which optimises keying and thus reduces freedom to move) and the robustness of the unit. Our armour system shapes offer an optimum combination of these factors. Alternative, more stocky units may be more robust in some applications; however, the loss of keying allows the units to move more easily, and that can induce extremely high dynamic forces as a result of unit-to-unit contact. Other units claim to have the same Kd coefficients as ours; however, their unit-to-unit motion has not been studied specifically. We urge designers to pay special attention to the selection of Kd coefficients, because not all testing objectives are equal.
The breakage rate during handling is extremely low. So far, very limited breakage of units in service has been reported, even following design storm conditions.
Field observations have led us to fine-tune our concrete strength requirements. For large units, the minimum tensile strength has been increased. Please refer to the values indicated in our brochures.
No steel reinforcement is required in any of our armour units.
7. I would like to compare CLI’s solutions with other armouring solutions. How can I do that?
At the design stage, designers can either: design their own armour solution and submit it to CLI for comments, or ask CLI to provide the outlines of a solution.
At the tender stage: contractors are welcome to send the relevant project data to CLI.
Within two weeks, CLI will make confidential recommendations, along with the financial conditions for sublicensing and technical assistance.
In any case, CLI will not charge any fees before the sublicensing agreement is signed, with the exception of rental fees for model units in the event that physical modelling is carried out.
8. Can moulds be fabricated anywhere?
For new moulds, preferred mould manufacturers are recommended. A list of possible formwork manufacturers can be provided at the tender stage.
Other qualified mould manufacturers can be appointed but will require more substantial assistance from CLI in the very early stages at the start of construction.
In all cases, CLI supplies the Contractors with formwork drawings after the sublicensing agreement comes into effect.
9. Does it take more time to obtain the right packing density?
With the help of GPS and assistance from divers whenever possible, placing requires as much attention as any other interlocking-based armouring system. The placement drawings provided by CLI are inserted into the Contractor’s positioning system. Each unit is generally placed in its theoretical position within the stated tolerance requirements. Once the staff have been trained, the units can be placed at a good rate. Estimated placing rates are given in our brochures.
Actual packing densities have been recorded within a range of 98% to 105%.
Nowadays there are electronic and acoustic placement systems providing a means of visualising the underwater placement of CLI units in real time. The POSIBLOC™ is one such tool, and can be used to install CLI units.
10. What is CLI’s fee?
CLI’s fee includes the rights to use the techniques, and technical assistance during construction. It varies mainly depending on the country, the volume of armour concrete, and the technical assistance to be provided.
For small-scale projects the minimum fee is a lump sum including at least two site visits by CLI.
The price includes the costs of R&D, which is conducted on an ongoing basis in order to make further improvements to the technique.
11. Do single-layer armours require any particular maintenance during the lifetime of the structure?
No maintenance is required, provided that:
- the design wave height is correctly estimated,
- CLI’s design standards are followed,
- the concrete characteristics are respected,
- the filter profile is within the tolerances before the armour facing is placed,
- and the minimum packing density is achieved.
The packing density of single-layer armours of this type has been observed to increase slightly over the first few years in service as the units find their optimum position, reducing their freedom to move in subsequent years.