1. Why is a single layer better than a twin layer armouring system?

The single layer system consumes far less concrete than a double layer system, significantly reducing construction time and cost. This economic benefit is used to build a more reliable and robust structure.

Consider that one of the most significant risks in structure design is armour unit motion initiated by wave action. Conventionally, this motion can cause displacement of an armour unit, which may lead to a failure. However, in larger size concrete units, motion or rocking of the unit, prior to displacement, may cause stresses that induce cracking in the unit. Cracking in turn leads to a smaller broken piece that can act as a projectile or a dynamic “hammer” that breaks other units, leading to a progressive failure. This type of failure has been documented in double layer concrete armour layers, and is a far more important mode of failure than simple “displacement” of a unit. Ironically, the “reserve stability” often thought of in double layer systems (such as for stone) may not really exist in concrete units, especially for twin layer systems as their size increases. On the opposite, single layers keep that advantage due to the fact that freedom of movement of each unit reduces with time, provided that their installation is performed according to CLI standards.

The main advantage of single layer armours is the great economic savings of a single layer allows the units to be sized more conservatively, reducing the chance of unit rocking, and thus unit breakage. Unit sizing is always such that unit removal is not an acceptable design basis. Further, the placement of the units in a single layer on a controlled grid provides a high degree of interlocking for every unit – unlike a double layer system where a unit may be less securely interlocked – leaving the chance for increasing motion and breakage.

In the end, the properly placed single layer system provides a greater protection against wave displacement or motion, at less cost to the Owner.

Overall, the single layer remains much more cost-effective than any equivalent twin layer, even if crest height may have to be adjusted. Typical savings of armour concrete: .

  • 40% less than Tetrapods . 
  • 50% less than Antifer cubes

On top of these savings, it must be stated that no maintenance is expected when single layers are properly designed and built.

2. Stability values at design stage?

Recommended design Kd values deducted from extensive physical modelling are shown in our brochures.

Note that these Kd values are generally conservative, and may be improved with site specific model studies. A conservative approach is recommended because of the discussion outlined in question 1 above. However, as a practical guideline, the values stated above are recommended for preliminary design as far as simple project cases are concerned.

Alternatively, Hs/∆Dn values by J. Van Der Meer can be followed for design purpose.

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 particularly exposed to wave action, or where overtopping is a governing factor whenever the number of rows in the slope is excessive and when seabed slope exceeds 1%.

It is recommended not to exceed 20 rows of units along the armour slope.

3. Are model units available?

A large inventory of model units is made available for rental to laboratories for stability testing of specific projects. The model units are rented from CLI on first-come-first-served basis at 2 € per kg per week.

The required number of model units can be sent out by air freight or road depending on their location and availability. Expedition costs are borne by CLI, and the return shipping costs are borne by the hirer.

In the case the required model units sizes are not available, new model units can be fabricated at cost depending on the time allowed and our work load.

This rental fee does not include the technical assistance and training for initial placement in the laboratory by a CLI specialist. A standard rental agreement can be obtained from cli@concretelayer.com.

4. Don’t single layer armours usually yield greater overtopping discharge?

They do yield slightly more runup and overtopping than more absorbing twin layers. However, with higher percentage air voids, a single layer armour has a greater energy dissipation capability than any other armouring system.

Overtopping coefficients have been given by in the paper by Pearson following et al. (2005) the CLASH tests performed in Edinburgh.

5. Which armour slope is best?

Both 3H/2V or 4H/3V armour slopes are optimum and suitable for the single layer.

Adopting a milder armour slope however will not necessarily yield a greater hydraulic stability (contrary to conventional wisdom).

The slope should be selected based on construction considerations as well as other site factors such as wave exposure during construction and subsurface conditions

6. How about robustness of armour units?

There is a balance between the slenderness of the unit (which optimises interlock and thus reduces motion) and the strength of the unit.

Our armour systems have proven to be an optimum combination of these factors. Alternative, more blocky units, may have greater strength in some applications, however the loss of interlock allows the units to move more easily and thus induce extremely high dynamic stresses as a result of unit to unit contact.

Claims have been made by other units to have the same Kd factor as ours, however unit to unit motion is not documented. We urge designers to consider Kd claims very carefully – not all testing objectives are equal.

The breakage rate in handling is extremely low. So far, very limited breakage of units in service have been reported, even following design storm conditions.

Field observations led us to refine concrete strength requirements. For larger units, the min. tensile strength has been increased. Refer to values indicated in our brochures.

No steel reinforcement is required in any of our armour units.

7. I would like to compare CLI solutions with other armouring solutions. How to proceed?

At design stage:

Designers can either:

  • make their own armour solution and submit it to CLI for comments, or 
  • request CLI to provide an outline of a solution. 

At tender stage:

Contractors are welcome to send the relevant project data to CLI. Within two weeks, CLI will confidentially make recommendations, along with sublicense financial and technical assistance terms.

In any cases, CLI will not charge any fee before sublicensing, except rental fee of model units when physical modelling is carried out.

8. Can moulds be made anywhere?

For new moulds, preferred mould manufacturers are recommended. A list of possible formwork manufacturers can be given at tender stage.

Other qualified mould manufacturers are possible but will require more assistance by CLI in the early stage of construction.

In all cases, formwork drawings are supplied by CLI to the Contractors after signing of the Sublicense Contract.

9. Does it take more time to obtain the right packing density?

With the help of GPS and assistance of divers whenever possible, placement requires as much attention as any other interlocking armouring system. Positioning drawings provided by CLI are inserted into the Contractor’s positioning system. Each unit is generally placed within the tolerance requirement of their theoretical position. Once the learning process is understood, the units can be placed at a good rate. Estimated placement output rates are given in our brochures.

Actual packing densities have been recorded within the specified range: 98% to 105%.

Nowadays, electronic and acoustic placement systems exist, allowing to visualize in real time the underwater placement of CLI units. The POSIBLOC™ is one of these tools that can be exclusively used to install CLI units.

10. What is CLI fee?

CLI fee includes the rights and technical assistance during construction. It basically varies in function of the country, the volume of armour concrete, and the technical assistance to be provided.

For projects with small volumes the minimum fee is a lump sum, allowing for at least two site visits by CLI.

Fees generated by projects are used for providing top quality services, quality control assistance as well as on-going project specific research. The Licensors US Army Corps of Engineers and SOGREAH continue to invest in research for improving and updating our single-layer techniques and developing efficient tools, based on the license fees they each receive.

11. Is any maintenance of single layer armours required over time?

No maintenance is expected, provided that the :

  • design wave height is correctly estimated 
  • CLI design standards are followed, 
  • concrete characteristics are respected, 
  • filter profiles are within tolerances before placement of the primary armour, 
  • minimum packing density is achieved. 

As storms pass, single layers of this interlocking type have been observed to slightly increase their packing as units eventually find their optimum position against each other, which diminishes the risk of rocking with time. Ultimate breakage becomes therefore less likely as time goes.