The ACCROPODE™ II unit
Developed in France by Sogreah in 1981 based on the experience gained with the Tetrapod since 1953, the ACCROPODE™ technique has become the benchmark for single-layer armour systems to protect rubble-mound breakwaters. The experience acquired on a substantial number of projects led to the invention of a new-generation armour unit. This new unit retains the legendary attributes of its predecessor, the first-generation ACCROPODE™ unit, with a modified shape making more efficient use of concrete. The ACCROPODE™ II unit, which was launched on the market in 1999, thus improves the original concept by optimising its use. This new design has also considerably simplified the placing methods.
As is the case with all CLI armour units, it has a high stability coefficient derived from its shape as well as from the associated placing techniques. It can be used on structures of all types from coastal defences in shallow water to large breakwaters designed to protect ports or nuclear power plants. Last but not least, this is a technology with minimal maintenance costs since these structures are normally intended to withstand the design wave without sustaining damage.
Practical aspects of ACCROPODE™ II units
1. A simple formwork principle based on:
- Two steel symmetrical half-shells that are assembled for easy concrete casting through the top.
- Simple fabrication, avoiding the need for leading-edge technologies.
- A proven shape ensuring seamless stripping.
2. Concrete strength specifications that are simple and open to the continuous improvements being made to this technology
The concrete used for ACCROPODE™ II units is a simple mass concrete that is by its very essence less costly than the high-performance concretes. The specifications for producing this concrete are brief, to leave the project players free to choose the concrete characteristics best suited to their project. As stated in the specifications, CLI has confirmed on the basis of experience that the following characteristic strength values give a safety factor that is sufficient for the construction phases and during the working life of the structure:
& ACCROPODE™ II
|Units ≤ 4 m³||5 m³≤ units ≤ 15 m³||Units > 15 m³|
|Concrete class required||C25/30||C30/35||C30/35|
|Tensile strength||2.5 MPa||3.0 MPa||3.0 MPa|
for form stripping Fck Cyl
|6 MPa||7 MPa||10 MPa|
for handling Fck Cyl
|15 MPa||20 MPa||25 MPa|
for placing Fck Cyl
|25 MPa||30 MPa||30 MPa|
3. Unit fabrication
Fabrication of ACCROPODE™ II units is a simple process calling on methods that are either highly rudimentary or, conversely, highly sophisticated. With the traditional fabrication method, the following parameters are adopted:
- Minimum area required to fabricate one unit: 1.55H² (where H = ACCROPODE™ II unit height).
- Fabrication of one unit per day and per mould. Depending on the methods used, this rate can be doubled to two units per day.
4. Storage and handling
- Forklifts can be used to transport the units. For some very large units, handling equipment such as cranes and slings will be required.
- Units can be stored on one or two levels depending on their size, on levelled ground with a sufficient bearing capacity.
- Minimum area required to store 10 units on one level: 7.1H² (where H = ACCROPODE™ II unit height).
5. Unit placing
The ACCROPODE™ II unit benefits from the experience gained with the first generation of units, and the most significant improvements made are with regard to placing techniques. Indeed, the placing rules have been simplified and reduced to enable simpler and quicker implementation. As is the case with the first-generation ACCROPODE™ units, each ACCROPODE™ II unit must be placed on a suitable underlayer.
A placing grid defines the theoretical target positions in order to place the units easily; according to the feedback we have received, this ensures that the packing density required by the technique is obtained. Today, DGPS / GPS RTK-type positioning systems are systematically used. Unit keying is achieved easily thanks to the random orientation of the units.
The unit placing speed is determined by the resources used, which must take the weather conditions at the site into account. The use of a hydraulic shovel and a special grab can double the standard output of a cable crane.
Placement rate (using cable cranes) – monthly average
|Unit size||Average placing time per unit|
|Small||4 to 6 min|
|Medium||7 to 10 min|
|Large||10 to 20 min|
Higher rates can be obtained using a “custom” method.
Using the ACCROPODE™ II technology in association with technical assistance geared to the construction site makes it possible to:
Control and reduce the cost of building the facing, by using a well-proven technology.
Reduce environmental impacts by limiting the footprint of the structure.
Control and reduce the volumes of materials required, especially concrete.
Minimise risks and construction times, enabling the structure to be put into service more quickly.
Significantly reduce the costs of inspecting and maintaining the structure throughout its working life.