Accropode

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Accropode blocks are man-made unreinforced concrete objects designed to resist the action of waves on breakwaters and coastal structures.

History

Accropode (1981)

The Accropode is a single-layer artificial armour unit developed by Sogreah in 1981. Accropode concrete armour units are applied in a single layer. It is the most widely used single layer interlocking concrete block in coastal structures.

Sogreah's predecessor, the Laboratoire Dauphinois d'Hydraulique, was the inventor of the Tetrapod block in 1950-53. This block was the first interlocking monolithic concrete block used for breakwaters. It enabled structures to be protected with much larger blocks than before, when only natural rock was used (with a unit weight of up to around 10 tons) or concrete cubes. The Tetrapode was used all over the world, especially in Japan. The Tetrapod was used in a double layer with a unit weight of up to around 50 tons.

Ecopode (1996)

The Ecopode armour unit with a rock-like appearance was developed by Sogreah to enhance the natural appearance of concrete armourings above low water level. A patent application was filed in 1996. The color and type of rock-like appearance can be specified to match the surrounding landscape.

Accropode II (2004)

In 1999, Sogreah modified the original Accropode shape by chipping away excess materials and adding friction features in the form of small pyramids. A patent application was filed for this modified shape. In 2004 further modifications to the 1999 shape were made, resulting in the Accropode II. The shape modifications are intended to increase interlocking.

According to the developers, the modified shape results in factors comprising minimized rocking and settlement, maximized energy dissipation, reduced wave reflection and run-up/overtopping, and improved structural strength.

Widely used, the Accropode technology has been applied on over 150 breakwater and seawall projects worldwide.

Design

Hydraulic stability

Specified stability coefficients at design stage:

• Hudson’s design K_D values:
  • 15 on trunk sections (16 for Accropode II)
  • 11.5 on roundheads (12.3 for Accropode II)

• Van der Meer stability number:

N_S = H_S/(∆ D_n50)= 2.7 (2.8 for Accropode II)

where:

H_S = significant wave height

∆ = relative mass density

D_n50 = nominal diameter

These coefficients are valid for armour slopes from 3H/2V to 4H/3V and for seabed slopes up to 3%.

The uneven surface of the Ecopode improves interlocking by friction, thereby increasing hydraulic stability.

Advanced Numerical Study

3D Numerical Simulation - MEDUS 2009

The Maritime Engineering Division University Salerno (MEDUS) developed a new procedure to study, with a more detailed and innovative approach, the interactions between maritime breakwaters (submerged or emerged) and the waves, by an integrated use of CAD and CFD software.

In the numerical simulations the filtration motion of the fluid within the interstices, which normally exist in a breakwater, is estimated by integrating the RANS equations, coupled with a RNG turbulence model, inside the voids, not using a classical equations for porous media.

The breakwaters were modelled, as it happens in the full size construction or in physical laboratory test, by overlapping three-dimensional elements and the numerical grid was thickened in such a way to have some computational nodes along the flow paths among the breakwater’s blocks (AccropodeTM, Core-locTM, Xbloc).

Implementation

Large Accropode units are lowered into a position offshore by aid of a crane.

Fork-lifting is effective for handling the small to medium size units, whereas large units are handled by sling. Placement for breakwaters generally requires a crane or a barge-mounted crane.

The units can be stored one on top of the other, and placed in a random attitude to obtain the specified packing density. The proper packing method provides an adequate coverage on breakwater slopes.

The use of a remote-release hook is used for placing the unit, while underwater placements may be enhanced by GPS, adhering to a theoretical grid, and allowing for reliable QA/QC procedures.

Diver inspecting blocks

See also

• Breakwater (structure)
• Erosion control
• Riprap
• Artificial reef
• Coastal erosion
• Ocean surface wave
• Seawall

References

• Ciria-CUR (2007) - Rock Manual - The use of rock in hydraulic engineering.[1]
• K. d'Angremond (2004) - Breakwaters and closure dams.
• N.W.H. Allsop (2002) - Breakwaters, coastal structures and coastlines.
• J.W. Van der Meer (1988) - Rock slopes and gravel beaches under wave attack.
• Delft Hydraulics Laboratory (1987) - Stability of rubble mound breakwaters - Stability formula for breakwaters armoured with Accropode (report H 546).
• U.S. Army Corps of Engineer Waterways Experiment Station (WES) - Shore Protection Manual (1984) - Hudson formula based on Hudson's extensive work in the fifties.

Gallery

• 

  Accropode blocks on offshore breakwater roundhead.

• 

  Storage of Accropode II blocks.

• 

  Accropode II on forklift.

• 

  Accropode on truck.

• 

  Placing Accropode blocks from land.

• 

  Accropode blocks on breakwater.

• 

  Placing with a crane

• 

  Monitoring

• 

  Inderunder water inspection

External links

• Concrete Layer Innovations
• MEDUS (Maritime Engineering Division University Salerno)
• Specialized Company for ACCROPODE(TM) and one layer armouring breakwaters IDMER.NET

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