Xbloc

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Large Xblocs (8.0 m or 280 cu ft) on a trial placement area Large Xbloc on trial placement area.JPG
Large Xblocs (8.0 m or 280 cu ft) on a trial placement area

An Xbloc is a wave-dissipating concrete block (or "armour unit") designed to protect shores, harbour walls, seawalls, breakwaters and other coastal structures from the direct impact of incoming waves. The Xbloc model was designed and developed in 2001 by the Dutch firm Delta Marine Consultants, now called BAM Infraconsult, a subsidiary of the Royal BAM Group. Xbloc has been subjected to extensive research by several universities. [1]

Contents

Benefits vs other systems

Concrete armour units are generally applied in breakwaters and shore protections. The units are placed in a single layer as the outer layer of the coastal structure. This layer is called the armour layer. Its function is twofold: (1) to protect the finer material below it against severe wave action; (2) to dissipate the wave energy to reduce the wave run-up, overtopping and reflection. These functions require a heavy, but porous armour.

Common factors to apply single layer concrete armour units are:

Also compared to older concrete armour units, as e.g. tetrapod which are normally placed in double layer as for rock protection, modern single layer armour units (like the Xbloc and Accropode), involve significantly less concrete. Therefore, less construction material (cement, gravel) is required, [2] reducing costs and also the carbon footprint of coastal protection works. [3]

Like Xbloc, most of these blocks are commercial developments and patented [4] [5] as such, Xblocs are not produced by the patent holder, but are fabricated and installed by a contractor who in return pays a license fee. Such an agreement involves certain technical support activities to ensure the correct application of the protection system. The patent expires in 2023, but although after that date anyone can make a block with this shape, one is not allowed to call it Xbloc, because the name is a protected trademark.

Hydraulic stability and interlocking mechanism

The Xbloc armour unit derives its hydraulic stability from its self-weight and by interlocking with surrounding units. Due to the highly porous armour layer (layer porosity of almost 60%) constructed with Xbloc units, the energy of the incoming waves will be largely absorbed. The Xbloc armour layer is therefore able to protect the rock in the under layer from erosion due to waves. Besides empirical formulae derived from physical model testing, the interaction between breakwater elements (submerged or emerged) and waves as well as the filtration of the fluid into the porous breakwater has been investigated amongst others by MEDUS, based on RANS equations coupled with a RNG turbulence model. [6]

Xblocs are typically applied on an armour slope steepness between 3V:4H and 2V:3H. Unlike natural rock, the hydraulic stability does not increase at shallower slope inclinations, because, in that situation, the interlocking effect is reduced. Standard Xbloc sizes vary between 0.75m3 (significant wave height up to Hs = 3.35m) and 20m3 (Hs = 10.0m). It is noted that the given relation between design wave height and volume size is valid for the concept stage only. Further parameters as foreshore slope, crest configuration, construction equipment, etc. can have an important effect on the recommended unit size. [7] For detailed design, in particular for non standard situations, physical model tests are essential and normally carried out to confirm overall stability and functional performance of a breakwater (wave overtopping and/ or wave penetration).

The effect of interlocking is apparent when comparing a rock revetment with a modern single layer unit for average boundary conditions, while taking into account the lower specific density of concrete compared to most natural rock commonly used in breakwater construction. Assuming that natural rock would be placed at identical slope steepness, the individual rock weight would require to be three times as high, compared to Xbloc units. Rock is generally to be placed as double layer, thus the volume of armour material which needs to be quarried, stored, handled, transported and installed can be enormous for a larger breakwater exposed to significant wave action. Due to the interlocking effect the weight, and thus the volume, of single layer armour units is considerably less compared to an armour consisting entirely of rock. In addition, units are normally fabricated near or at project site, so that transport issues are less critical.

Production of armour units

The Xbloc consists of non-reinforced concrete, similar to other single layer armour units. Ordinary concrete C25/30 is normally appropriate for the production of Xbloc armour units. However, often concrete of higher strength is applied for other reasons, e.g. early strength for faster de-moulding, ice loads, etc. By omitting reinforcement, time and costs are cut and the armour units are less vulnerable to long term corrosion damage. The optimal shape of a single layer armour unit combines the robustness of a compact concrete body with the slenderness required for interlocking. The structural integrity is normally confirmed by finite element calculations (FEM) and prototype drop tests.

Although both wooden and steel moulds can be used to construct the Xbloc formwork, steel moulds are preferred as they can be used repeatedly to produce large numbers of armour units. Various mould designs, consisting of 2 sections, are used. The moulds are either vertically or horizontally assembled. Pouring and compaction of concrete is done simultaneously. An appropriate formwork design is facilitating the stripping of the moulds at an early stage and largely prevents honey combing, surface bubbles and striking damage.

Due to the shape of the Xbloc unit, relatively simple formwork can be used which is made of a limited number of different steel plates. Since a single Xbloc unit can weigh up to 45 tons, the construction is done as close as possible to the area of application. [8]

Placement

In contrast to the placement of other interlocking concrete blocks, the Xbloc unit does not require stringent specifications about the orientation of each unit on a breakwater slope. Because of the shape of the Xbloc, each of the 6 sides of the unit is efficiently interlocking. Hence, the blocks easily find a position that fully utilizes the interlocking mechanism. This increases the efficiency of placing armour units on a slope. [9] Due to the random structure and high porosity of an Xbloc breakwater, an artificial reef habitat is created for marine fauna and flora.

XblocPlus

DMC came to the market in 2018 with the XblocPlus. This is not merely an improved version of the Xbloc, rather it is a block that functions differently, and has its own advantages and disadvantages. The XblocPlus needs to be placed regularly and, has characteristics found in placed blocks such as natural basaltic columns or concrete placed blocks like Basalton. [10] DMC saw opportunities for this block in the Afsluitdijk improvement that began in 2018. Here this block is used in the wave impact zone. The block in this usage is called the ‘Levvel-block’, after the joint-venture that improves the Afsluitdijk. The Basalton Quattroblok is placed in the wave run-up zone on the Afsluitdijk. The XblocPlus is also used in the Vistula Spit canal in Poland. [11]

See also

Related Research Articles

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The Afsluitdijk is a major dam and causeway in the Netherlands. It was constructed between 1927 and 1932 and runs from Den Oever in North Holland province to the village of Zurich in Friesland province, over a length of 32 kilometres (20 mi) and a width of 90 metres (300 ft), at an initial height above Amsterdam Ordnance Datum of between 6.7 metres (22 ft) along the section at Friesland, and 7.4 metres (24 ft) where it crosses the deep channel of the Vlieter. The height at the greater sea depths west of Friesland was required to be a minimum of 7 metres everywhere when originally constructed.

<span class="mw-page-title-main">Groyne</span> Structure extending into a body of water to alter water flow

A groyne is a rigid hydraulic structure built perpendicularly from an ocean shore or a river bank, interrupting water flow and limiting the movement of sediment. It is usually made out of wood, concrete, or stone. In the ocean, groynes create beaches, prevent beach erosion caused by longshore drift where this is the dominant process and facilitate beach nourishment. There is also often cross-shore movement which if longer than the groyne will limit its effectiveness. In a river, groynes slow down the process of erosion and prevent ice-jamming, which in turn aids navigation.

<span class="mw-page-title-main">Breakwater (structure)</span> Coastal defense structure

A breakwater is a permanent structure constructed at a coastal area to protect against tides, currents, waves, and storm surges. Breakwaters have been built since Antiquity to protect anchorages, helping isolate vessels from marine hazards such as wind-driven waves. A breakwater, also known in some contexts as a jetty or a Mole, may be connected to land or freestanding, and may contain a walkway or road for vehicle access.

<span class="mw-page-title-main">Pavers (flooring)</span> Stone or tile structure which can serve as floor; pavement type with solid blocks

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<span class="mw-page-title-main">Riprap</span> Rock or concrete protective armour

Riprap, also known as rip rap, rip-rap, shot rock, rock armour or rubble, is human-placed rock or other material used to protect shoreline structures against scour and water, wave, or ice erosion. Riprap is used to armor shorelines, streambeds, bridge abutments, foundational infrastructure supports and other shoreline structures against erosion. Common rock types used include granite and modular concrete blocks. Rubble from building and paving demolition is sometimes used, as well as specifically designed structures called tetrapods or similar concrete blocks. Riprap is also used underwater to cap immersed tubes sunken on the seabed to be joined into an undersea tunnel.

<span class="mw-page-title-main">Formwork</span> Molds for cast

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<span class="mw-page-title-main">Tetrapod (structure)</span> Concrete breakwater element

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<span class="mw-page-title-main">Iribarren number</span> Dimensionless parameter

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<span class="mw-page-title-main">Ramón Iribarren</span> Spanish civil engineer

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References

  1. https://repository.tudelft.nl/islandora/search/Xbloc Various reports and papers by TUDelft
  2. http://www.vandermeerconsulting.nl/downloads/stability_c/1999_vandermeer.pdf Van der Meer, J.W. - Design of concrete armour layers (table 1, page 8)
  3. http://resolver.tudelft.nl/uuid:861a421f-3fc7-423e-ba49-fe15d759b482 Labrujere, A.L. Analysis of the Carbon Footprint of Coastal Protection Systems
  4. Xbloc.com (pdf)
  5. https://worldwide.espacenet.com/patent/search/family/030768268/publication/EP1540087A2?q=03765408.4 European patent
  6. MEDUS
  7. Guidelines
  8. https://journals.tdl.org/icce/index.php/icce/article/download/7729/pdf_873/ Richard de Rover, Bas Reedijk and Pieter Bakker - XBLOC INNOVATIONS AT SWINOUJSCIE BREAKWATER, ICCE 2014
  9. http://resolver.tudelft.nl/uuid:73c70362-8e00-42d9-8988-2457a199c866 Theoretical and Experimental study on the placement of Xbloc
  10. http://resolver.tudelft.nl/uuid:fbe797ef-5944-4bc1-9d25-e7448dce3d1b Belen Rada Mora Hydraulic Performance of Xbloc+ Armor Unit
  11. Sikes, Bryanna (21 September 2022). "BESIX in Poland: Official opening of the Vistula Spit shipping channel". Civil + Structural Engineer magazine.