An innovation developed in 1963 in South Africa and first deployed in 1964 at a breakwater has proven capable of repeatedly withstanding forces imposed by severe ocean waves. All along the South African coast you find dolosse wherever coastal management is necessary to prevent tides and rough seas from eroding breakwaters, harbour walls, coastal construction.
Dolosse construction yard - Yzerfontein, South Africa
The dolos (plural: dolosse) is a wave-dissipating concrete block used in great numbers as a form of coastal management. It is a type of tetrapod. They are used to protect harbour walls, breakwaters and shore earthworks. They are also used to trap sea-sand to prevent erosion.
How Dolosse Work
Dolosse work by dissipating, rather than blocking, the energy of waves. Their design deflects most wave action energy to the side, making them more difficult to dislodge than objects of a similar weight presenting a flat surface. Though they are placed into position on top of each other by cranes, over time they tend to get further entangled as the waves shift them. The individual units are often numbered so that their movements can be tracked.
Dolosse East London South Africa
The breakwater at the Port of East London, South Africa was the first to include an array of dolosse that could maintain structural integrity during severe wave conditions.
History and Design
The design of the dolos is usually credited to the South African Eric Mowbray Merrifield, one-time East London Harbour Engineer (from 1961-1976). Eric Merrifield lead a team which included Piet Grobbelaar and Jack Badham-Thornhill who collaborated for many years perfecting the design. These blocks were designed to 22 tonne in later years.
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In the late 1990s the claim of Aubrey Kruger gained more prominence. Kruger's claim is that he and Merrifield had considered the shape of concrete blocks to be used to protect East London's extensive breakwaters for the City's non-natural harbour, following a major storm in 1963. Merrifield wished to design a block that did not break up or shift when struck by the sea; that was cheap; and that did not require precise placement.
Kruger stated that he went home for lunch, cut three sections from a broomstick, and fastened them with nails into an H-shape with one leg turned through 90 degrees to create the distinctive dolos shape. Merrifield was intrigued by the object and had Kruger draw a plan. Kruger never formally received credit for the invention.
The dolos (plural dolosse) is a South African invention for which the inventors received no payment or recognition. They did not take out a patent either because they were employed by the South African Railway & Harbour Services at the time they designed it.
The Name "Dolos"
The name is derived from the Afrikaans word dolos (plural: dolosse). This word has two given derivations:
- Rosenthal (1961) states it to be a contraction of 'dobbel osse', or 'gambling' (Afrikaans) 'bones' (from Latin). The first is a meaning-shifted reference to knucklebones used in divination practices by sangomas, Southern African traditional healers.
- Boshof and Nienaber state it to be a contraction of 'dollen os', or 'play' (old Dutch) 'oxen' (Afrikaans). The second is a reference to the knucklebones used by African children to play.
Application Design
Dolosse surface protection need to lay two layers, because the upper and lower end of the bar is perpendicular to each other, when placed, one end should be parallel to the embankment slope, which placed above the embankment slope. The other end is perpendicular to the embankment slope, which placed below the embankment slope for increasing the anti-tipping moment of the block and is conducive to the stability of the protection layer.
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According to the relevant specifications, when the dolos is placed regularly, the vertical bar should be placed below the slope and pressed on the cross bar of the front block, which is placed on the bedding blocks, and the waist bar is straddled on the cross bar of the adjacent block.
Application design
In the precast construction of dolos, horizontal construction is superior to vertical one in equipment. The vertical construction efficiency is better than the horizontal one in the formwork disassembling and assembling process. Vertical wipe speed is higher than horizontal, but wipe quality control is more difficult than horizontal. Vertical prefabricated surface bubbles is slightly more than horizontal. Vertical precast construction efficiency is higher than horizontal precast. It is known that: vertical and horizontal prefabrication have applicability, which should be selected according to the site situation, resource allocation and construction period requirements.
Main Features
The purpose of the invention of the dolos is to take advantage of the characteristics of the slender bar block, which requires less concrete, large voidage and good interlocking. These advantages are more prominent where the water is shallower and the waves are smaller.
Dolos is a block composed of three slender bars, and its stability is mainly shown by their interlocking action. Therefore, the placement of the block largely determines the stability. There are two types of block placement: random placement and regular placement. The former is convenient in construction and fast in cycle, but the block interlocking effect is poor.
Application Scenario
Since its appearance, dolos has experienced more than 20 years of research and use, the basic characteristics have been obvious. Research methods, design process, construction and maintenance experience has gradually matured. Although dozens of new types of blocks have been proposed worldwide, dolos is still widely used and further studied and improved for its unique advantages.
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The biggest flaw of the dolos is that it is easy to break in the weak place when it wobbles and rolls under bad sea conditions. However, in the coastal area of China, due to the natural shielding effect of the dry vast continental shelf and coastal islands, the wave is relatively small, the wave height is generally less than 7m, and the stable weight of the dolos is basically within 10t, so its weakness is not very obvious. It has been applied in a large number of projects such as Qingdong Artificial Island of Shengli Oilfield, Majishan Port dike project of Baosteel, and the third phase of Qinhuangdao Port coal Wharf.
Dolosse in Rivers
Dolosse are also being used in rivers in the Pacific Northwest of the United States of America, to control erosion, prevent channel migration and to create and restore salmon habitat. Examples are engineered log jams, or ELJs, that may aid in efforts to save stocks of salmon.
While each dolos used to build an ocean breakwater can weigh up to 80-tons, much smaller dolosse would likely be sufficient along river tributaries where seasonal flooding of valleys could occur without causing any significant property damage in the immediate surrounding area.
Installing low-height dolosse dams at strategic locations along tributaries could encourage beavers to further build on such dams and prevent water from flowing downstream. At strategic upstream locations where beaver dams already exist, there may be scope to install an array of dolosse across the stream either upstream or downstream of the beaver dam, to reduce catastrophic flooding in the event of a breach of the beaver dam. The natural instinct of the animal would be to repair a breach of its own dam to stop water from flowing.
SANRAL Project in Port Elizabeth
The roads agency which has a responsibility to protect its asset against destructive wave action is manufacturing the dolosse which are used to protect the sea-facing N2 and the parallel railway line adjacent to the Settlers Interchange in Port Elizabeth from destructive waves. The dolosse are interlocking blocks of concrete and are manufactured at the Markman Casting Yard in Markman.
“Twenty people, five each from the four SMME subcontracting companies, received training. The four SMME subcontracting companies were selected for the 17 SMME packages which were identified and advertised as part the N2 Bramlin to Soutwerke (PE Bypass) special maintenance project,” said Thabiso Ngozwana, SANRAL Southern Region project manager.
The one-month training was held in July this year and included practical sessions on cleaning and assembling concrete formwork, basic concrete technology, concrete mixing, placing, finishing and testing. The training was provided by Meroe Skills Development, at the Heart Lights Training Centre in Walmer.
Portia Ngcwabe, who has a post graduate degree in management and is the owner of Phefong Construction, had five of her staff attend the concrete training programme. Nwabisa Mgubasi, a qualified architectural draftsman and co-owner of Sokhazimla Trading CC also attended the concrete training and said she now knows how to mix concrete properly. Nathi Lusizi who works for Magna Civils said the course was very informative.
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