Dolos
From Wikipedia, the free encyclopedia
A dolos (plural dolosse) is an unusually-shaped concrete block weighing up to 20 tons which is used in great numbers to protect harbour walls from the force of the sea. They were developed in East London, a port city in South Africa, in 1963 and are found in their millions around the world.
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[edit] Construction
Dolosse are normally made from un-reinforced concrete, poured into a steel mould. The concrete will sometimes be mixed with small steel fibers, to strengthen it in the absence of reinforcing. Construction is done as close as possible to the area of application as a result of their great weight.
[edit] Use
They are used to protect harbour walls, breakwaters, and shore earthworks. They are also used to bind sea-sand to prevent erosion. In the order of 10 000 dolosse are required for a kilometre of coastline.
They work by dissipating, rather than blocking, the energy of waves. Their design presents very little surface area to the wave action, making them difficult to dislodge, unlike rectangular blocks that can be swept away by heavy seas. 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 about. Their design ensures that they form an interlocking, but porous, wall.
They are often numbered on the outside so that satellites can periodically track their movement. This helps engineers gauge whether they need to add more dolosse to the pile.
[edit] Credit for their invention
Their design is usually credited to the South African Eric Mowbray Merrifield, one-time East London Harbour Engineer (from 1961 – 1976).
This went more or less unquestioned until the late 1990s, when the claim of Mr Aubrey Kruger gained more prominence.
Kruger's claim is that he and Merrifield had been playing with the idea of 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. He said in later years that he wanted a block designed in such a way that it could be "sprinkled like children's jacks". Kruger has stated that he went home for lunch on a certain day and cut three sections off the end of his wife's broomstick and knocked them together with nails into an H-shape with one leg turned through 90 degrees to create the distinctive dolos shape. Returning from his lunch Merrifield was intrigued by the object and asked Kruger to draw a plan, which he did.
Kruger has never formally received credit for the invention of the dolos. Merrifield went on to win the prestigious Shell Design Award and the Associated Science and Technology Societies of South Africa's Gold Medal.
The death of Merrifield (in 1982) has put this controversy beyond proof either way.
[edit] Origin of shape
What induced either man to choose such a shape is not known, though there is a legend that it was inspired by the difficulty the East London harbour authorities had pulling apart a large pile of similarly-shaped anchors for salvage.
[edit] Design protection
Unusually, their design is not subject to design protection, Merrifield not having taken the necessary steps to protect them.
The reason for this is also uncertain as two reasons for this failure have been put forward: one by Merrifield; the other by Kruger. Merrifield stated that he did not protect them as he wished them to benefit humanity. Kruger alleges that Merrifield received incorrect legal advice that as they were designed in office hours while employed by the state he was unable to protect their design.
[edit] Origin of name
Their name is derived from the Afrikaans word dolosse. This word has two given derivations. Rosenthal states it to be a contraction of 'dobbel osse', or 'gambling' (Afrikaans) 'bones' (Latin). Boshof and Nienaber state it to be a contraction of 'dollen os', or 'play' (old Dutch) 'oxen' (Afrikaans). The first is a meaning-shifted reference to ox knuckle-joint bones used in divination practices by sangomas, Southern African traditional healers. They somewhat resemble these bones. The second is a reference to ox or lamb knuckle bones used by African children at play. The name was attached to the objects when Kruger's father, Joe Kruger, who also worked in the harbour, came upon his son and others playing with small models of the objects and asked him Wat speel julle met die dolos? (English: What are you playing at with the dolos?).
[edit] Similar objects
Concrete blocks used for the same purpose come in various shapes:
- pierced-pyramid (four equilateral triangles, joined edge to edge)
- A-jack
- Akmon
- Tetrapod (structure), sometimes called concrete jacks (named for the children's toy, but shaped like caltrops)
[edit] References
- Eastern Cape Dispatch, 28 June 1999
- Martin Creamer's Engineering News, 16 February 2001
- South African Financial Mail, 3 December 2004
- John McPhee, Giving Good Weight, "The Atlantic Generating Station", Macfarlane Walter and Ross, 1975, ISBN 0-374-51600-6
- Encyclopaedia of Southern Africa, Eric Rosenthal, Frederick Warne and Co. Ltd, London and New York, 1961, definition of 'dolos' (published prior to the invention of the subject of this article)
