Did you know that the most successful concrete structure to have protected coastlines from the mighty ocean force is a South African invention? The dolos, or dolosse in its plural form, is an oddly shaped concrete structure closely piled in its thousands to arm harbours, ports and shorelines from strong erosive ocean waves. The dolos was born out of necessity to better protect Port Elizabeth’s Port from South Africa’s rough eastern coastline. Today, this concrete structure is renowned for its long-lasting and effective ability to protect shorelines around the world from oceanic damage.
Concrete is a widely used building ingredient in the construction of resilient coastal engineering. The role of concrete mixers in producing the dolos and other protective coastal structures is essential in constructing concrete armour to safeguard the harbours, ports and coastal walkways in South Africa and across the world.
This article will discuss the invention of the globally used dolos and, more broadly, other engineering feats in protective coastal design. If you need to mould your concrete to fit your designs, BS Power imports and manufactures the best quality concrete mixers and have you covered.
The Dolos: A Tale Of Two Local Coastal Engineers
There is debate as to who gets to claim the title of inventor of the dolos which is narrowed down to two South Africans who once worked at the port of the country’s eastern coastline. Eric Merrifield, harbour engineer and his young draughtsman, Aubrey Kruger, worked at East London Port from the 1960s till the mid-seventies.
Merrifield served as chief engineer to East London Port and is often given the majority of the credit for the invention of the dolos. However, Kruger’s involvement should not be overlooked. The draughtsman, then still only 28, had a conversation with the harbour’s chief engineer that would set in motion the idea behind today’s most successful concrete device to provide safety against forceful ocean waves for harbours across the world.
Originally, the dolos as we know it today was once called “the Merrifield block”. However, to give fair credit to Kruger’s stake in the dolos, the draughtsman’s involvement was recognised and replaced by the initial name.
Neither Merrifield nor Kruger took out a patent on the design of the dolos. In the end, it was Eric Merrifield who was awarded for the concrete invention and received awards for its design from Shell and the Associated Science & Technology Society SA.
The Dolos And Protective Coastal Engineering
The invention of the dolos speaks of a larger need for coastal protection of harbours and ports against the incredible force of ocean waves. Up until the use of the concrete dolos, inadequate engineering techniques and construction were used for coastal protection that could not last the erosive power of the ocean.
Very large rocks or huge concrete blocks were generally used as a protective measure against ocean waves, and also against the infamous waves that roll along the south African eastern coastline. But these large rocky boulders and concrete blocks eroded over time and washed away under strong ocean currents. East London Port required more durable coastal protection. An inexpensive construction that could withstand and lessen ocean wave force while remaining firmly in position was needed.
The Story Behind The Oddly-Shaped Dolos
Dolosse are commonly referred to as having an odd geometric shape. The shape that a dolos embodies is no accident. Merrifield and Kruger were discussing the creation of a concrete structure with the capability of protecting East London’s harbour from the pounding waves. Their discussion gave Kruger food for thought and he decided to design a prototype for this concrete structure. Kruger broke off the wooden handle of a broomstick and cut off pieces that he nailed together to form an “H”. One of Kruger’s “H” prototype’s legs was cut off. The story goes that the draughtsman got the idea for the shape from what is known in Afrikaans as a “dubbeltjie” thorn.
The peculiar geometry inspired by the dubbeltjie thorn and developed from a wooden “H” functions in a collective manner. The geometry of the dolos allows them to interlock and stay firmly in position. That way, many Dolosse can stand their ground locked in position to withstand ocean waves and water erosion to protect industrial coastal development.
The Making Of The Dolos
Usually, Dolosse are made of unreinforced concrete prepared in a concrete mixer and poured into a steel frame mould. The steel frame replicates the particular geometry of the dolos in its exterior form, with a hollow inside to fit the concrete inside for the mould to be constructed. Concrete mixers stir together un-reinforced concrete and are added with small steel fibres in order to strengthen the concrete mixture for a stable dolos structure. A single dolos can weigh up to 8 tonnes, or 8000 kilograms. These effective protective concrete structures are indeed incredibly heavy and play a role in production logistics. Due to their incredible heaviness, dolosse are moulded and stored close to harbours and ports.
Material And Protective Advantages Of The Dolos
There are many engineering strategies and accompanying structures to keep coastlines safe from the potentially damaging forces of the ocean. However, the dolos has advantages that make it a reliable and widely used coastal protection structure. Here are some advantageous properties of the dolos:
The geometry of the dolos emulates the dubbeltjie thorn. This geometry essentially combines two anchors in a single-shaped structure. The interlocking of these double-edged anchors provides highly stable structures that won’t be swept underwater by ocean waves.
• Wave-dissipating performance
Dolosse drastically reduces ocean wave pressure on seawalls close to coastal development and wave runup that ensures protected coastlines. Their large rough surfaces are very porous, in other words, the concrete absorbs large volumes of seawater. The porous concrete consistency made possible by concrete mixers in dolos structures effectively takes the biggest blow out of ocean waves, keeping the ports and harbours behind the oddly shaped structures unharmed by oncoming waves.
• Economic production
Due to their highly stable structure, dolosse require considerably smaller mass per unit than conventional concrete block protection. This implies smaller concrete mixers and other machinery for the transportation and installation of the dolos and thus more cost-effective hardware. Furthermore, highly porous dolos structures imply that less concrete is needed than otherwise dense concrete blocks. Dolosse require less concrete, and in turn less production per unit for the same level of effective protection. A cost-effective structure indeed.
Concrete Mixers: Other Significant Concrete Coastal Protection Structures
The dolos is most certainly incredibly effective at protecting harbours and ports. However, there are many other engineering structures that provide marine protection and other important functions. Here are three other common concrete marine protection structures:
Seawalls are large protective coastal structures that can be made from reinforced concrete and are built parallel along shorelines. Seawalls are often built along promenades, coastal roads and houses that face seaward located on edges of beach profiles. The protective purpose of seawall construction is to prevent flooding caused by storm surges and the overtopping of ocean waves.
Jetties, unlike seawalls and dolosse which are strategically placed along coasts and harbours, are constructed from the coastline running into the sea. Jetties are commonly built from concrete and steel, or even stone and wood. Jetty construction is most often used to prevent the formation of sandbars and restrict the forceful impact of ocean currents. Based on the use of the jetty, this extended protective concrete walkway into deep ocean water is found at estuaries where river mouths meet ocean water or at harbour entrances.
These concrete protectors have wide use in developed marine environments and have many types of build. Breakwaters are long-term permanent marine protective structures that armour harbours and coastlines but also provide shelter for still-standing ocean vessels docked at harbours. Breakwaters are usually part of a much larger coastal management system that encompasses a harbour, anchorage, urban coastal development and a beachfront, all requiring reliable marine protection.
Breakwaters, just like the dolos, seawalls and jetties are solid concrete constructions and provide an array of oceanic protection services that tell us about the nature of the concrete as well as the construction design. Breakwaters are very similar to jetties and seawalls when we look at the interaction between concrete and seawater and what dangerous oceanic force is being cushioned.
Porous concrete makes for good absorption of seawater and sand that minimises harbour and coastal erosion. Breakwaters, like the other concrete marine protectors, diminish ocean force by catching the majority of wave water that safeguards the harbours and coasts behind it. Another important purpose of breakwaters is the isolated anchorage space they create for mooring ocean vessels. Breakwaters that connect to large harbours that receive and dock cruise ships and large cargo vessels need quiet waters to safely remain in anchorage.
Semi-enclosed concrete walls connected to harbours that extend into deep sea water create marine anchorage space for vessels armoured from crashing waves and stormy sea water.
Mixers: Marine Concrete Durability
Protective marine concrete degrades over time, regardless of how strong the concrete is or the quality of concrete mixers used in its manufacture. Here are the top reasons for concrete’s deterioration in marine environments:
• Physical: freezing-and-thawing
Harbours and coasts in the northern Hemisphere located in either the Atlantic and Pacific oceans or at the polar regions far up north reach temperatures below freezing. Ocean waters in these cold geographic areas freeze against any marine concrete structures. Once temperatures rise, frozen seawater on concrete structures melts and thaws as it starts to move in a liquid state, also expanding concrete particles. Over time on a large scale, the freezing-and-thawing of seawater creates contracting and expanding concrete creating cracks that weaken the structure and the protective ability of any concrete marine construction.
• Chemical: concrete breakdown
The interaction between seawater with marine concrete protection structures creates a constant chemical abuse of the concrete material. Water-covered concrete dries as the moisture dissolves and leaves a thick salt layer rich in chloride, sulfate and magnesium. The composition of the salt leads to the chemical breakdown of concrete particles creating cracks that weaken the integrity of marine concrete protection structures.
BS Power Mixers For Your Protection
Degrading seawater, forceful ocean waves and storm surges can create horrible damage to coastal development. Don’t let the quality of concrete construction be the reason for a wave of damage. Look to concrete mixers you can trust. BS Power manufactures and imports only the finest quality, high-performance concrete mixers for your coastal construction, agricultural or forestry building needs.
BS Power is a Cape Town-based construction equipment supplier that keeps South Africa and the rest of Southern Africa satisfied with top-quality building equipment, helping you build construction that lasts a lifetime.