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Dolos
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 | This article includes a list of general references, but it lacks sufficient corresponding inline citations. (July 2012) |
A dolos (plural: dolosse[1]: 10 ) is a wave-dissipating concrete block used in great numbers as a form of coastal management. It is a type of tetrapod. Weighing up to 8 tonnes (8.8 short tons), dolosse are used to build revetments for protection against the erosive force of waves from a body of water.[2][3] The dolos was invented in 1963, and was first deployed in 1964 on the breakwater of East London, a South African port city.[4][5]
Construction
[edit]
Dolosse are normally made from non-reinforced concrete, poured into a steel mould.[1]: 11 The concrete will sometimes be mixed with small steel fibers to strengthen it in the absence of reinforcement.
They are used to protect harbour walls, breakwaters and shore earthworks. In Dania Beach, Florida, dolosse are used as an artificial reef known as the Dania Beach Erojacks.[6] They are also used to trap sea-sand to prevent erosion. Roughly 10,000 dolosse are required for a kilometre of coastline.[citation needed]
They 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. Their design ensures that they form an interlocking but porous and slightly flexible wall.
The individual units are often numbered so that their movements can be tracked. This helps engineers gauge whether they need to add more dolosse to the pile.[citation needed]
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. The sheer mass of the dolosse provides ballast for logs and slash ("wrack" or "rack" organic debris) to create a stable, complex habitat structure, all the while precluding the need for excessive, environmentally-invasive and costly excavation for their placement into substrate.[citation needed]
Credit for invention
[edit]The design of the dolos is usually credited to the South African Eric Mowbray Merrifield, one-time East London Harbour Engineer (from 1961–1976).[1]: 10 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. In the late 1990s the claim of Aubrey Kruger[7] 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. 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 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. Merrifield won the Shell Design Award and the Associated Science and Technology Societies of South Africa's Gold Medal.[1]: 11 The death of Merrifield (in 1982) has put this controversy beyond proof either way.[citation needed]
Aubrey Kruger died in East London on 19 July 2016.
Design protection
[edit]The design of the dolos is not protected by any form of patent. Merrifield did not take the necessary steps to protect the concept.[1]: 11
The reason for this is uncertain. Two reasons for this 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.[1]: 11 Kruger alleges that Merrifield received incorrect legal advice: to wit, that as the blocks had been designed during office hours while he was employed by the State (South African Railways and Harbours Administration), he was unable by law to protect their design.[citation needed]
Origin of name
[edit]
The name is derived from the Afrikaans word dolos (plural: dolosse).[1]: 10 This word has two given derivations. Rosenthal (1961) states it to be a contraction of 'dobbel osse', or 'gambling' (Afrikaans) 'bones' (from Latin). Boshof and Nienaber state it to be a contraction of 'dollen os', or 'play' (old Dutch) 'oxen' (Afrikaans).[1]: 10 The first is a meaning-shifted reference to knucklebones used in divination practices by sangomas, Southern African traditional healers.[1]: 10 The second is a reference to the knucklebones used by African children to play.[1]: 10 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?).[1]: 10
See also
[edit]
Wikimedia Commons has media related to Dolosse.
- Riprap
- Humboldt Bay – Bay on the North Coast of California – jetties were reinforced using dolosse in the 1980s maintaining the entrance at one of the world's most treacherous harbor entrances
- Czech hedgehog – Static anti-tank obstacle defense
- Xbloc – Concrete breakwater element
- KOLOS – Concrete breakwater element
- Tetrapod – Concrete breakwater element
- Accropode – Concrete breakwater element
References
[edit]- ^ a b c d e f g h i j k "Bastions of the Sea". South African Panorama. 19 (3). Information Service of South Africa. 1974 – via Internet Archive.
- ^ Smith, Jane McKee, ed. (April 2007). Coastal Engineering 2006. World Scientific Publishing. pp. 4806, 4810, 4812. ISBN 978-9812706362. Retrieved 26 February 2018.
- ^ Edge, B.L.; Baird, W.F.; Caldwell, J.M.; Fairweather, V.; Magoon, O.T.; Treadwell, D.D. (1882). "Failure of the breakwater at Port Sines, Portugal". Isbn 978-0-87262-298-2. Retrieved 17 February 2019.
- ^ Terry Hutson (10 August 2016). "The full story behind the dolos and its SA creator". IOL. Archived from the original on 26 February 2018. Retrieved 26 February 2018.
The drawings for the first dolos were completed in 1963, based on the shape devised in wood by Aubrey Kruger. [...] The following year, 1964, the first dolos was laid on the port breakwater [in East London].
- ^ Barbara Hollands (27 July 2016). "SA inventor of dolos sea buffer system dies aged 82". TimesLIVE. Archived from the original on 26 February 2018. Retrieved 26 February 2018.
Aubrey Kruger‚ the former East London harbour draughtsman who designed the ingenious dolos sea buffer system 50 years ago‚ died in the city last week.
- ^ "Dania Beach Erojacks". Sink, Florida, Sink!. Retrieved 3 March 2013.
- ^ Hollands, Barbara (26 July 2016). "SA world-famous dolos designer dies in home city". DispatchLIVE. Retrieved 2016-07-27.
Further reading
[edit]- Eastern Cape Dispatch, 28 June 1999
- Martin Creamer's Engineering News, 16 February 2001
- South African Financial Mail, 3 December 2004
- McPhee, John (2011). "The Atlantic Generating Station". Giving Good Weight. Farrar, Straus and Giroux. ISBN 978-0-374-70857-3.
- Rosenthal, Eric (1961). Encyclopaedia of Southern Africa. London and New York: Frederick Warne.
External links
[edit]
Dolos
View on Grokipediafrom Grokipedia
Overview
Definition and Purpose
A dolos is an interlocking concrete armor unit designed specifically for use in rubble-mound breakwaters and revetments.[6] It features a distinctive three-dimensional shape resembling an X or H, consisting of a central shank with flukes that extend in multiple directions to facilitate mechanical interlocking when placed in layers.[7] This precast concrete structure is engineered to withstand severe marine conditions and may be reinforced or unreinforced depending on size and application, with reinforcement common in larger units to prevent breakage.[8][9] The primary purpose of the dolos is to absorb and dissipate wave energy, thereby protecting coastal structures from erosion and overtopping. Through its rocking motion under wave impact and the interlocking of adjacent units, it creates a porous armor layer that breaks up incoming waves more effectively than smoother surfaces, reducing the transmission of hydraulic forces to the underlying mound.[6] This mechanism allows dolosse to settle and readjust dynamically, enhancing overall stability while minimizing displacement.[7] In coastal engineering, dolosse serve as an optimized alternative to traditional rock armors, particularly in high-energy environments such as harbors, seawalls, and exposed shorelines where wave heights can exceed 10 meters. They offer superior performance in dissipating energy due to their higher porosity and interlocking efficiency, requiring less material volume per unit area compared to quarried stone while providing greater resistance to wave run-up and erosion.[8] This makes them suitable for tetrapod-like applications but with enhanced adaptability to steep slopes and intense storm conditions.[7]Physical Characteristics
The dolos is a precast concrete armor unit distinguished by its complex geometry, consisting of a central waist or shank from which four flukes—elongated arms—extend outward, creating a symmetric, X-like profile. These flukes are typically configured with two protruding at each end of the shank, promoting interlocking when placed in breakwaters. The ends of the flukes are rounded and filleted at the junctions with the shank to reduce stress concentrations and breakage risks during transport, installation, and wave exposure.[10][11] In terms of dimensions, standard dolos units feature fluke lengths ranging from approximately 1 to 2 meters, with the overall height (along the shank) scaling accordingly to match the unit's mass and project requirements. For instance, a representative 200 kg prototype model has a shank height of 80 cm, a waist width of 26 cm, and fluke heads about 16 cm wide, with these proportions maintained in larger units through geometric scaling. The waist-to-height ratio, often around 0.32 to 0.37, influences the unit's slenderness and is adjusted during design for balance between hydraulic performance and structural integrity.[10][9] Unit mass typically spans 10 to 50 tons for full-scale applications, such as harbor breakwaters, where heavier units (e.g., 20–42 tons) provide greater stability against wave forces; lighter variants (2–10 tons) suit milder conditions. Dolos are manufactured from high-density concrete, typically unreinforced but sometimes reinforced with steel bars for tensile strength in larger units, resulting in a uniform gray appearance and textured surface that aids in energy dissipation. In visual representations, such as engineering schematics, dolos are illustrated in random or quasi-random placements to highlight their interlocking behavior, with the four-fluke design enabling high porosity (packing densities of 0.61–1.0) in double-layer armor systems.[12][13][6]History and Development
Invention and Early Adoption
The dolos, a concrete armor unit designed for coastal protection, was invented in 1963 by Aubrey Kruger, a draughtsman at the Port of East London in South Africa, under the supervision of harbor engineer Eric Merrifield.[14][15] Kruger developed the initial concept using a wooden model inspired by the interlocking shape of a local dubbeltjie thorn, aiming to create a stable, wave-dissipating structure that could replace unstable rectangular concrete blocks and large natural rocks.[14] Merrifield, who served as East London Harbour Engineer from 1961 to 1976, oversaw the refinement of the design, which was completed in drawings by late 1963.[15][16] The invention addressed key challenges in coastal engineering at the time, particularly the difficulty in sourcing and handling large natural rock blocks for breakwater armor, which were both scarce and labor-intensive to transport.[16] This need was heightened by post-World War II expansions of harbors in southern Africa, including East London, where increasing trade volumes demanded more efficient and mass-producible protective structures to withstand severe wave action.[17] The dolos shape was selected for its high void-to-solid ratio, promoting interlocking and energy dissipation while allowing economical concrete casting without specialized equipment.[16] Prototype testing began in 1964 with the placement of the first full-scale concrete dolosse—each weighing approximately 19.5 tons—on the East London Harbour breakwater extension, where they were subjected to waves up to 18 feet high.[14][16] These units demonstrated exceptional stability, showing minimal movement and a tendency to self-embed into the structure, outperforming existing armor in preliminary field trials.[16] By the end of 1965, around 450 dolosse had been deployed at the breakwater's end and along a short seaward section, paving the way for full-scale implementation in 1966 as part of the harbor's ongoing reinforcement.[16][15]Evolution and Improvements
Following the initial deployment of dolos units in the 1960s, early experiences revealed vulnerabilities to breakage during severe storms, prompting refinements in the 1970s to improve structural integrity and interlocking performance. The catastrophic failure at the Sines breakwater in Portugal in 1978, where 42-tonne dolos units suffered extensive fracturing under extreme wave conditions, highlighted stress concentrations at the fluke-shank intersection as a primary cause of failure, with breakage rates exceeding expectations in hydraulic models.[18] In response, engineers introduced shape modifications, such as large fillets or chamfers at the fluke-shank junction, which reduced peak stresses by over 60% compared to sharp intersections, enhancing resistance to dynamic loads and promoting better post-storm interlocking by minimizing fracture propagation.[19] These adjustments addressed initial interlocking deficiencies observed in stormy conditions, where broken units failed to maintain armor layer stability. By the 1980s, scaling efforts advanced the dolos design for deeper-water applications, culminating in the development of "super dolos" units weighing up to 80 tonnes to withstand higher wave energies in exposed sites. This evolution was supported by significant improvements in hydraulic model testing, including the incorporation of irregular wave spectra and three-dimensional simulations to better replicate prototype conditions, as demonstrated in post-Sines investigations that refined stability coefficients (K_D) to 15-40 for larger units.[18][20] Such testing advancements allowed for optimized packing densities (φ ≈ 0.73-1.47) and reduced manufacturing breakage rates to 1-2%, enabling safer deployment in demanding environments.[18] The widespread adoption of dolos units from the 1970s to the 1990s extended across Europe and Asia, with notable implementations in Portuguese harbors like Sines and Japanese ports such as Naha, where 40-50 tonne units protected against typhoon-prone coasts.[18][9] This global proliferation, driven by the units' high stability factors, influenced subsequent innovations, including hybrid designs that integrated dolos-like interlocking with tetrapod geometry for enhanced porosity and wave dissipation in varied coastal settings.[21]Design Principles
Geometry and Stability Mechanisms
The geometric design of the dolos unit features a complex, three-limbed structure resembling an H-shape, with projecting flukes that facilitate interlocking and controlled motion under wave loading. The fluke angle promotes optimal rocking by allowing the unit to pivot effectively without excessive translation. The slenderness of the limbs enables the units to nest closely while maintaining sufficient flexibility for energy absorption. This configuration, with a standard waist ratio of around 0.32 (waist diameter to overall height), balances hydrodynamic performance and structural integrity, as higher ratios improve strength but slightly reduce interlocking efficiency.[22][23] Stability in dolos armor layers relies on mechanisms that dissipate wave energy through dynamic interaction rather than rigid resistance. The primary mode is controlled rocking, where individual units roll and resettle in response to wave forces, absorbing kinetic energy via friction and minor rotations without displacing the overall armor profile. This movement is limited by the interlocking of flukes, which creates a network of mutual restraints that prevents cascading failure. The resulting voids from this arrangement reduce the armor layer's porosity to 40-50%, compared to higher values in non-interlocking systems, thereby minimizing wave run-up and overtopping while allowing internal turbulence to further dissipate energy through porous flow paths. Experimental studies confirm that this combination yields packing densities around 0.83, enhancing layer cohesion under irregular wave attack.[24][23] A fundamental metric for dolos stability is the stability number , where is the dolos weight, is seawater density (typically 1025 kg/m³), is gravitational acceleration (9.81 m/s²), and is the significant wave height. This parameter originates from the Hudson formula for rubble-mound armor sizing, empirically derived from flume tests on various units under monochromatic waves. The derivation starts with balancing the destabilizing wave force, proportional to per unit width but cubed for three-dimensional scaling in stability criteria, against the unit's inertial resistance , yielding the cubic dependence on . For sloped structures, the full Hudson equation incorporates slope angle as , where is the specific gravity of concrete (≈2.35); however, isolates the core hydrodynamic coefficient. For dolos on a 1:1.5 slope with random placement, experimental values of range from 10 to 25 (no damage to minor movement), significantly higher than quarrystone (≈3-4), reflecting superior energy dissipation. To compute , select from validated curves based on wave steepness and placement density, then solve the full equation; for example, with m, , cot θ = 1.5, and (S-1)^3 ≈ 2.46 (mass units, ρ_w ≈ 1 t/m³, g omitted), tonnes, establishing scale for deep-water applications.[23][9]Sizing and Scaling Factors
The sizing of dolos units for coastal structures is primarily determined using environmental parameters such as significant wave height , wave period , and water depth , to ensure hydraulic stability under design conditions.[25] The standard approach employs the Hudson formula, which relates the armor unit mass to these factors via , where is the water density (≈1.0 t/m³), is the slope angle, is the stability coefficient (10-25 for dolos depending on conditions and placement), and is the relative density.[23] This formula ensures the nominal diameter , with mass scaling as for fixed geometric and material properties, often resulting in units up to 30 tonnes for around 5–7 m.[23] Wave period influences the surf similarity parameter , adjusting for plunging or surging breakers, while water depth determines if conditions are deep () or shallow, affecting wave transformation and required mass. Structural integrity must also be considered in sizing, as dolos units are prone to breakage at the shank-fluke junction under repeated rocking and impact loads; finite element analysis or empirical stress limits (e.g., max principal stress < 30% of concrete strength) are recommended to supplement hydraulic design.[25][23] Scaling dolos designs from hydraulic models to full-scale prototypes presents challenges due to the incompatibility of Froude and Reynolds similitudes in wave tank tests.[26] Froude scaling ( linear scale 2–60) is applied for gravity-dominated wave flows, but the resulting low Reynolds number () in models (often <10^5) exaggerates viscous effects like boundary friction and porosity flows compared to prototypes, leading to overestimated stability by 10–20% for small-scale tests ( m).[23][26] To mitigate, large-scale models () or numerical corrections are used, with empirical adjustments for dolos interlocking based on tests showing scale-dependent packing density (0.83–1.15).[23] Environmental factors necessitate site-specific adjustments to dolos sizing for optimal performance. Storm surge elevates water levels, increasing effective at the structure toe by up to 20–30% during extreme events, requiring a safety factor in mass calculations (e.g., 1.5 for interlocking units).[23] Seabed slope influences underlayer stability and scour, with recommended armor slopes of 1:1.5 to 1:2 (cot = 1.5–2.0) for dolos; steeper slopes (1:1) demand 10–15% heavier units to counter sliding, while gentler (1:3) reduce size needs but increase material volume.[23] Layer thickness is typically 2–3 dolos units deep (), enhancing permeability and reserve strength against overtopping, with underlayers sized at to support the primary armor without settlement exceeding 1–2% of total height.[23]Construction and Materials
Manufacturing Process
The manufacturing process of dolos units begins with the preparation of reusable steel formwork molds, typically constructed from 3/16-inch thick mild steel plates that are flanged, ribbed, and fixed in casting pits with open tops to accommodate the complex, anchor-like geometry.[27] These molds are designed for durability and efficiency in large-scale production, often featuring multiple shells or sections to facilitate easy demolding.[28] Prior to pouring, reinforcement is installed within the molds, consisting of rebar grids or steel wire loops arranged to enhance tensile strength, prevent cracking under handling stresses, and provide lifting points; early designs used heavier steel rail frames, but modern practices favor lighter configurations for cost and weight reduction. Alternatively, steel fibers may be added to the concrete mix at rates of about 50 kg/m³ to enhance tensile strength and durability.[27] [28][29] Concrete is then mixed in batches with a low water-cement ratio (typically 0.35-0.5) and cement content around 300-400 kg/m³, often including supplementary cementitious materials like slag or fly ash, and poured into the molds, where it is vibrated to achieve high density, eliminate air pockets, and ensure uniform compaction around the reinforcement.[27] [30] [29] Vibration is particularly critical in the narrow waist and fluke areas to avoid voids that could compromise structural integrity.[31] After casting, units are demolded after 12 to 24 hours, depending on ambient temperatures and concrete formulation, and transferred to a curing yard for initial hydration.[27] [28] Full curing requires a minimum of 28 days under controlled conditions—often 7 days in a moist environment followed by 21 days in a final yard—to attain the necessary compressive strength, typically exceeding 6,000 psi for handling and deployment.[27] [28] Quality control is integral throughout production, involving visual inspections, compressive and flexural strength testing on sample cylinders from each batch, and non-destructive methods such as ultrasonic testing or core sampling to detect internal voids or segregation.[28] In dedicated facilities, batch production can achieve rates of 25 to 130 units per day, depending on the number of molds (e.g., 8 to 54 forms) and operational scale, enabling thousands of units over a season for major projects.[28] Cost factors favor efficiency, with low material consumption—approximately 0.42 m³ of concrete per metric ton of unit weight, given a typical density of 2.4 t/m³—but require significant upfront investment in robust mold sets.Material Composition and Durability
Dolos units are typically constructed from high-strength concrete with a target compressive strength of 35-60 MPa at 28 days, achieved through a mix design featuring a low water-cement ratio of 0.4-0.5 and a cement content around 300 kg/m³.[32][33] This concrete incorporates sulfate-resistant Portland cement, such as Type V, to mitigate chemical attacks from seawater sulfates prevalent in marine environments.[34] Aggregates consist primarily of crushed stone meeting ASTM C33 standards for grading, soundness, and deleterious substances, ensuring structural integrity under dynamic wave loads. Additives like pulverized fly ash or silica fume are commonly included at 10-15% replacement of cement to enhance impermeability by refining the pore structure and reducing water absorption.[35][36] Durability is paramount for dolos units exposed to aggressive marine conditions, with the concrete formulation providing resistance to abrasion, corrosion, and biofouling. Abrasion resistance is verified through the Los Angeles abrasion test on aggregates, targeting less than 30% loss to withstand wave-induced wear over extended periods.[37] For reinforced dolos designs, epoxy-coated steel rebar is employed to protect against chloride-induced corrosion, forming a barrier that extends the service life by preventing rust expansion within the concrete matrix.[38] The low-permeability mix, bolstered by pozzolanic additives, also limits biofouling by minimizing substrate availability for marine organism attachment.[32] Overall, these properties contribute to long-term durability in severe offshore conditions, assuming proper manufacturing integration such as controlled curing.[39] Testing protocols emphasize material quality and long-term performance, including ASTM C33 compliance for aggregates to ensure uniformity and resistance to degradation. Accelerated weathering simulations, such as chloride penetration tests per ASTM C1202, assess resistance to ion ingress, confirming the concrete's ability to maintain integrity under simulated decades of exposure. These standards collectively verify the concrete's suitability for dolos applications, focusing on sulfate resistance and overall environmental resilience without compromising structural demands.[40]Applications and Performance
Implementation in Coastal Structures
Dolos units are deployed in double-layer armor configurations on rubble-mound breakwaters and revetments to provide hydraulic stability and wave energy dissipation.[41] Placement typically achieves 55-65% coverage, balancing interlocking and porosity to facilitate water flow through the layer while preventing excessive movement under wave attack.[41] Units are positioned using cranes mounted on barges or land-based platforms, allowing for precise deposition in offshore environments.[28] To optimize stability, dolos are oriented randomly during placement, often guided by GPS systems such as Real-Time Kinematic (RTK) technology for accuracies of approximately 8-11 inches, which promotes natural interlocking and maximizes voids without aligned patterns that could create preferential wave paths.[28] Integration of dolos into coastal structures involves layering over a prepared underlayer of smaller stones or core material, typically sized at 1/10 to 1/5 the weight of the primary armor units to provide a stable foundation and reduce differential settlement.[41] At the structure's toe, protection berms constructed from graded, stable stones—often weighing 800-1,300 pounds—are essential to prevent scour and undermining, ensuring the armor layer remains intact against base erosion from currents and waves.[41] Sizing of these underlayer and toe elements is determined based on site-specific wave conditions, as outlined in established scaling factors.[41] Post-installation monitoring is critical to verify proper seating and detect early displacements, commonly conducted using divers for visual inspections in shallow waters or remotely operated vehicles (ROVs) for deeper or hazardous areas, supplemented by side-scan sonar for broader surveys.[42] Logistical aspects include transportation via specialized flatbed trucks to loading ports and subsequent barge vessels for offshore delivery, minimizing breakage during handling.[28] Under calm sea conditions, installation rates typically range from 100 to 200 units per day, depending on unit size, equipment capacity, and weather windows.[28]Case Studies and Failures
One prominent case of successful dolos implementation is the Sines Harbor breakwater in Portugal, constructed in the 1970s with initial plans for 30-ton units that were later adjusted to 42-ton dolosse to enhance stability against expected significant wave heights of up to 10 meters.[31] Following initial storm damage and subsequent repairs incorporating reinforced units and improved placement, the structure has demonstrated long-term resilience.[43] A more recent application occurred at Barnegat Inlet, New Jersey, where approximately 18,000 6.5-ton dolosse were placed between 2015 and 2016 to repair rubble-mound breakwater damage caused by Hurricane Sandy in 2012.[28] The project utilized fiber-reinforced concrete units and precise GPS-guided placement, highlighting the continued, though selective, use of dolos for rehabilitation in areas with suitable wave conditions. In contrast, a significant failure occurred at Crescent City Harbor, California, during severe winter storms in the early 1980s, with approximately 29% of the 240 dolosse in the outer breakwater section broken by August 1982, escalating with additional losses in the 1983 season.[44] This breakage, amounting to over 70 units in the monitored area, was attributed primarily to inadequate underlayer support and exposure to extreme wave forces exceeding design expectations, leading to structural fracturing and displacement of the slender unreinforced units.[44] The incident prompted extensive rehabilitation efforts, including the installation of 680 reinforced 38- to 42-ton dolosse in 1986, which incorporated fiber reinforcement to mitigate fatigue and improve interlocking.[45] Key lessons from these and similar dolos failures have driven industry-wide shifts toward rigorous core testing of concrete units for structural integrity under dynamic loads, as well as the adoption of hybrid armor systems combining dolosse with interlocking units like Core-Loc for enhanced repair and stability.[46] Modern assessment metrics now emphasize a damage index based on the percentage of broken or displaced units, with levels below 5% considered acceptable for ongoing operational integrity before initiating major interventions.[47] These advancements, informed by prototype monitoring and model studies, have reduced recurrence risks in subsequent designs.[45]Naming and Intellectual Property
Etymology
The term "dolos" derives from Afrikaans, where it denotes the knucklebones or small bones (often from sheep or goats) used in traditional South African children's games and divination practices, resembling irregular, interlocking play pieces.[48] This linguistic root is commonly attributed to a contraction of "dobbel osse," translating to "gambling bones" in Afrikaans, reflecting the dice-like tossing and chance element in such games.[49] The name evokes the bones' deceptive, multifaceted shapes that shift and lock unpredictably when thrown, mirroring the complex geometry of the concrete units.[50] In South African cultural context, these dolos bones feature prominently in indigenous pastimes among Zulu and Xhosa communities, where they serve as toys for simulating animal herding or as tools in ritualistic throwing games that symbolize strategy and fortune.[50] The etymology ties directly to this heritage, with some sources suggesting an additional influence from the Xhosa word "idolo," meaning "knee," alluding to the anatomical origin of the knucklebones.[50] This naming choice honors local traditions while highlighting the units' ability to interlock like the bones in play, enhancing stability against wave forces.[48] While occasionally termed "dolos blocks" in informal descriptions, the units are frequently compared to tetrapods—four-legged concrete armor developed in Japan for similar coastal protection—but distinguished by their more elongated, three-pronged form.[51] In professional coastal engineering literature, however, "dolos" (plural: dolosse) remains the official and retained designation, underscoring its South African provenance.[52]Patents and Recognition
The original Dolos design was intentionally not patented by its developers to promote its free and widespread adoption in coastal engineering projects globally, avoiding restrictions that had limited earlier armor unit innovations.[12] This decision by the South African Railways and Harbours Administration, under whose authority the design was developed, allowed for unrestricted use without licensing fees or legal barriers.[53] The primary credit for the Dolos invention is attributed to Eric Mowbray Merrifield, the harbour engineer at East London Port who oversaw its creation in 1963, with substantial contributions from draftsman Aubrey Kruger in conceptualizing the interlocking shape.[1] While Merrifield received formal recognition, including the 1972 Shell Design Award and the Gold Medal from the Associated Scientific and Technical Societies of South Africa, Kruger later publicly contested the attribution, asserting his leading role in the prototype's development; however, no litigation ensued, and the design's collaborative nature within the Portnet authority was acknowledged.[4] Subsequent adaptations and modifications to the Dolos, such as reinforced or scaled variants, have been patented independently by other engineers and organizations to address specific performance issues.[54] The Dolos garnered significant professional acclaim through presentations at key international forums, including the 1972 ECOR Symposium on the Ocean's Challenge to South African Engineers, where its stability was detailed in technical papers. Its influence extended to shaping global design standards, notably in the U.S. Army Corps of Engineers' Coastal Engineering Manual (EM 1110-2-1100), which incorporates Dolos-specific stability coefficients and placement guidelines for rubble-mound structures.[55] This integration reflects the unit's high-impact role in advancing economical and effective wave-dissipating technologies.References
- https://coastalwiki.org/wiki/Scaling_Issues_in_Hydraulic_Modelling