Municipal Solid Waste Disposal - a decades old dilemma
By John Jeapes
Back in the early 1970s, the old Rutherford County Court was faced with a
dilemma in the form of new state laws dealing with the disposal of solid waste.
Solid waste was a new name for an age-old problem. Before then, most people
didn‘t really give much thought to garbage or trash disposal. If you lived in the
city of Murfreesboro, you dragged your trash cans to the curb and sanitation
engineers did the rest. The trash was transported to the aptly named ―city dump‖
where the combustibles were burned and the remainder buried.
I can still remember seeing the black, acrid smoke and the sad sight of poor folks
sifting through the trash looking for useful items. Seeing that was a real eye-
If you lived outside the city limits, you were on your own when it came to
disposing of your trash. Most burned it in a metal barrel. Others placed it in
galleys and sinkholes. A few just pitched it out the back door. Many dumped their
trash on other‘s property, in ditches and at the end of dead end roads.
Tennessee‘s Solid Waste Management program, instituted in 1971, was designed
to bring all of those nasty practices to an end. The program followed the creation
of the EPA during the Nixon administration.
You can trace America‘s environmental movement back to 1962 when Rachel
Carson published the earth-shaking book ―Silent Spring,‖ that dealt with
pesticides and pollution. The book had a huge following and made people think,
for the first time, the consequences of messing up the environment.
The youth movement, activated by opposition to the Vietnam War, adopted
―ecology‖ as an issue and by 1969, the environment was becoming a hot button
issue. Sen. Gaylord Nelson of Wisconsin became the leader of a bipartisan push in
Congress. On the national level, 1970 was a crucial year, marking the first Earth
Day on April 22.
New federal laws, like the Clean Air Act, quickly flowed from the feds to the state
level and onto local governments and communities in Tennessee had to draft
plans to deal with solid waste disposal.
Rutherford County drafted a plan that called for strategically placed dumpsters
where citizens could dispose of their trash and began the search for a site for
what was called then, a ―sanitary landfill.‖
Geology played the chief role. Rutherford County with its limestone rock and karst
landscape with sinkholes and caves was a difficult place to find a site with the
proper (and sufficiently deep) soil. Ultimately, less than a handful of sites were
discovered off Manson Pike, Cherry Lane, Manchester Pike and Jefferson Pike.
The late Ben Hall McFarlin was county judge then and arranged a tour of all the
sites by the magistrates on the county court. They boarded a school bus and
ultimately a site off Jefferson Pike was selected as the repository for all the
county‘s trash. Geologically, it was the best site, and the location provided easy
access off of U.S. 231 and Jefferson Pike (for Smyrna and La Vergne).
From day one, operation of the Rutherford County Landfill was problematic as
was the dumpster plan. The dumpsters quickly became a blight on their
communities, especially when people left furniture, car parts and used tires.
They also became a dumping area for unwanted pets. It was a nightmare. The
county tightened its laws, increased enforcement, but didn‘t come up with a
satisfactory answer until it devised the convenience center concept.
By the 1980s, the county found itself facing an even bigger problem. It was
running out of landfill space. The proposed Middle Point Landfill seemed like an
answer to the county‘s prayers ... or at least an easy out. The Rutherford County
Landfill could be closed, getting rid of a major problem, and the county could
dump its trash, for free, at the new facility.
Of course, nothing is ever free and taking the easy way out may be harming
citizens in other ways. But no one ever considered that ―free‖ price might include
radioactive and other toxic waste.
Another Rutherford County (this one in North Carolina) was a bit smarter. Its
code pointedly bans dead animals, radioactive waste and sewer sludge among
other things. Unfortunately, it seems after all these years it seems not only
Rutherford County, but every county still faces a dilemma. What to do with
Municipal Solid Waste?
Americans produce more and more solid waste each year; they generate more
per capita than any other nation. But, at the same time that they generate more
waste, they are running out of places to dispose of it. Landfill capacity in some
places is almost filled to the saturation point, and solid waste facilities continue to
be difficult to site because of public resistance, commonly known as the ―Not in
My Backyard‖ (NIMBY) syndrome. Public resistance is often based on
environmental concerns, unpleasant smells, noise, and truck traffic. Public
resistance is not limited to landfills and combustors. Even materials recovery
facilities and recycling centers can be difficult to site. (The feckless voyage of the
―garbage barge‖ in 1987 and the ash barges last year have become national
symbols of America‘s solid waste dilemma).
Although solid waste management is primarily a local responsibility, the problem
is international in scope, and we all need a strategy to solve it. In response to
this burgeoning problem, we have to fashion a strategy for improving the
management of municipal solid waste. The following report was developed after
extensive consultation with a variety of knowledgeable groups and individuals and
it offers a number of concrete suggestions. It calls for a ―systems‖ approach to
managing municipal solid waste; that is, the complementary use of source
reduction, recycling, combustion and landfills to comprehensively manage
municipal solid waste. It also underscores the need for a fundamental change in
the nation‘s approach to producing, packaging and disposing of consumer goods.
In the past, ―business as usual‘‘ meant an accelerating trend toward disposable
products, convenience packaging, and an ―out-of-sight, out-of-mind‖ attitude
toward solid waste.
As a nation, the US can no longer afford this kind of ―business as usual.‖ Together
we must adopt a new solid waste management ethic that minimizes the amount
and toxicity of waste created by the products we make and purchase, produced
during the manufacturing process, and generated by our day-to-day activities as
consumers. That ethic must also maximize the amount of waste materials that
are reused and recycled so that we achieve a fully integrated system for waste
management. In short, we need to change the way we do business. This change
will not be easy, but if we work diligently together, we will achieve our goal. I
hope this paper will serve as a center piece for this change.
What the government calls municipal solid waste, and almost everyone else calls
garbage, generates about 160 million tons of solid waste last year; by the year
2012, we are projected to generate 190 million tons.
How should we handle this outflow of refuse - the cans, the bottles, the leaves
and lawn clippings, the paper and plastic packages, the broken furniture and
appliances, the uneaten food and the old tyres. This deluge of garbage is growing
steadily and we must find ways to manage it safely and effectively. Eighty
percent of garbage is land filled. But we‘re running out of space to bury it in
existing landfills; more than one third of the nation‘s landfills will be full within the
next few years and many cities are unable to find enough acceptable sites for new
landfills or new combustors. To eliminate this growing capacity gap, all levels of
government, the public and industry must forge a new alliance to develop and
implement new integrated systems for solid waste management.
In this article, I describe the construction of reinforced concrete caissons, which
once filled with waste that would otherwise occupy landfill, can be placed on the
sea bed, thus becoming an aid to waste management in the United Kingdom,
Malta, Greece, Cyprus and Ireland. These countries are after all surrounded by
water. In fact even in the USA. True, being composed of granular glacial till, using
the sea bed will require careful placing of such caissons, but by using a Global
Positioning System (GPS), along with lasers, caisson builders should be able to
place caissons within inches of their final approved location despite tides and
I also describe below the configuration of an orthotropic deck using this very
technology, which is in reality a private yacht pier, built near Glen Cove, Long
Island, USA. This construction has brought out a few points which may be of
interest, but it is an example of a small engineering structure, which, though of
no great moment in itself, illustrates the adoption of a means to an end that may
be capable of very great extension.
The problem, as I understand it, is the fact that the world faces a period of
unparalleled change in its approach to waste management. In addition to wider
environmental and sustainability considerations, the need to meet targets under
relating to the diversion of non Biodegradable Municipal Solid Waste (MSW) from
landfill represents a particular challenge. It is estimated that by 2012, depending
on the rate of growth of the MSW stream, England for example will need to divert
between 8.4 and 14.4 million tonnes of MSW away from landfill. Failure to meet
this target could result in EU penalties of up to £0.5M per day.
Combustion of municipal waste can be a viable waste management alternative for
many communities, To increase the viability of this option, it is important to
ensure that combustors are designed, operated, and controlled to minimize risks
to human health and the environment from both air emissions and ash. Options
for improving the Combustion of waste can be a viable waste management
alternative if it is designed, operated, safety of combustion include upgrading
combustor and controlled to performance standards, increasing education and
technical minimise risks to human health and the environment.
Landfills are still used to dispose of the majority of most nations municipal solid
waste, and will continue to be essential in the future. Although increased source
reduction and recycling will not only reduce the volumes of waste going to
landfills, but may also make some waste more benign, large amounts of money is
required to increase the safety of landfills to ensure protection of human health
and the environment. Public support is a problem when new land fill areas must
be sited. Operator certification, minimum design and operation standards,
education and technical assistance, and studies on potential bans of some wastes
from landfills all contribute to reaching the goal of increased safety and reduced
volumes of waste needing land filling.
Properly designed and operated landfills require knowledgeable operators in order
to ensure efficient and safe compaction of waste without damage to liners,
leachate collection systems, or other design features. In addition, the monitoring
required at municipal landfills requires an experienced operator.
Not only to provide safe disposal of waste, but also to efficiently reduce the bulk
or toxicity of the waste. However, waste disposed in landfills degrades very slowly
and safe design and operation throughout the life of the landfill is crucial, but
remedial action for existing and/or closed landfills are posing potential threats to
human health and the environment. So we should perhaps be looking for an
alternative method of MSW disposal.
Many countries are surrounded by water, and quays and harbours abroad often
consist of large concrete caissons sunk in line to form a wall, which is then filled
with concrete or rubble before being capped with a reinforced concrete deck.
They could just as easily be filled with non Biodegradable Municipal Solid Waste
instead of the same Waste being buried in landfill.
The construction of the yacht landing at East Island, which involved the use of
reinforced concrete caissons, was on an exposed southern shore of Long Island
Sound, and constructed at that point in support of an elaborate country
residence. The slope of the beach at this point is very gradual, and it was
specified that there should be a depth of at least 4 ft. of water at low tide.
Soundings indicated that this necessitated a pier 300 ft. long. It was further
specified that the pier should be to some extent in keeping with the scale of the
place being created there, and that a wooden pile structure would not be
Besides these aesthetic conditions, wooden piles were rejected because the
Teredo, a genus of shipworm that bores holes in wood, is very active. At the
same time, the owner did not care to incur the expense of a masonry pier of the
The pier was also to be used to unload all material for the house and grounds
during construction, and later coal and other supplies, thus necessitating a pier
wide enough not only to allow access for vehicles but also to provide enough
room for vehicles turning at the pier head.
Comparative designs and estimates were prepared for (a) a pier of ordinary
construction, but with creosoted piles; (b) a concrete pier on concrete piles; and
(c) for a series of concrete caissons with reinforced connections. The latter plan
was very much the best in appearance, and the calculated cost was less than that
of the pier of concrete piles, and only slightly more than that of creosoted piles,
the latter being only of a temporary nature in any case, and because it has been
found that the protection afforded by creosote against the Teredo is not
At this point on the Sound, the mean range of the tide is about 8 ft., and it was
determined that at least 5 ft. above mean high water would be required to make
the dock safe from wave action. There is a northeast exposure, with a long reach
across the Sound, and the seas at times become quite heavy.
These considerations, together with 4 ft. of water at low tide and from 2 to 3 ft.
of toe-hold in the beach, required the outer caissons to be at least 20 ft. high.
To construct such piers in the ordinary manner, i.e. behind coffer-dams, and in
such an exposed location, would have involved expenditure far beyond that which
the owner cared to incur. The writer's attention had shortly before been called to
the successful use of reinforced concrete caissons on the Great Lakes for
breakwater construction, by Major W.V. Judson, M. Am. Soc. C.E., under patents
held by that officer.
Our nations have choices as to how they are going to deal with their ever-growing
MSW problem. As we continue to create more and more garbage, we can no
longer continue to bury most of our waste. We must, sooner or later, find feasible
ways to recycle more of it. We can design products and packaging without
considering disposal or we can design for source reduction and recycling.
We can wait for local crises to occur or we can plan now to avoid them. In short,
we can ignore the issue and hope it goes away, which it will not, or we can act
now to deal with it. But whether we like it or not, our garbage is no longer ―out of
sight and out of mind.‖
This paper outlines a ―game plan‖ for addressing our garbage problem which
underscores the need for an effective integrated waste management approach,
including source reduction, recycling, combustion, but doing away with landfill. It
is not a panacea, but I believe that its implementation will go a long way in safely
eliminating the gap between the generation of garbage and our capacity to handle
it, as well as providing for waste management that protects both human health
and the environment.
It seems to me that concrete caissons could be a solution to the Municipal Solid
Waste problem world wide.
Concrete caissons could be constructed on shore using a cement additive for
inhibiting concrete deterioration in salt water. An effective inhibiting component,
a powder mixture of an inorganic cation exchanger, such as a highly calcium-
substituted zeolite, and an inorganic anion exchanger, such as hydro-calumite
could be used.
Cement additives will inhibit the alkali-aggregate reaction and the corrosion of
reinforcing steel, thus protecting concrete constructions from deterioration. After
thorough inspection and seasoning, the caissons can launched in a manner
somewhat similar to a boat, towed into position, sunk in place, and then filled
All we need is a structure that can be constructed safely and cheaply on land, can
then be allowed to harden thoroughly, and then placed in accurate position on the
sea bed. The weights to be supported are not great, so any beach of good gravel
and sand and fairly level will do, and in good weather, the placing of the caissons
promises to be a simple matter.
In the construction of the yacht landing at East Island, an effort was made to
preserve some element of the original yachting area was included in the design.
So bow-string trusses, being merely enlarged gang planks, were used to connect
The pier was originally laid out as a letter "L," with a main leg of 300 ft. and a
short leg of 36 ft.
The pier head consisted of eight caissons in close contact, and was intended to
form a breakwater, in the angle of which, and protected from the wave action,
was to be moored the float and boat landing.
After the first bids were received, the owner wished to reduce the cost, and every
other caisson in the pier head was omitted, so that, as built, the pier contains
eight caissons and five 53-ft. trusses.
The caissons supporting the trusses were 8 ft. wide and 12 ft. long, and those in
the pier head are 12 by 12 ft. On account of the shoal water and the great height
of the outer caissons in comparison with their cross-section, it seemed advisable
to mould them in two sections. The reinforcement in the side walls consisted of
round 1/2-in. rods horizontally, and 3/8-in. rods vertically, spaced.
The caissons were reinforced for exterior pressures, which were to be expected
during the launching and towing into position, and also for interior pressures,
which were to be expected at low tide, when the water pressure would be
nothing, but the filling of the caissons would be effective. The corners were
reinforced and enlarged.
In order to secure proper bedding into the seabed foundation, a 12-in. lip was
allowed to project all around the caisson below the bottom. In the bottom there
was cast a 3-in. hole, and this was closed by a plug while the lower section was
being towed into place.
The question of the effect of sea water on the concrete was given much thought.
I have been unable to find any authoritative opinions on this subject, which are
not directly controverted by equally authoritative opinions of a diametrically
opposite nature, but concrete caissons have been used in seawater for decades,
the Mulberry Harbour for example, with excellent performance. However, special
care in mix design and material selection will be necessary for the most severe
A structure exposed to seawater or seawater spray is most vulnerable in the tidal
or splash zone where there are repeated cycles of wetting and drying and/or
freezing and thawing. Sulphates and chlorides in seawater require the use of low
permeability concrete to minimize steel corrosion and sulphate attack. A cement
resistant to sulphate exposure is helpful. Proper concrete cover over reinforcing
steel must be provided, and the water-cementitious ratio should not exceed 0.40.
Concrete exposed to sea water is wetted by a solution of salts--principally sodium
chloride and magnesium sulphate, but any damage to concrete, if it occurs,
usually results from failure to use good practices in concrete construction, and
often is the result of freezing and thawing or wetting and drying as much as or
more than the results of the effects of sea water as such.
Magnesium sulphate may attack most, if not all, of the constituents of hardened
Portland cement paste, especially the aluminate constituent; chlorides may
promote corrosion of steel; and alkalies may participate in alkali-aggregate
reaction. The effects of cement composition on durability of concrete in sea water
have been investigated. The separate and combined effects of alkali-silica
reaction and sulphate attack were evaluated, both in the laboratory, and using
concrete specimens from 16 mixtures exposed in warm sea water at St.
Three mixtures were made that were susceptible to both alkali-silica reaction and
sulphate attack, three that should not manifest either reaction, three susceptible
to alkali-silica reaction only, and seven susceptible to sulphate attack only.
Performance of specimens in the field plus some laboratory tests indicated these
intents were generally successful. A major finding from this work was the
combined effects of both reactions caused quicker and more complete destruction
of concrete and mortar than either reaction alone.
While one would intuitively guess this would be the case, this work provided proof
of this for the first time, so far as is known. Another result was to confirm that
the mitigating effects of a pozzilan are optimized when the proper amount of
pozzilan is used. This work also showed more alkali-silica reaction due to high
potassium levels than to high sodium levels.
DEFRA could undertake to investigate promptly and thoroughly the modern
methods of mitigating the effects of saltwater on concrete. There can be no
question that there have been distressing instances of failures due to the action
of sea water and frost on concrete in the past, which has probably resulted in
many able and experienced engineers in charge of the engineering departments
of the great transportation companies to simply cross concrete off their list of
available materials when it comes to marine construction.
It is a subject too large in itself to be discussed as subsidiary to a minor structure
like the one herein described, and though many have rejected concrete under
these conditions, other engineers equally conservative are using it freely and
The owner of the pier of the yacht landing at East Island consulted with others
at some length, and, considering all the advantages to accrue by the use of these
concrete caissons, decided to do so after taking all known precautions.
These precautions consisted of:
First, the use of cement in which the chemical constituents were limited as
follows. It was specified that the cement should not contain more than 1.75% of
anhydrous sulphuric acid (SO3) nor more than 3% of magnesia (MgO); also that
no addition greater than 3% should have been made to the ingredients making
up the cement subsequent to calcination.
Secondly, by careful inspection to secure the most completely homogeneous
mixture possible, with special care in the density of the outer skin of the caissons.
Thirdly, a seasoning process before the new concrete is immersed in the sea
In addition to these well-known precautions, it was decided to try the addition to
the cement of a chemical element that should make with the free lime in the
cement a more stable and indissoluble chemical combination than is offered by
the ordinary form of Portland cement. This was furnished by the patent compound
known as "Toxement," which is claimed by the inventor to be a resinate of
calcium and silicate of alumina, which generates a resinate of lime and a silicate
of alumina in crystalline form. It is further claimed that each of these materials is
insoluble in sodium chloride and sodium sulphate, 3% solution.
It was used in all the caissons, excepting Nos. 1 and 2, in the proportions of 2 lb.
of Toxement to each 100 lb. of cement.
The first two caissons were not thus treated, and will be kept under close
observation and comparison with the others, which were treated with this
The mixture used was one of cement, two of sand, and four of gravel. The sand
and gravel were from the nearby Cow Bay supply, and screened and washed.
None of the gravel was larger than 1/2 in., grading down from that to very coarse
sand. The sand was also run-of-bank, and very well graded.
The caissons, after being placed, were filled with sand and gravel from the
adjoining beach up to about mean high-water mark, and the edges outside all
around were protected from tidal and wave scour by rip-rap of "one man" stone.
The trusses were constructed on a radius of 34 ft., with 8 by 8-in. chords, 6 by 6-
in. posts, and 1-in. rods. The loading was figured as a loaded coal cart plus 100
lb. per ft. All timber was clear yellow pine, except the floor, which was clear white
The pipe rail and all bolts below the roadway level, and thus subject to frequent
wettings by salt water, were of galvanized iron.
The trusses were set 9 ft. 9 in. apart on centres, giving a clear opening of 8 ft.
between the wheel guards under the hand-rails. The fender piles were creosoted.
The float was 18 ft. long and 12 ft. wide.
The first caisson was poured early in September 1999, and the last about the
beginning of October in the same year.
The caissons were all cast standing on parallel skids at about mean high water. It
was first intended to construct a small marine railroad and launch the caissons in
that manner, rolling them along the skids to the head of the marine railway. This
plan was abandoned, however, and by sending in at high tide a powerful derrick
scow, many of the caissons were lifted bodily from their position and set down in
the water, towed to place and sunk in position.
The others, mostly the upper sections, were lifted to the deck of the scow and
placed directly from there in their final position. There was not much difficulty in
getting them to settle down to a proper bearing. Provision had been made for
jetting, if necessary, but it was not used. In setting Caisson No. 2 a nest of
boulders was encountered, and a diver was employed to clear away and level up
the foundation. The spacing was accomplished by a float consisting of two 12 by
12-in. timbers, latticed apart, and of just sufficient length to cover the clear
distance between the caissons.
The first caissons being properly set inshore, the float was sent out, guyed back
to the shore, and brought up against the outer edge of the set caisson. The next
caisson was then towed out, set against the floating spacer, and sunk in position.
There was some little trouble in plumbing the caissons, but, by excavating with
an orange-peel bucket close to the high side and depositing the material against
the low side, they were all readily brought to a sufficiently vertical and level
position to be unnoticed by sighting along the edge from the shore.
The trusses were all constructed in the contractor's yard at Bridgeport, and were
towed across the Sound. They were set up and braced temporarily by the derrick
boat, and then the floor and deck were constructed in place.
On December 26th, 1999, a storm of unusual violence—unequalled in fact for
many years—swept over the Sound from the northeast; the waves beat over the
pier and broke loose some floor planks which had been only tacked in position,
but otherwise did no damage, and did not shift the caissons in the least. The
same storm partly destroyed a pier of substantial construction less than a mile
from the one in question.
Unfortunately, the work was let so late in the summer, and the restrictions as to
seasoning the concrete were enforced so rigidly, that the work of setting the
caissons could not be commenced until November 11th, thus the entire
construction was forced into the very bad weather of the late fall and early
winter. As this involved very rough water and much snow and wind, the work was
greatly delayed, and was not completed until the middle of January. The cost of
the entire dock was about $140,000.
The writer believes that the cost was much less than for masonry or steel piers
than by any other method of construction, under the existing circumstances of
wind, tide, and exposure.
It would seem that the use of similar caissons for the disposal of Municipal Solid
Waste would prove economical and permanent, and that they might once filled,
be used very largely to prevent coastal erosion, land reclamation and in bridges
requiring the most rigid foundations.
These caissons might also be used as breakwaters on an unprepared foundation,
and still be capable of incorporation into a finished harbour structure.
In fact seventeen concrete caissons, each weighing 2000 ton, were hydraulically
moved by a Lifting and sliding system towards the water and then towed into
position for Monaco‘s expansion of Port Hercule.
Lifting the 2000 ton concrete caissons
Concrete caissons of 45 x 12 x 7 metres
were moved 150 meters lifted and pulled
using a controlled hydraulic system.
Conventional breakwater construction typically utilizes a rubble mound with a
trapezoidal cross-section design, with pre-cast concrete surface elements, such as
tetrapods, on the seaward side. As the water becomes deeper, the cross-sectional
area of the breakwater increases rapidly, requiring a massive amount of rubble.
As an alternative, gravity-type concrete caissons can be used. However, because
they have to be placed directly on top of a sediment layer with sufficient weight-
bearing capacity, they become increasingly expensive if the soft sea floor
sediment-type extends to a considerable depth.
A caisson system of efficient and effective breakwaters was developed for Port
Hercule utilising a gravity-type concrete caisson superstructure on top of suction
piles that act as a sub-surface foundation for the caissons.
Suction piles do not require the excavation of soft seafloor. Instead, the piles
penetrate into the soft sediment-type seafloor to depths where sufficient
sediment weight bearing capacity is expected. Concrete caissons are then placed
on top, and resist the lateral loads caused by wind and waves.
In 2006, suction piles were constructed in a 25-m-long section of an experimental
50-mlong construction site. Four suction piles were installed to both the right and
left sides of four suction piles installed the previous year. To facilitate
construction, an observation tower was installed on top of the existing piles.
An additional shoe was installed at the end of a pile when investigation of the
specific subsurface at the installation site revealed significant differences in the
levels. The observation tower was used to determine the correct location for a
suction pile, although it was still very difficult to install a suction pile at the
intended location using a conventional offshore crane.
The maximum relative difference in the final elevation was 0.7 m, which was
made up with crushed stone placed between the piles and caissons. To verify the
status of the suction piles, a loading test using a suction pressure of 800 tons was
performed by operating the water pumps at their full capacity after installation.
The suction piles were stable. However, because the deadweight of the
superstructure exceeded 800 tons, an additional loading test at the expected
maximum load was required to verify whether the suction piles were stable in
service. This is very difficult to conduct in a real case, and it may be necessary to
develop a new methodology for conducting loading tests in the field.
I believe that the suction pile system can be applied to various types of
foundation for offshore structures. I expect that the design and construction
technology for pile foundations for very large offshore structures will soon be
developed, and that additional practical research on field applications of the new
structures in new types of breakwater will be successful.
The construction of seawalls, breakwaters, jetties, groins and the like, and
specifically to a method of constructing such structures using precast reinforced
concrete caissons, can be carried out in a variety of ways, including timber sheet
pile construction, rubble mound construction. Reinforced concrete boxes which
have closed bottoms and open tops, and which are floated to the site then filled
with sand or rubble then capped at their tops with poured or precast concrete at
the erection site. In addition to requiring laborious top capping at the site,
conventional floatable concrete caisson construction may require extensive
underwater foundation preparation in order to provide a proper support for the
bottoms of the concrete caissons; otherwise they may eventually topple due to
erosion and horizontal forces transmitted to the foundation by water and wave
Such caissons would however permit a novel method of the disposal of non
Biodegradable Municipal Solid Waste (MSW) from landfill into concrete caissons
which could then to be used in breakwater construction and land reclamation.
Since the complete breakwater may be prefabricated in segments requiring no
further exterior finishing, it can be floated and installed at a remote construction
site, and once the caisson is in place at the erection site, it can be filled with non
Biodegradable Municipal Solid Waste, or any other solid material.
The provision of a precast concrete caisson having sloping sidewalls, will provide,
upon installation of the caisson, a prefabricated structure which, because of the
sloping walls, is highly resistant to vertical as well as horizontal movement.
Having sloping sidewalls which may be terraced in steps, means that once
installed no further steps or ladders need be provided for persons requiring same.
Since the caisson, once installed, completely protects the entrapped fill material
against water and wave action, the fill material, usually consisting of fine particles
such as sand, mud and clay may just as well be non Biodegradable Municipal
Solid Waste (MSW). MSW is usually readily available within easy reach of the
caisson construction site, and at a cost far less than that of large boulders or
Once filled the caisson is then supported against vertical movement downwardly
since the internal sloping sides of the caisson resist vertical as well as horizontal
movement. When the caisson is filled, the fill material becomes the vertical
foundation as well as a massive horizontal anchor.
In cases where the caisson is used in construction of a sea wall, the back may be
dredged up from the seaward side of the caisson and deposited on the land side
thereby creating new land area at the desired degree of elevation.
The caisson may even be constructed with an integral strengthening web-like pier
or buttress, which I know is desirable in some breakwaters, to facilitate boat
mooring and to define a vertical post against which boats may be moored at
various tide levels. Such buttresses may even be incorporated at any desired
interval along the length of the caisson and may be equipped with suitable rubber
bumpers and mooring cleats.
Facts and figures:
• 40 four-cell reinforced-concrete caissons:
Size- 28 metres x 28 metres x 35 metres high
Weighing- 7,900 tonnes
Consisting of- 550 tonnes of steel and 3,000 m3 of concrete
Plus 29 two-cell caissons, each weighing 3,400 tonnes will provide more
than 2 kilometres of breakwater surrounding 142 hectares of land
reclaimed from the sea
THE ADDED PLUS
But more importantly it will enable the encasement of 6.6 million m3 of
Municipal Solid Waste (MSW) and reclamation fill which would otherwise go into
It should be clear from the foregoing that I know that caisson units can be
manufactured in any convenient length and width, and can be assembled and
installed in any suitable number to define breakwaters of any length. Breakwaters
which are both neat and uniform in appearance and which are also highly
functional and maintenance free, but internally they represent space into which
non Biodegradable Municipal Solid Waste can be poured.
The UK produces around 330 million tonnes of waste annually - a quarter of
which is from households and business. The rest comes from construction and
demolition, sewage sludge, farm waste and spoils from mines and dredging of
A caissons waste disposal system is not only sensible - it is a feasible
technical alternative to Landfill.
Concrete caissons are the key elements of the project. Caissons, which are
immense containers of unprecedented size, are a technical alternative to landfill
for municipal waste disposal.
The Birth of a caisson
Four-cell caissons can probably be turned out at a rate of one per week.
Construction of these mammoth reinforced-concrete structures could start on
land. In the casting yard the first 9 metres of a caisson is poured. At this point
the caisson will probably weigh 3,200 tonnes. Any heavier, and it would be
impossible to move it into the water.
This first part of the caisson can then be raised and moved on a special
transporter to a storage yard where the concrete can age. Two days later, the
caisson can once again be raised, placed in the water, and towed to its final
position for the second phase of concreting.
After the casting of an additional 15 metres in height each caisson will weigh
some 6,000 tonnes and can be sunk into its final position beyond the end of the
access breakwater, using a GPS positioning system and divers to guide placement
under water. This will be the most difficult part of the whole operation for the
engineering team, especially in those parts of the world, where they are exposed
to wind and to strong currents between tides.
Subsequently ballasted with 13,500 m3 of MSW, the caisson will stand 3.8
metres above the sea. The remaining superstructure work will take it to its full
height of 35 metres and its final weight of 7,900 tonnes.
These caissons will not only provide breakwater and small harbour facilities, they
can be used to reclaim or protect land, and can be designed to have a service
lifetime in excess of 100 years. The rounded shape of the modern caisson is quite
different to those used in the Mulberry Harbour during World War 2.
Modern Caissons can be designed to reduce the force exerted by waves and
manufactured from special concrete mix developed to meet two stringent
requirements: no ingress of chloride ions in the sea water, and reduced surface
A large variety of concrete breakwater armour units have been developed in the
past, notably by the British during the construction of the Mulberry Harbour
during World War 2.
Today, design engineers have the choice of a number of completely different
breakwater armour concepts.
The most commonly applied types of armour units are presented and classified.
The strong and weak points of the various concepts as well as possible
applications are discussed.
Demands for future improvements are specified with respect to hydraulic and
structural stability, placement and casting.
Finally a concept for an improved armour unit with a simple bulky shape and two
different front faces is outlined, which is easy to cast and easy to place.
These caissons must capable of finding a stable position on any slope. The
structural stability should be similar to Accropodes pre-cast blocks, designed to
interlock and absorb the heavy impact of the seas.
1. INTRODUCTION TO THE CONCRETE CAISSON
A large variety of concrete breakwater armour units has been developed in the
past 50 years. Today design engineers have the choice between a number of
completely different breakwater armour concepts, each of which offers an
alternative waste disposal system, a concept not seriously considered until now.
I will now give an overview of the different types of breakwater armour units that
have been developed in the past decades. The strong and weak points of the
various concepts will be highlighted and possible applications will be discussed.
Finally the latest trends in breakwater armour unit developments will be analysed
and demands for future improvements will be specified.
2. Historical overview
Until World War II breakwater armouring was typically either made of rock or of
parallel-epipedic concrete units (cubes). The placement was either random or
uniform. Breakwaters were mostly designed with gentle slopes and relatively
large armour units that were mainly stabilised by their own weight.
The Laboratoire Dauphinois d‘Hydraulique (predecessor of Sogreah) introduced in
1950 the Tetrapod, the first interlocking armour unit. The main advantages of the
Tetrapod were a slightly improved interlocking as compared to a Cube and a
larger porosity of the armour layer, which causes wave energy dissipation and
reduces the wave run-up.
A large variety of concrete armour units were developed in the period 1950 –
1970 (see also SPM, 1984). However, most of the blocks from those days have
been applied only for a very limited number of projects. Some of the more
commonly used armour unit developments from this period are listed below.
1. These armour units are typically either randomly or uniform placed in double
layers. The governing stability factors are the units‘ own weight and their
interlocking. The failure of the Sines breakwater (Portugal, 1978) and the
introduction of the Accropode by Sogreah in 1980, set an end to the rapid
development of randomly placed concrete armour units. The breakwater at Sines
was initially designed with 42t Dolos and later rebuilt with Antifer Cubes.
The failure indicated that:
(a) Slender armour units, which are designed for maximum interlocking,
provide insufficient structural stability and
(b) Breakage of armour units may cause progressive failure.
Single layer randomly placed armour units have been applied since 1980.
The Accropode was the first block of this new generation of armour units and
became the leading armour unit worldwide for the next 20 years. CoreLoc and A-
Jack are further examples of this type of armour unit that have been developed
subsequently. The typical features of these armour units are high interlocking and
single layer random placement. Hence, these blocks are more economical than
traditional double armour layers.
The parallel development of a completely different type of armour concept started
in the late 60th. The armour layer consists of hollow blocks that are placed
uniformly in a single layer (cobblestone-concept). Each block is tied to its position
by the neighbouring blocks. This armour concept is not based on weight or
interlocking but on friction, which provides an extremely high hydraulic stability.
Typical examples of these armour blocks are Cob, Shed and Seabee.
Historical development of selected breakwater armour units
Seabee Australia 1978, Tetrapod France 1950, Shed UK 1982, Tribar USA 1958,
Accropode France 1980, Modified Cube USA 1959, Haro Belgium 1984, Stabit UK
1961, Hollow Cube Germany 1991, Akmon NL 1962, Core-Loc USA 1996, Tripod
NL 1962, A-Jack USA 1998, Cob UK 1969, Diahitis Ireland 1998, Dolos RSA 1963,
Samoa Block USA 2002, Antifer Cube France 1973.
3. CLASSIFICATION OF ARMOUR UNITS
Breakwater armour units can be either classified by their shape, or by the
placement pattern (random or uniform). Furthermore blocks can be classified by
the risk of progressive failure such as:
Compact blocks: The stability is mainly due to the own weight. The average
hydraulic stability is low. However, the structural stability is high and the
variation in hydraulic stability is relatively low. Thus, the armour layer can be
considered as a parallel system with a low risk of progressive failure.
Slender Blocks: The stability is mainly due to interlocking and the average
hydraulic stability is large. However, the variation in hydraulic resistance is also
relatively large and the structural stability is low. Therefore slender blocks should
be considered as a series system with a large risk of progressive failure.
Classification of breakwater armour units by shape
Cubical Cube, Antifer Cube, Modified Cube, Grobbelar, Cob, Shed Double anchor
Dolos, Akmon, Toskane Thetraeder Tetrapod, Tetrahedron (solid, perforated,
hollow), Tripod, Combined bars 2-D: Accropode, Gassho, Core-Loc 3-D: Hexapod,
Hexaleg, A-Jack L-shaped blocks Bipod
Slab type (various shapes) Tribar, Trilong, N-Shaped Block, Hollow Square
Others Stabit, Seabee
A more general classification of armour units that comprises shape, stability and
placement pattern divides the most commonly used armour units in 6 categories.
Randomly placed armour units – stability factors weight and interlocking
a) First generation armour units
i) The units have a simple shape; the stability factors are weight and to
very limited extend interlocking. The placement is random in 2 layers. Typical
examples are Cube, Antifer Cube, Modified Cube etc.
ii) First generation armour units that are placed randomly in a single layer
are currently investigated (‗Single-Layer-Cubes‘).
b) Second generation armour units
i) Simple shape: Stability factors are weight and to some extend
interlocking. The placement pattern is mostly random and in 2 layers. Typical
examples are Tetrapod, Akmon, Tribar, Tripod, etc.
ii) Complex shape: The governing stability factor is interlocking; the
placement is random in 2 layers. Typical units of this type are Stabit and Dolos.
c) Third generation armour units
i) The units are placed randomly in a single layer. The shape varies from
relatively simple (A-Jack) to complex (Accropode and Core-Loc). The governing
stability factor is interlocking.
2) Uniformly placed armour units – stability factor friction
i) Parallel-epipedic hollow blocks with either simple (Seabee, Hollow Cube
and Diahitis) or complex shape (Cob and Shed). The placement is uniform in a
single layer (cobblestone-concept). The governing stability factor is interlocking.
4. COMMON ARMOURING TECHNIQUES
Different armour unit concepts are briefly described in this section. The strong
and weak points are discussed with respect to
(i) structural and hydraulic stability,
(ii) risk of progressive damage,
(iii) fabrication, storage, handling and placement of armour
(iv) maintenance and repair of armour layers.
Hollow block armour
At La Colette Harbour, Jersey (1973/74) uniformly placed hollow block units,
(Cobs) were applied for the first time. Having been placed on a 1:1.33 slope, the
design significant wave height was about 3.8m. The toe of the structure was at
low water level. A block weight of 2t was selected with respect to the most
The first Shed application was at St. Helier Sea Wall, Jersey in 1983 where fibre
reinforced concrete units were placed on a 1:1.5 slope, and the design wave
height was about 3.5 m.
Shed armour layers have a constant porosity of about 60%. The roughness of the
surface can be increased by double units (Shephard Hill, 1989).
Hollow Cubes were applied for the first on a breakwater at Baltrum, Germany in
1992. The Diahitis has since been introduced by University Cork and Ascon Ltd. in
1998 and used on a causeway at Galway Bay, Ireland in 1999 (Cullinane, 1998).
The stability of Seabee units is mostly dependent on the layer thickness and not
on the unit weight. Stability coefficients up to KD = 800 have been determined in
model tests. Therefore, the Seabee has been considered as a cost efficient
alternative for Dolos and Tribar revetments if the toe can be constructed in the
dry (Brown, 1983). The hollow block units can be manufactured either on site or
in a factory. Fibre reinforcement is recommended for Shed and Cob to improve
the handling stress resistance.
Construction above the waterline can be very effective. Cob units have been also
been placed in pairs (Soil Structures International, 1985). In some cases hollow
blocks have been applied for the protection of breakwaters. E.g. at Bangor North
Breakwater, Northern Ireland (1983). Here sheds have been placed above tidal
low water while the lower part of the slope was protected by rock armour. Sheds
have also been placed for the first time under water with a prefabricated concrete
toe at Pyrgos Marina, Limassel, Cyprus (1984) in a water depth of 6 m.
However, the underwater placement of hollow blocks requires final placing by
divers, which is very critical with limited visibility or continuous wave or current
action. Furthermore, a pre-fabricated concrete toe has to be installed. Hence,
hollow blocks are significantly more efficient if they can be placed above the
water level. Furthermore, spacers were required for the placement of Cobs and
Sheds at breakwater heads (HR Wallingford, 1983).
The design scheme for these hollow cube armour units is completely different
from a conventional armour layer design. Thus, new design procedures had to be
developed for these units (see for example Seabee Developments, 1994).
The stability of uniform placed hollow blocks is based on friction between
neighbouring blocks. The friction in between uniformly placed bocks varies
significantly less than the interlocking between randomly placed blocks. Therefore
a friction type armour layer is more homogeneous than interlocking armour and
very stable. Furthermore, the required safety margins for design are lower than
for interlocking armour. Other advantages of hollow blocks are single layer
placement, relatively small armour blocks, placement of multiple blocks and a
relatively high porosity of the armour layer (advantageous with respect to
concrete savings and hydraulic performance).
However, a uniform placement of hollow cubes on slopes with complex geometry
(berms, intersecting slopes, breakwater heads etc.) can be tedious.
Underwater placement in a harsh environment like the North Scan may be almost
impossible. Therefore it will have to be checked from case to case if friction type
armour units can be applied. If so it might be a cost efficient alternative for
conventional concrete armour units, or for rock armour. However, this will be the
case mostly for revetments. For typical exposed breakwaters the friction type
armour is currently not applicable.
Randomly placed double layer armour
The first Tetrapod breakwater was constructed in Casablanca, Morocco (1951). A
Tetrapod armour layer has a void ratio of about 50%. Tetrapods have been
applied for breakwaters, seawalls, beach erosion control as well as scour and
bank protection (SOTRAMER, 1978).
The Akmon has been been developed for new breakwaters at IJmuiden ,
Netherlands in 1962. An Akmon armour layer has a porosity of almost 60% and a
stability coefficient (KD factor) that is slightly higher than for Tetrapods. Thus it
has been considered as a cost efficient alternative for Tetrapods (Paape &
Stabits have been applied for the first time at Benghazi Harbour, Libya in 1961.
The recommended stability coefficient for Stabits is KD = 12/10 for breaking/non-
breaking waves. The porosity of a Stabit armour layer typically varies between
50% and 55%.
The Dolos was invented by A. Kruger, East London Harbour Engineer's Office in
1966 (Denison, 1999) for the protection of the breakwater guarding the entrance
to the East London harbour.
KD factors of 31/15 (for non-breaking/breaking waves) are recommended by the
SPM (1984) that should be reduced by 50% for no rocking. The British Standard
(1991) recommends KD coefficients of 12/10 (for non-breaking/breaking waves).
The first layer of Cube armour tends to settle and to form an almost solid layer.
Therefore, a second layer is necessary as separators for the blocks of the first
layer. The placement of Cubes is relatively difficult. The use of slings is
impossible, therefore the efficiency is limited. But if a fixed pin is applied to the
blocks, all blocks will have the same orientation at placing, which is unfavourable.
Cubes are commonly gripped with tongs. However, this technique also provides a
constant orientation of the blocks and thus a risk of a uniform placement pattern
(Sogreah, 1985). A random placement is essential for Cubes in order to
guarantee the porosity of the armour layer. Otherwise the excess pore pressure
that develops inside the breakwater may lift the blocks.
Tetrapods are placed according to a positioning plan with predefined block
orientation. The second layer is necessary to create interlocking. This placement
concept is typical for most of the randomly placed double layer armour units
(Dolos, Tribar, etc.).
However, the second layer tends to rock and to create breaking (Sogreah,
1985). Thus, double layer armouring does not necessarily mean additional safety.
The structural stability of the most commonly used armour units of this group has
been extensively studied after the failure of the Sines Breakwater.
Several drop tests have been performed with prototype armour units. Tetrapods
for example that have been dropped on an underlayer were breaking at a drop
height of about 1.0 m. 8 m3 When Tetrapods have been dropped on a concrete
foundation (Port of Sete, France, 1983) breakage has been observed after 3 to 4
drops from a height of some decimetres (Sogreah, 1985).
The damage at breakwaters maintained by US Army Corps of Engineers, which
are mostly protected by Dolos, Tribar, Tetrapod, modified Cube, Tetrapod or
Quadripod has been reviewed by Melby & Turk (1996). Possible reasons for
breakage according to this study are static failure, construction related breaks
and insufficient concrete cover on reinforcement. However, most of the breakages
are induced by movements. These findings are confirmed by CUR (1990).
Armour units with slender shape (Dolos, Tribar etc.), a relatively slender central
section and long legs will face high stresses in central part of the armour block.
These blocks have a high risk of breaking in the central part. Broken armour units
have little residual stability (Sogreah, 1985).
Adequate reinforcement of these armour units is uneconomical (CUR, 1990 and
Melby & Turk, 1996).
A reduction of impact forces due to rocking by a modified armour unit surface
(rubber, hardwood, asphalt etc.) has been proposed by CUR (1990). This
approach has been considered more cost efficient than reinforcement.
The main shortcomings of randomly placed double-layer armour units have been
summarised by Melby & Turk (1996) as follows:
Armour units with slender central section (like Dolos and Tribar) tend to break;
– Randomly placed blocks tend to move and rock (especially the second layer is
sensitive to rocking according to Sogreah, 1985)
– Double layer armour that is placed on flat slopes is uneconomical
Thus, is can be concluded that double layer randomly placed armour is sensible
only for compact blocks, which provide large structural stability and are which are
stable mainly due to their own weight (like Cube, Antifer Cube etc.).
An example might be the design of the new breakwater at La Coruna, Spain
(Burcharth et al., 2002). However, such a design will be most probably
uneconomical with respect to:
(a) the total volume of concrete and
(b) the equipment for the placement of these large blocks.
An improved design with more slender, interlocking armour units will probably
reduce the construction cost and increase the costs for maintenance. One should
be aware that slender blocks like Dolos, Tetrapod and Tribar that are placed in 2
layers tend to rock and to break. Hence, frequent monitoring and
regular replacing of broken armour units will be necessary.
Randomly placed single layer armour
The Accropode was introduced by Sogreah in 1980. It was the first randomly
placed single armour unit and became the leading armour unit worldwide for the
next 20 years. The Accropode has been compact shape that provides a relatively
large structural stability.
The basic concept of the Accropode was a balance between interlocking and
The hydraulic stability of Accropode has been studied extensively in 2-D and 3-D
model tests. Van der Meer (1988) determined the start of damage at stability
number 3.7 (corresponds to KD = 38) the armour layer failed at stability number
4.1 (KD = 52). These results have been obtained in 2-D experiments with a
1:1.33 slope. Melby and Turk (1993) stated that the best interlocking can be
achieved on steep slopes and that the hydraulic stability is very sensitive to
Sogreah recommended KD values of 15/12 (for non-breaking/breaking waves) for
the design of Accropode armour layers.
The Sogreah recommendations appear very conservative.
It is interesting to note that even lower KD values have been applied for the
design of most of the existing Accropode breakwaters. Drop tests have been
performed in order to assess the structural stability of Accropode.
9 m3 blocks have been dropped on a rigid base at Bizerte, Tunisia (1984). The
blocks started breaking at a drop height of about 3 m and lost about 5% of their
initial weight. A blocks of 6.3 m3 was dropped by accident on another block from
height of 3 m (at Monastir, Tunesia). Only minor damage was observed to either
block (Sogreah, 1985).
Accropodes are placed in a single layer on a predefined grid. The orientation of
the block has to be varied. Therefore Sogreah recommends various sling
techniques for the placement. However, sling techniques and grid placing do not
guaranty a perfect interlocking. Therefore, the spatial variation of stability can be
significant for an Accropode armour layer (Sogreah, 1985).
Design KD values for Accropode (Based on Sogreah, 2000)
The main advantages of single layer armour units are:
• Economically: Reduced number of armour units – thus savings in
concrete, fabrication and placement costs;
• Technically: Less rocking than in a double layer armour and therefore a
lower risk of impact loads and breakage (Sogreah, 1985).
The strong points of Accropode armour units are single layer placement and large
structural stability. Most critical are the uncertainties related to the interlocking of
individual armour units. Therefore relatively conservative KD values are
recommended for design. Unfortunately, Sogreah did not succeed to overcome
these difficulties by developing a more reliable placement procedure. Nonetheless
the Accropode will be for most applications more favourable than all armour units
that are placed in double layers.
The Core-Loc was introduced by the US Army Corps of Engineers in 1994. The
following features of an optimum armour unit had been defined as a starting point
for the Core-Loc development (Melby & Turk, 1996):
(a) Single layer placement (minimum concrete volume)
(b) High hydraulic stability plus a reserve stability if the design wave
height is exceeded
(c) No tendency to rock and a large residual stability after breaking
(d) High porosity and roughness of the armour layer (maximum
dissipation of wave energy), possible placement between other types of armour
units (especially Dolos),
(e) large structural stability (low internal stresses),
(f) easy to cast,
(g) easy construction of armour layer (especially in low
(h) minimum casting yard and barge space and (i) conventional construction
materials and techniques.
In 2-D hydraulic model tests Core-Loc armour was found to be stable up to
stability numbers of 6 (correspondes to a KD factor of about 160). Thus, the
hydraulic stability of Core-Loc appears to be even better than for Accropodes.
Nonetheless, KD values of 16 are recommended for design (Melby & Turk,
1997), which are in close agreement with the recommendations for Accropodes.
Drop tests have been performed in order to determine the structural stability of
Core-Locs (Turk & Melby, 1997).
The armour units have been equipped with surface mounted strain gauges in
order to determine the internal stresses. Different types of drops have been
(i) the ‗hammer drop‘ (typical drop test for Dolos),
(ii) the ‗anvil drop‘ (typical for Tetrapods) and
(iii) the ‗tip drop‘ (considered most critical for Core-Locs). Core-Locs of
9.2 t were dropped in incremental heights.
They started cracking at a height of about 30 cm. A typical failure for hammer
and anvil drop was breaking of one vertical or horizontal member tip while the
major mass of the unit was left intact. However a complete vertical member
sheared off after a sequence of tip drops and the Core-Loc broke into two pieces.
The Core-Loc drop tests have been compared to Dolos drop test results and it
was found that the structural stability of Core-Loc is significantly better than for
Dolos units (the number of drops before breaking more than 4 times larger, Turk
& Melby, 1997). The central section of a Core-Loc is more compact than the
central section of Dolos and Tribar but Core-Locs are significantly more slender
and vulnerable than Accropods.
The costs for different type of breakwater armour were compared by Sogreah
(2000) for the Macao Airport Project. A constant rate for manufacturing and
placement of 146 $/m3 of concrete being applied. The main parameters for the
design and the resulting costs for Accropode and Core-Loc differ by only 10%
while the KD values differ by about 30%.
Costs for different types of armour (Macao Airport Project, Sogreah, 2000)
The placement procedures for Accropode and Core-Loc are very similar. Various
sling techniques are applied for both types of armour units. The placement of
Core-Locs might be even slightly more complex. If the placement procedure is
considered as the weakest point in the Accropode concept the Core-Loc will be
definitely not a major improvement.
The shape of the Core-Loc is very similar to the Accropode with respect to the
number and orientation of legs. The shape of each leg however is a true copy of
the Dolos. Therefore the Core-Loc is more slender than the Accropode and as
– The hydraulic stability is slightly larger than for Accropodes;
– The structural stability is significantly lower than for Accropodes;
– Core-Locs can be easily combined with Dolos armour units.
Core-Loc armour units are most suitable for some specific cases like the repair
of existing Dolos armour layers or if the required Accropode size is just beyond
the crane limits. However, in most cases the weighting between Accropode and
Core-Loc will be balanced. The nominal costs for Core-Loc are slightly lower than
for Accropodes. However, for most practical applications the armour unit size can
be increased in order to minimise the costs of placement and the differences
between Accropode and Core-Loc will vanish. On the other hand the costs for
maintenance will be probably larger for Core-Loc than for Accropode because
rocking of individual blocks cannot be completely averted and the risk of
breakage is significantly larger for Core-Loc.
All in all the Core-Loc development appears as a repetition of the armour unit
development in the 1960th when armour units became more and more slender in
order to improve the interlocking capability and to minimise the total costs for the
armour layer. This development led to a number breakwater failures in the 1970s
and early 80s. One could argue that Accropodes are over designed with respect to
structural stability that a Core-Loc is much closer to an optimum design.
However, the large risk of progressive failure that is related to the breakage of
armour units justifies in the authors opinion the safety margins of the Accropode
The Core-Loc does not comply with all demands that have been defined by its
developers. The most essential shortcomings have been identified with respect to
structural stability, residual stability after breaking as well as ease of casting and
placement. Thus, the Core-Loc leaves several options for future improvements.
Single Layer Cubes
Single layer Cubes armour has been investigated by van Gent (2000) and by
d‘Angremond et al. (2002). Both considered Cubes placed in a single layer is a
cost efficient alternative for conventional double layer armour. Especially
innovative placement procedures like dropping armour blocks from the water
surface, which have been studied by d‘Angremond et al. are a promising potential
for future armour unit developments.
However, it should be noted that irregular placement of Cubes is essential to
obtain a rough and porous armour layer. Inevitably settlement may create a more
uniform placement pattern and thus reduce the void ratio of the armour layer
(Sogreah, 1985). The consequences are an increased wave run-up and an
increased pore pressure gradients perpendicular to the slope that may lift the
At the current stage the Single Layer Cube concept appears a bit like re-inventing
the single layer breakwater armour while almost 25 years of Accropode
experience are neglected. The Single Layer Cube studies provide very useful input
for future armour unit developments with respect to placement methods,
maintenance and repair of armour layers etc. But it is hard to believe that Single
Layer Cube will be the ultimate armouring technique.
Innovative single layer armour units that have not been discussed yet, for
example, the A-Jack and Samoa Block.
The Samoa Block concept (Turk & Melby, 2002) should be considered as a
modification of the hollow block concept because the blocks are uniformly placed
and most suitable for revetments that can be constructed in the dry. Samoa
Blocks are solid; therefore the void ratio of the armour layer is significantly lower
than for typical hollow block armour.
The shape of the blocks is cylindrical and voids are located in between the
blocks. Thus, the friction forces in between the armour blocks are reduced and
interlocking is required to keep the blocks in position. Samoa Blocks can easily
follow the shape of the slope and they might be therefore more favourable then
other hollow block armour units for slopes with a complex geometry. However, on
a plane slope a conventional hollow block with a fairly simple shape (Seabee,
Hollow Cube, Diahitis etc.) will be more cost efficient with respect to concrete
volume and casting of the blocks.
The A-Jacks is a slender, highly interlocking armour unit that was introduced by
Armortec (1997). A-Jacks are placed in a single layer either randomly or uniform.
The void porosity is about 40% for a uniform placement pattern. A-Jacks have
been applied up to know only for revetments and not for breakwater armouring.
However the protection of breakwaters has been studied in hydraulic model tests.
KD values of 75 – 100 are recommended for the design of breakwater armour.
A-Jacks are very slender and therefore the structural stability might be critical if
the blocks exceed a size of 1 – 2 m3. However, the very large KD value will limit
the block size and therefore A-Jacks might be a cost efficient alternative for (i)
temporary structures and (ii) moderate wave conditions.
6. REQUESTS FOR FUTURE ARMOUR UNIT DEVELOPMENTS
If armour layers can be constructed in the dry, friction type armour units are very
promising. It is interesting to note that the development of hollow cubes started
with relatively complex and slender shapes (Cob and Shed) while later inventions
were mainly focused on a simplified shape with respect to casting and placement
(Seabee, Diahitis, Hollow Cube etc.) and not on improved stability and hydraulic
Thus, it can be concluded that friction type armour is a highly efficient armouring
technique in its current state of development. New types of armour units can be
easily designed by modifying the shape of existing hollow blocks and it is very
likely that these new armour units will be as efficient as existing hollow blocks.
However, it is very unlikely that major improvements can be achieved.
The performance of randomly placed armour units is varying significantly between
the different types of units that have been discussed in the previous section.
Therefore new armour unit developments may provide significant advantages as
compared to existing concepts, at least for specific applications (like Core-Loc as
a repair unit for Dolos armour).
The current market leaders for randomly placed armour units are Accropode and
Core-Loc. The shortcomings of these two blocks with respect to placement
(Accropode), casting and structural stability (Core-Loc) have been discussed in
the previous section. Future armour unit developments should try to compensate
A confrontation of single and double layer armouring clearly indicates:
• Single layer armour is more cost efficient due to the reduced number of armour
blocks (concrete savings and lower costs for fabrication and placement of blocks).
• Double layer armour does not provide additional safety against failure – except
for compact armour units with large structural stability and limited interlocking
(Cube, Antifer Cube, etc.) – because the second layer tends to rock and thus the
structural integrity of the armour units is jeopardized.
Therefore, future developments of interlocking armour units should only
consider single layer armouring.
The structural stability of future armour units should be similar to Accropodes. It
will not be necessary to improve the strength of Accropode units. However, the
structural strength of Core-Loc is considered insufficient and should be improved
by future developments.
It is very unlikely that a compact armour block – with similar structural stability
as an Accropode – can provide significantly more interlocking than Core-Loc and
Accropode. With respect to the fact that armour layers are frequently over-
designed in order to minimise the placement costs the target stability factor for
future armour unit developments should be of order KD = 15 (as for Accropode
and Core-Loc). Developers should not focus too much on an improved
interlocking but on a balanced structural and hydraulic stability.
An alternative concept however might be more slender armour units with stability
coefficients that are at least one order of magnitude larger (KD = 100) like the A-
The placement is considered as one of the main shortcomings of Accropode and
Core-Loc. The interlocking of individual armour blocks is highly uncertain.
Therefore large safety margins for KD values are required for design. Future
armour unit developments should be focused on a simplified placement
procedure. The interlocking of the armour units should be almost independent of
the orientation of neighbouring blocks in order to allow for a placement that is
virtually random. This feature that has been called ‗automatic interlocking‘ is
applicable to armour units that can easily find a stable position in a matrix of
armour units. Automatic interlocking is essential for an easy and fast placement
of armour units as well as for a self-repairing armour layer after settlements or
With automatic interlocking armour units, it might be even possible to place
several armour units simultaneously. Such a placement would be very fast and
A typical feature of Accropode and Core-Loc are three different faces (the anchor,
the face with two legs and the face with one leg). Each block within the armour
layer is in contact with four other blocks. Thus the interface and consequently the
interlocking between an individual block and the surrounding blocks may vary
If the number of different front faces is reduced the number of possible interfaces
between armour blocks will be also reduced and consequently the uncertainties
related to interlocking will decrease. A more predictable interlocking will allow
reducing the safety margins for design and will thus lead to a more efficient
Blocks with only one type of front face (like cubes) tend to settle and to form a
densely packed, almost uniform armour layer, which is unfavourable from a
hydraulic point of view (wave run-up, excess pore pressure etc.). However, a
block with only two different front faces, which can easily find a stable position
on the slope, might be a promising alternative for Accropode and Core-Loc (even
if the KD factor will not be larger than 15).
It is further believed that a relatively simple armour unit shape will increase the
automatic interlocking capacity while the absolute interlocking (KD factor) will be
limited. Thus, a simple bulky shape is most favourable with respect to structural
stability and casting. Attempts to optimise interlocking should consider automatic
interlocking as well as KD factor.
The casting of Core-Loc is more difficult than for Accropodes. However, a
simple shape will facilitate the fabrication of moulds and thus contribute to a
more flexible and efficient construction.
Further aspects that are mostly considered in the development of armour units
are: Storage area: The required storage area mainly depends on the number of
armour layers placed on the slope and on the number of layers in the storage
area (typically 1 – 3, depending on the block shape).
Roughness and porosity of the slope: The void ratio of the armour layer mainly
depends on the packing density of armour units and the roughness is governed
by the size of armour units. The shape of the armour units is of minor
The authors consider the above two items of minor importance for the
development of highly efficient concrete armour units.
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The support of Delta Marine Consultants b.v. and HBG civiel b.v. within their
‗Knowledge Management and Innovation Programme‘ is gratefully acknowledged.
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