07 September 2011

An engineer's view of reclamation and dredging

What actually goes on during the dredging and reclamation that are happening on many of our shores?
Dredging near Cyrene Reef
Massive dredging near Cyrene Reef, July 2011
I came across some articles in Port Strategy with engineering details on such coastal works. Including how environmental concerns are met.

For example, I learnt that for areas with "underlying soil that is very weak, with soft clay materials", this requires the removal of "20 foot or so of soft material" before reclamation can start and "a lot of drainage work afterwards". In areas "where environmental regulation may require soft deposits to remain", deep cement mixing may be used. "Deep cement mixing is a bit like using a giant food blender and dosing the area with cement. Once the cement in the ground goes off – it takes seven to 28 days to come up with reasonable strength – you then have a breakwater or embankment which can be used to retain the soft soil behind."

There's an article about how reclamation can be speeded up. Also another article on how instead of dumping dredged materials, these are made into geotubes which are used in breakwater and protection against coastal erosion.

Beating land pressures
Felicity Landon Port Strategy 7 Sep 11;

High land prices, particularly in coastal areas, make reclamation a relatively ‘cheap’ option for many port expansion projects. Felicity Landon looks at the latest developments.

These may be quieter times in terms of land reclamation, when compared with giant projects such as Dubai's The Palm, or the Rotterdam port expansion now nearing completion – but there is still steady demand around the world, and an expectation that another flood of major projects may be just around the corner.

“It all depends on the economic climate,” says Robert de Bruin, the new spokesman for Dutch dredging and marine contractor Van Oord. “Major dredging companies are investing a lot in heavy self-propelled cutter suction dredgers, because we see in certain markets – for example, the Middle East and Australia, a boost in port projects. That is the result of demand generated by, for example, China. So we are looking at economic or geographical changes that can push forward projects.”

Van Oord highlights the large number of port construction or expansion projects going ahead in South America, and says demand is also picking up in Africa. “These are far from the very large-scale projects we have seen in the past in Singapore, Hong Kong and the Middle East. But we expect in the long run, further ahead, climate change will drive the need for large reclamation projects again.

"Low-lying delta around the world will be affected very dramatically by sea level rises, so we expect that large quantities of material will be needed.”

With increasing environmental pressures, lack of space for expansion, and high land prices, Mr de Bruin says: “Everybody seems to forget that reclamation is relatively cheap. Of course, when that is said by a dredging contractor, people say ‘he would say that’ – but if you compare current land prices, in particular in coastal areas, with prices for land reclamation, there is large difference.

“If you are looking to buy square metres in Dubai or Singapore, you have to pay large sums of money. The average land reclamation costs €125-150 ($179-$215) per m2, no matter what the water depth is, even in Dubai. It is very, very cheap. So for new development projects, it is a good choice.”

Land reclamation isn’t a sector for constant dramatic technological advances – there have been some new techniques, but recent years have mostly seen the refining or developing of old technologies, according to Van Oord.

“The scale of the equipment has increased dramatically over the past decade and we have installed power equipment and introduced the use of rainbowing techniques. We have also introduced barge-loading facilties onboard trailers, so that barges can be loaded during the dredging operation. But apart from longer, deeper, wider, more capacity, including the size of vessels, there are no significant changes,” says Mr de Bruin. “Demand always drives innovation but the technology at present meets our needs. The dredging rates we can achieve today would have been completely unbelievable ten years ago.”

Every reclamation project is different, of course, and a particularly challenging factor is when the site has underlying soil that is very weak, with soft clay materials. This requires very careful reclamation and a lot of drainage work afterwards – and in some instances Van Oord says it has had to first remove 20 foot or so of soft material before it can start reclamation.

“In other areas we have to remove very hard material. Normally designs are drawn up by consulting engineers – we look at it differently. From the engineering point of view it might be very good design but from the construction point of view there might be better alternative. Early contractor involvement is vital.”

A solution to the problem of soft underlying soil – the removal of which is increasingly restricted by environmental regulation – is ‘deep cement mixing’. Colin Spinney, of Royal Haskoning, says: “Recently we have been working on this method, which is fairly new to us, of mixing cement into existing soft soils, so leaving all the soils in situ. Deep cement mixing is a bit like using a giant food blender and dosing the area with cement. Once the cement in the ground goes off – it takes seven to 28 days to come up with reasonable strength – you then have a breakwater or embankment which can be used to retain the soft soil behind.”

In estuary areas where environmental regulation may require soft deposits to remain, infilling on top can be very tricky, he points out. “Yes, you do get settlement but you can also have stability problems – and failure is cost and time. Deep cement mixing can provide an edge structure around the area to reclaim.”

He says this technique was prohibitively expensive in the past and therefore not adopted to a great extent by European contractors, but it has increasingly been used in Asia. Not only does it offer an environmentally acceptable answer, but it can speed up the process of achieving the required pre-ground conditions and therefore construction.

However, the process is far from universal in terms of application – the ‘recipe’ must be right, and that depends on the soil type, sea water and other conditions. Laboratory trials and in situ tests are needed to ensure the right dosage of cement.

The use of geotubes – large bags of geotextile filled with soft sediment and perhaps polymers – is also increasing in land reclamation. The bagged material can be placed on the seabed and used to form an edge structure or breakwater, again providing stability relatively quickly.

Belgian group Jan De Nul will use geotubes for a different purpose in its contract for the extension of the Brazilian naval base at Itaguai, in the bay of Sepetiba, in Rio de Janeiro state.

The first stage will be to dredge the contaminated soils – the dredged material will be stored in several layers of geotubes on navy land. Once all the contaminated soil is removed, Jan de Nul will bring the whole area to a sufficient depth so that hoppers can gain access and start dredging the deeper layers. All dredged material will be dumped 58 kms from the site.

Sand for the reclamation of quay surfaces and shipyard areas will be dredged from outside the bay; the reclamation will be by means of rainbowing (a method for discharging huge quantities of sand in shallow locations) and a floating pipeline with spray pontoon.

About 250 kms northeast of Rio de Janeiro, Jan De Nul is working on the construction of a new port and massive industrial services zone for LLX, the logistics branch of EBX group. The breakwater and quays of the port – to accommodate capsize vessels – are being completely built on the Atlantic Ocean. In total, 5.3m m3 of sand will be dredged out at sea and pumped ashore by the hopper dredge Cristobal Colon, via a submerged pipeline and discharge pipe up to 7 kms long.

In Australia, the Baulderstone-Jan De Nul consortium recently completed the reclamation for the Port Botany Bay container terminal expansion. A key design feature of the new port infrastructure is the quay wall, built up with 200 massive counterfort units, 9 metres wide, 20 metres high and weighing 640 tons each.

All units were cast on site and installed on an engineered trench excavated to 30 metres. Behind the concrete wall they form, nearly 8m m3 was pumped to form the new land.

Settling for stability
Stevie Knight Port Strategy 30 Aug 2010;

In the area of land reclamation, size matters – and so does money, of course. The downturn and uneven economic recovery has led to the withdrawal of a good number of smaller ships, fuelling the relentless drive for size.

At the same time, ports in some areas face more difficulty than ever in securing finance for – or even justifying – major capital projects involving land reclamation and the construction of deepwater quays capable of handling ever larger vessels.

Further, if you want a quay suited to the loading and unloading of the larger ship sizes, it can mean building a structure capable of supporting 1,000 tons on each crane leg.

But these factors are also mixed with growing environmental awareness, explains Colin Spinley of Royal Haskoning. “Recent changes in legislation have meant increased pressure to leave poor quality soil in situ. While this reduces dredge and dump quantities and reduces the amount of fill needed, it also creates its own problems as the poor material needs to be improved and stabilised in some way,” he says.

The need for deeper berths coupled with heavier cranes has also created an engineering issue, he says. “It is very difficult to be certain that increased loads and deeper dredging can be applied safely to existing structures.” Existing quay structures were often designed economically and have little extra margin against deepening or increased loading, he explains.

For part of its resistance to sliding, a quay wall is dependent on the dredged or “passive” side of the wall. Increasing the crane loads on top of the wall, while at the same time removing passive resistance, can reduce the factor of safety against failure of the wall, says Mr Spinley. Further, for quay walls that have been operational for many years, it can be difficult to determine the existing structural condition. Although it is possible to make visual inspections, there is a limit to what you can see underwater, making it difficult to ascertain much about the structure’s integrity as a whole.

Owing to the limitations of existing quay walls and because of uncertainties, it is often necessary to reclaim land either directly in front of the original quay or build out into a completely different area of the port to gain the required deepwater.

Another development for Royal Haskoning has been a greater emphasis on seismic design issues, from the inclusion of seismic structural design to undertaking liquefaction assessments for reclamation fills. This increase has been driven by a shift towards large port developments in locations subject to earthquake loading and from a heightened awareness of the potentially devastating effect on structures. Mr Spinley adds: “You can see this in its most dramatic form by looking at the quay and attendant crane collapse at Haiti.”

At the other end of the scale, there are major challenges for engineering projects as emerging economies show explosive growth.

Cai Mep International Terminal (CMIT), one of a handful of new deepsea terminal developments at Cai Mep, 80 km from Ho Chi Minh, Vietnam, has needed large-scale reclamation involving 3-3.5m cu m of earth. “However, fast tracking a development like this brings up a few issues,” says Paul van Weert, head of civil engineering at APM Terminals.

“Ideally you want all of the settlement to happen before you start on the infrastructure building. This isn’t always possible in an economic way, so in projects like these we set up a number of vertical dewatering drains together with a six to nine metre high surcharge load to speed the process.” Although these measures are effective in places like Cai Mep, which is a low-lying tidal area, some 15% of the settlement process is still left to take place while the terminal is in operation.

This, on a project the size of Cai Mep, makes for a huge amount of extra soil. Fortunately, most of this could be pumped in hydraulically from the dredge, which was also sizeable.

To optimise the required quantity of surcharge material, the site was split into two areas. The first has to be completed six months prior to the second. The surcharge material from the first area is used as fill and surcharge in the second. The issue of surcharge loading in an area with a very soft subsoil is trickier than many realise. When a site is completely covered with a surcharge mound (which can require an enormous amount of earth in one go), there is a serious risk of landslides. To minimise this risk, the contractor had to increase the surcharge in three to five steps, adding some two metres of material each time. Along the waterfront it was decided to use vacuum drainage in order to speed up the consolidation, limit the total surcharge height and thus limit the risk of landslides.

As Mr van Weert explains, you cannot always be sure that the effect will be even. “We measure the differential settlement with care. If the reclamation settles unevenly, it could later cause a number of serious problems, including undermining things like the stability of the box stacks – so if we see it begin to happen, we put extra loading on to that part of the construction.”

Mr Spinley agrees: “The ground is often highly variable, meaning that there is a risk of differential settlements on any site, even where surcharging has occurred. Where fill is placed against existing structures in port expansion projects, you have to pre-load the interface, just to avoid getting a step.”

It is essential to think about stability issues during this part of the build, he adds, because if the surcharge is loaded up too high too quickly, there is a possibility that a failure could occur during construction – with disastrous consequences for programme and budget. The decision is often taken to work in stepped phases to minimise the risk as well as the amount of earth needed.

Both Mr Spinley and Mr Weert say it is a finely tuned balancing act between trying to achieve settlement as quickly as possible in order to get the terminal operational, and the need to maintain stability throughout the works.

Other measures in consideration of a minor differential settlement have been taken up by the Cai Mep facility. Taking into account future maintenance and repair cost, a flexible pavement system consisting of a cement-based mixture with concrete pavers on top was selected.

However, the client often has its own settlement operational limits which are related to pavement levels and controlling slopes for handling equipment. This can be down to a number of millimetres over three decades.

Colin Spinley concludes that along with the growing environmental awareness, projects will just have to plan around the issues that are thrown up when putting down foundations on a softer base.

“Increasingly ports will recognise that you can’t always construct new ports economically on very short timelines,” he says. “It is becoming ever more important to understand the programme and cost risks posed by poor ground conditions and to identify ways in which these risks can be managed. This may mean lengthening construction times, increasing the operational tolerance to total and differential settlements, or innovative approaches that blend all of these risks together.”

Dredging can unlock a valuable raw material
Port Strategy 7 Aug 2009;

Just as Dickie Attenborough and Steve McQueen had to invent ingenious ways to dispose of spoil as they worked on tunnels through which to escape Stalag Luft III in The Great Escape, so dredging work requires ingenious solutions to find alternative uses for the spoil it creates.

For a start we could stop calling it 'spoil', says a trio of Irish authors in a paper for Terra et Aqua, the journal of the International Association of Dredging Companies. Call it dredged material, say Colm Sheehan, Joseph Harrington and Jerry D Murphy, and it might be seen as a useful product, rather than inconvenient waste.

Although tiny by world standards, Ireland's ports and their dredging requirement are a microcosm of global activity, as a balance is sought between economic and environmental concerns.

It's recognised that the potential for using what's been dredged in the past may not have been explored as well as it might have been, and the country's Department of Agriculture, Fisheries and Food has changed its dumping-at-sea licence application form to reflect the fact. It says: "The dumping of dredge spoil at sea is only acceptable when other means of disposal are ruled out for ecological or sound social or economic reasons. Even so, for ecological/environmental reasons, the dumping of the waste may not be permissible in all cases."

That puts Ireland between a rock and a hard place. Its dozen main commercial ports form the conduit for 99% of the country's imports and exports, with an annual value of up to ¢120bn ($170m). Dredging is obviously vital to sustaining that, so how to use the material it produces?

At the tiny Fenit Harbour, to the west of Limerick in the country's south-west, a capital dredging project is being planned that it's hoped can be linked to maintenance dredging in the future. The clever part is the use of geotubes, into which dredged material with at least 40% solids content is pumped. The water flows out through the tube walls, and the remaining sediment forms the basis of, in this case, a new breakwater and protection against coastal erosion.

Geotubes were used extensively to create protection from storms and the base of beaches for residents in the Amwaj Islands, an artificial island development in Muharraq, Bahrain. The island perimeter uses 30km of 13m-circumference GT 1000 geotubes to create the perimeter, which was then infilled with more than 20 million m³ of sand and stone.

Sheehan, Harrington and Murphy advocate geotubes as a potentially-viable solution providing a means of using the dredged material sustainably not only to prevent at-sea dumping, but also to make the most of economic benefits secured by not having to quarry and move rock that would otherwise be needed. They say: "This research may be applicable to a variety of sites around Ireland once the coastal structure being considered is within pumping distance of a port or harbour with a dredging requirement."

IADC president Koos van Oord believes it's part of his industry's challenge to develop innovative solutions. He says: "In an island nation such as Ireland where dredging is essential, the search for more cost-efficient means of achieving coastal security, as well as port development, can be combined by using, rather than disposing of, dredged material."

More articles about land reclamation on Port Strategy.

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