Beating land pressures
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.
“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.
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