A leg to stand on
Stevie Knight explains the merits of getting dredging contractors onboard from the off
Short of resources, and probably short of time. It’s an ongoing challenge of our age, but both of these issues can clash badly when it comes to land reclamation.
Phill Sanderson of Royal Haskoning explains it’s usually the rapidly developing nations that are most limited with the best kinds of fill to put underneath a new quay.
Generally, ports want to be able to use the material available to them that has come from channel or pocket deepening. However, this stuff is usually not that suitable for foundation work as there can be a number of issues, running from contamination to simply how much water it picks up. This last point is connected to the biggest issue - settlement.
“Normally you put in wick drains or similar, and use additional loading to surcharge the surface, but it can still take a year to settle,” explains Mr Sanderson.
No time like the present
However, ports can be impatient - they often ‘want it now’, but using it as rough storage area with temporary flexible surfaces like gravel and putting on the hard paving later is not ideal as there will be some flex in the surface. Also, the interaction with reclaimed land and its critical areas like rails or gantry tracks also needs attention as piles have to be driven through the fill and a concrete deck cast on top.
Obviously you need to calculate the settlement very carefully alongside the placing of fill, says Mr Sanderson. For example, clay balls need to be laid in uniform layers, so one layer compresses the next. Silt too has to avoid being placed in a ‘wedge’ and dredgers will often just tend to rainbow out some materials if left to their own devices instead of using a spreader pontoon.
Further, actions such as vibro-compaction also have to be carried out in their specified design sequence. All the extra interventions, including the monitoring, add to the expense.
From this point of view its going to be much easier if the people laying down the fill really ‘get’ the importance of doing it right. This is much easier to achieve if you have got together from the beginning as a team and this includes the contractor. If the contractors come in from the cold late on, it’s a recipe for disputes, says Mr Sanderson.
Even harder to sell to the client than the need to wait is the involvement of the contractors up front. So much can go slightly askew, beyond the soil itself, if the contractors haven’t been brought in early. But this kind of move tends to ring alarm bells with customers who are less than happy with an outcome where think they are paying more, not less, than a straightforward competitive tender.
On the other hand, the procurement process still needs an element of competition, and the port’s stakeholders need to be shown that they are getting good value for money. It can, obviously, be a ticklish business, doing this at an early stage when the design hasn’t been progressed very far and the project isn’t yet set in stone. But operators should not underestimate what this kind of collaboration can bring, nor should they underestimate what happens if the contractors haven’t ‘bought into’ the project. The point is, while Early Contractor Involvem
ent (ECI) is on its way to becoming more accepted, the take up still isn’t all that it could be.
The results of ECI in not just pushing but creating a whole new envelope are outlined by Port Botany’s 63 hectare reclamation which literally rested on Jan De Nul’s early involvement.
Yves Bosteels of Jan De Nul explains the first challenge was putting down enough land to get a foothold – the company had been told it couldn’t use even a metre of the port’s existing land.
Secondly, “every single bit of the fill had to be reclaimed from the existing site ‘envelope’,” explains Mr Bosteels. “We had to move 11m cubic metres of build material. That’s four times the volume of the Pyramid of Cheops, from a relatively constrained area.
”Complicating it all was the nearby airport, which put the takeoff runway within 200 metres of some parts of the dredging site. “Occasionally the airport controllers would look over at how close the dredgers were, and get worried about the possibility of destabilising their runways,” he adds. “It wasn’t always easy to negotiate our way through. Compliance was paramount.”
Third was the tight timeframe. While it’s obvious that the vessels and machinery used for land reclamation are massively expensive so you can’t start working out the plans after getting them on site, Jan De Nul took this whole aspect a stage further into a massively pre-planned approach.
It’s a normal run of the mill process to pause and check what is actually coming through the pipe of the dredger, keeping tabs on the kind of material you are loading and its suitability. But Jan De Nul, with the client’s agreement, decided not to slow operations to do this, but rely completely on a predesigned 3D matrix to decide how to cut the sections and where to put the suitable fill.
To take this ‘blindfold’ approach the matrix had to be very, very accurate: the agreement was to have below 5% fines in the fill. Further, the lurking problem of acid sulphate soils had to be avoided, as these sulphate soils form sulphuric acid when exposed to oxygen.
So, rather than the more normal dozen or so explorative drillings, Jan De Nul took over 200 borehole and viborcore measurements of the substrate and used it to build up this 3D matrix of the layers beneath the seabed.
The construction of the walls were a little unusual too. Instead of the more usual sheet metal pilings, these were constructed from ‘L-shaped’ counterforts placed upright on the sea bed with the fill sitting on the horizontal leg.
These truly massive concrete blocks were not shipped in: each and every single one of the 200, 20m high, 9m wide and 640 tonne units were built in situ, on a wharf that Jan De Nul had to firstly reclaim from the sea, compact, surface and then build a batching plant on.
Further, since there was no time to waste, as the counterforts were being installed using a purpose built 700 tonne capacity shear leg barge the fill was being placed behind. The result, says Mr Bosteels, is on time, inside budget, and within the agreed 10cm tolerance.
Hutchison Port Holdings and Patrick will be taking possession of the quay soon, and will be finishing it off with a concrete deck. “When I went to visit, I was standing on a 63 hectare platform, flat as a pancake, that only two years before was, simply, sea,” said Mr Bosteels. “It was really impressive.”
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