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Looming Large

The London Gateway port will be a UK record breaker when it opens for business, but right now it’s the ground engineering that is making the headlines. Marissa Lynch reports.

The Thames Estuary near Stanford-le-Hope in Essex is a hive of construction activity as engineers work to build a new world-class port.

The London Gateway facility will be capable of handling some of the world’s largest container vessels (with capacities of about 18,000 twenty-foot equivalent units, or TEUs).

The port is being developed privately by UAE developer DP World, which has pumped £1.5bn into the project.

The government says the port will create 36,000 new jobs, contribute £3.2bn to the UK economy annually, and negate millions of road kilometres of freight transport each year by offering a port and logistics park in proximity.

It will also add 3.5M TEUs to the nation’s port capacity.

But it’s not just the scale of the investment from DP World that makes this project notable - the unusual technical aspects are also providing huge challenges.

The first part of the project was carried out by Dredging International and main contractor Laing O’Rourke and involved reclamation of 175ha of land to form the port.

Still to be completed in phase one is the construction of a new jetty for an operating Shell bitumen plant; the installation of a quay wall; and dredging to deepen the existing 100km long by 300m wide navigation channel.

This northern approach channel to the Thames Estuary will be dredged to a depth of 14.5m in the inner channel and 16.5m in the outer, to accommodate the container ships.

“The wall is the most complicated design I’ve been involved in. We have always had to find the worst combination of factors and design for it.”

Now that reclamation work has been completed, construction of the quay wall is well underway.

The wall is a complex beast, with two separate diaphragm walls tied together - one to act as the front port wall, and one to act as a rear anchor to resist various loading conditions.

Quay wall construction has to be timed carefully, as the front and rear walls are being constructed at the same time by subcontractor Bachy Solentache.

“You have to excavate the sand and connect the walls, before you use replace the material and use a vibro compactor again,” explains DP World head of engineering Andrew Bowen.

Tie rods are being installed by Laing O’Rourke.

But Bachy Soletanche has been responsible for the design of two 1.25km long diaphragm walls on the quay wall, which are not entirely common in a port construction.

Bachy Soletanche project manager David Puller says this is one of the most complex and difficult projects he’s worked on.

“The wall design is wonderfully complicated - the most complicated I’ve been involved in designing,” he says.

“It’s complicated because there are many different loadings to be considered. There is loading from the cranes, both when they are working and when they are stowed, and in conditions of high wind.

“There are pulling loads from ships when they are moored, pushing loads when they are berthing. There is the pressure behind from ground water and pressure from the tide, which fluctuates by up to 6m.

Event_Engineering_5

“It’s like when you were at school and you did the rolling dice game where you had to use probability to predict the number of times you’d roll a six and combine the different outcomes. Here, we have always had to find the worst combination of factors and design for it.

“Horizontal crane loads will exert a load of 130kN per metre run and there is mooring which is a further 100kN per metre. During ship berthing, the load against the wall is 170kN. But there are so many combinations.”

Puller says they are halfway through the diaphragm wall construction and he says they should have less than five months now to complete the construction.

Parallel construction

The process has involved the construction of 448 panel segments of the walls by first excavating from the reclaimed land using a hydraulic KS grab suspended from a crane.

This type of grab is used because of the dense Thanet sand present on the quay wall.

The front wall and the rear anchor wall are spaced 35m apart so two cranes are excavating for each wall simultaneously.

Puller says that as the panels of the walls are completed, Laing O’Rourke is following behind to complete the capping beam on top of each diaphragm wall and then excavating before installing the tie rods between the two walls at a very slight angle to the horizontal of only a couple of degrees.

“The front wall is doing most of the work as it’s the wall that will have deep excavation in front of it where the ships will berth,” says Puller.

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“It varies in depth between 42m and 48m and is 1,500mm thick. The rear wall varies between 22m and 26m and is 800mm. That rear wall is acting like a big anchor to
the front wall and tie rods are installed at every metre.

“The rear wall also acts as foundation for the gantry cranes, which is one of the reasons why the front and rear walls are 35m apart; this coincides with the gauge of the cranes.”

Once the bentonite slurry-supported panels are excavated to the required depth, steel reinforcement cages are inserted.

Around a third of these cages are being fabricated on site, while the rest are fabricated off site.

Puller also says the concrete used for the diaphragm walls, which is supplied by Laing O’Rourke, is a high grade, durable concrete designed to be resistant to sea water.

Another reason for the two diaphragm walls is because it is a safer option as well as the fact the construction is being done on dry land with the reclamation having being completed first.

Thames_Gateway_Port

“If you’re building a quay wall in the river which is tidal and more exposed to the elements and currents you usually would use divers for some elements,” says Puller. “Here we’re doing it all on dry land so no divers have been used for the structural elements.”

Progress is steaming ahead at the port. “For us it’s the sheer demands of the productivity we have to do,” he adds.

“We’re trying to do 13 panels [of the diaphragm wall] a week which is a phenomenal amount. We’re working 24-5 with a Saturday day shift, so it’s very demanding for our staff. The amount we have to do equates to 2,500m2 completed wall per week.

“On some jobs, this is the total amount, but we’re doing this week after week. For us, that’s the truly demanding part and we have 120 people on site working on this.”

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