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Gateway gets going

Construction of the £1.5bn London Gateway port is racing ahead and attracting Royal interest. Marissa Lynch visits the Thames Estuary to chart progress.

A helicopter flies low to the Thames Estuary, carrying Royal cargo on a fairly blustery autumn morning on its way to a huge construction site in Stanford-le-Hope in Essex. HRH Prince Philip steps out donned in PPE and surveys the area, nodding approvingly as engineers explain the work being carried out on one of the largest port projects under construction in the UK.

When complete, the London Gateway port will be capable of handling some of the world’s largest container vessels - new ships with capacities of about 18,000 twenty-foot equivalent units or TEUs.

£1.5bn investment

The Duke of Edinburgh is right to be impressed. The port is being developed privately by United Arab Emirates developer DP World. It is pumping £1.5bn into the project.

The government has said that the port will create 36,000 new jobs, contribute £3.2bn to the UK economy annually, and negate the need for millions of road kilometres of freight transport each year by offering a port and logistics park in close proximity. It will also add 3.5M TEUs to the nation’s port capacity.

It’s not just the massive investment from DP World that is exciting. The project also has unusual technical aspects

“The quay wall is complicated because there are so many different loadings to be considered”

David Puller, Bachy Soletanche

The first part of the project, land reclamation, was carried out by Dredging International and main contractor Laing O’Rourke (NCE 17 March). This involved a major reclamation programme to create the 175ha terminal area.

Still to be done 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, 300m wide navigation channel.

Quay wall underway

Now that reclamation work is complete, construction of the quay wall is well underway. The wall is being constructed within the reclaimed area, avoiding the need to build under water.

The wall is a complex beast. It has 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.
DP World head of engineering Andrew Bowen says it is one of the most exciting projects he has worked on.

“It’s the most exciting port development around the world to be involved with,” he says.

“It’s a very large project and there was a long planning process to get the port underway.”

Careful timing

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

“You have to excavate the sand and connect the walls, before you use replace the material and use a vibro compactor again,” Bowen adds. Tie rods are being installed by Laing O’Rourke.

But Bachy Soletanche has been responsible for the design and construction of the two 1.25km diaphragm walls for the quay wall, which are not entirely common in port construction. The diaphragm walls are being constructed within the reclamation. When they are complete, the material in front of the quay wall will be removed.

“The wall design is wonderfully complicated - the most complicated I’ve been involved in designing,” says Bachy Soletanche project manager David Puller.

Different loadings

“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 and 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.

“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 Bachy Soletanche is halfway through diaphragm wall construction and he says it should take less than five months to complete the work.

Reclamation excavation

He explains the process has been to construct 448 wall panel segments 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 35m apart so two cranes are excavating each wall simultaneously.

Puller says that as the wall panels are completed, Laing O’Rourke is following behind casting the capping beam on top of each diaphragm wall then excavating before installing horizontal tie rods between the two walls.

“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.

“It varies in depth between 42m and 48m and is 1.5m thick. The rear wall varies between 22m and 26m and is an 800mm wall. 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.”

As the material is excavated from each panel, a bentonite powder in water is used as the temporary support slurry.

French bentonite

The particular bentonite Bachy Soletanche is using comes from France. Steel reinforcement cages are then inserted. A large proportion of these cages are being fabricated on site, while about a third are fabricated offsite.

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

The use of two diaphragm walls means construction is safer because work is being done on dry land with the reclamation having being completed first.

“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,” he says.

No divers needed

“Here we’re doing it all on dry land so no divers have been used for the structural elements.”

Progress is steaming ahead but one of the biggest challenges is to maintain the pace of construction.

“For us it’s the sheer demands of the productivity we have to do,” he says.

“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.”

In the end, DP World will certainly have a world class port. One that’s even fit for a prince.

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