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Piling and foundations: Turning the tide

Complex piling within the River Orwell is underway to enable a tidal barrier to be built that will protect the town of Ipswich from flooding

 

Ipswich’s position on the estuary of the River Orwell in Suffolk makes it susceptible to both fluvial and tidal flooding. The town’s vulnerability was exposed in 1953 when it – like much of the east coast of the UK – was hit by a devastating storm surge in the North Sea.

Since then, east coast flood defences have been significantly strengthened including along the banks of the Orwell and within Ipswich town centre. As a result, the chance of fluvial flooding in the town centre has been reduced to around one in 1,000. But tidal flooding still remains a very real possibility although current piling work will help reduce that risk.

In some parts of Ipswich the existing flood defences only provide tidal protection against a one in 20 year storm; and without any intervention, this could fall to just a one in three annual flood probability over the next 100 years, due to sea level rise.

That puts more than 1,600 residential and 400 commercial properties at risk of flooding whenever there is a major storm surge.

However, for the last 10 years, the Environment Agency and Ipswich Borough Council have been working on a £50M project that has seen existing flood defences on the east and west banks of the River Orwell raised and improved. And now, the final part of the plan is being implemented: construction of a £21M tidal barrier across the river that is designed to prevent surge tides from going up river while allowing navigation and free fluvial flow to continue.

Barrier plan

Ipswich Borough Council and the Environment Agency worked together to secure funding, and in November 2014 the Environment Agency appointed VBA, a joint venture of Volker Stevin, Boskalis Westminster and Atkins to design and construct the barrier.

The 20m wide barrier will sit within the channel known as the “New Cut” on the west side of the Orwell. Work on the main structure started in October last year, with the barrier itself being built inside a cofferdam that is currently being installed as part of a £5.5M enabling works contract by Volker Ground Engineering.

This square cofferdam measures around 29m by 29m in plan, and sits close to the east bank of the channel. It is being constructed primarily using AZ46-700N sheet piles, with 84 crimped pairs of piles making up the walls, and C9 and C14 connector piles forming the corners. The sheet piled walls of the cofferdam extend to a depth of 20.6m through predominantly loose weathered chalk.

These sheet piles will form part of the permanent works, getting cut down to bed level and remaining in place once the barrier structure has been constructed. But Volker’s contract also includes extensive temporary works, including sheet piling for river diversion and scour protection.

While the barrier is being constructed, the river will be diverted so that it flows to the west of the cofferdam. This is being achieved by installing two sections of “deflector” piles – sheet piled walls that extend from the corners of the cofferdam to act as a temporary river wall.

As a result of deflecting the flow, the existing quay wall on the west side could have been at risk of scour, so Volker has installed a run of temporary sheet piles in front of the existing wall to provide scour protection. This temporary protection is made up of 64 crimped pairs of AZ26-700 piles, all 19m in length.

The company is also currently installing two permanent wing walls using a combination of 1,422mm diameter steel tubes and a range of sheet piles varying in length from around 15.1m to 20.3m. These link the cofferdam to the land on the east side of the channel, and will eventually form the new quay wall on that side, with the area between the new combi walls and the existing bank backfilled as part of VBA’s main contract. Two similar wing walls will be constructed on the west side once the main gate structure has been built, with the end result that the channel will narrow at this point, and all the water will flow through the barrier structure.

Test piling

The other element of Volker’s contract is to install bearing piles within the cofferdam for the barrier’s foundations. The size and layout of these tubular steel piles has been refined as a result of the firm installing a test pile close to the west quay wall.

“While we were working on the west bank, the designer was evaluating the bearing piles, so we decided to do a test pile,” explains Volker contracts engineer Steve Yates. He says that a 60m long steel tube was installed in a section of the river bed with the same soil profile as the centre of the cofferdam. “It went down a bit easier than expected,” he adds.

The tube was installed using a vibratory hammer, and then tested using an impact hammer. “That allowed the designers to go away and re-evaluate the lengths within the cofferdam,” says Yates. Two consultants are doing the pile design on the project: Tony Gee for the temporary piles and Atkins for the permanent piles. As a result of the test, the designers were able to reduce the maximum length of the 1,016mm diameter bearing piles from 60m to 56.5m, and the total number from 56 to 48.

Before starting the job, Volker had anticipated carrying out all the piling using barges and jack-up rigs. “When we looked at it in further detail, we realised we could do it by sitting a large crane on the bank,” says Yates. “It was more cost effective than using barges.” The river is still being used to deliver all the piles to site. They arrive at the Port of Ipswich, a short distance down river, and are then loaded onto a barge, which takes them to the barrier site.

The lifting and driving is being done using a 400t crawler crane, which was initially sited on the west bank to install the scour protection and the test pile. It was then de-rigged and taken to the other side of the channel to an area known as Middle Island, and used to install the first side of the cofferdam, closest to the east bank, and the combi wall at the north end of the cofferdam.

Crane considerations

While all this was going on, Volker constructed a temporary piling platform slightly further south, from where the crane could reach the furthest corner of the cofferdam and the southern section of combi wall. “In order to sit that crane over there, we had to put some temporary piles in and a crane mat,” explains Yates. “The quay wall is structurally sound, but we are not able to sit anything within 5m of it, so we had to put more bearing piles in and a temporary platform.”

The edge of this new platform has been constructed using sheet piles, some of which will remain in place as part of the works for the barrier’s control building. Volker will also construct a series of 450mm diameter concrete CFA bearing piles for the barrier control building on the Middle Island.

The crane size was dictated mainly by the reach required, but also because of its load capacity, which might be needed for some of the piles. “It is possible that we might have to impact drive some of the bearing pile tubes, which would require a heavy hammer,” says Yates. So far, with the cofferdam and combi walls almost complete, most of the piles have gone in under vibration alone through the soft chalk underlying the river bed.

Volker is due to start installing the bearing piles in June, and is expecting to finish on site in September. At that stage, VBA will come onto site and spend around a year building the barrier structure. Once this is complete, the river will be diverted through the barrier, and Volker will come back to extract all the temporary piles and install the two combi wing walls on the west side, ready for VBA to backfill and complete the works.

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