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Flood defences: Defending the line

boston flood defences 1

Protecting more than 14,000 properties in the Lincolnshire town of Boston from tidal flooding requires major piling work along the River Witham.

Risk of tidal flooding in Boston, Lincolnshire is set to be significantly reduced by 2020 when new flood defences, which include a moveable gate on the River Whitham, are completed.

Work on the design for the long-awaited £100M Boston Barrier has now been underway for just over a year and the project has made significant strides forward since construction started in January. The work will not defend the area against extreme tidal surges, but will reduce the probability of tidal flooding to a one in 300 risk in any one year compared to the current one in 50 risk.

Delivering this flood risk reduction requires some major ground engineering work for the flood gate itself, plus work to enhance there silience of the surrounding area.The client for the Boston Barrier is a partnership between the Environment Agency, Lincolnshire County Council, Boston Borough Council and Black Sluice Internal Drainage Board.

boston flood defences 2

boston flood defences 2

Piling work is well underway

The barrier itself is being delivered under a design and build contract by a joint venture formed by Mott MacDonald and Bam Nuttall, which was awarded the work in August last year. The project comprises a 25m wide moveable flood gate across the River Whitham, which will lay flat on the riverbed when not in use, plus new flood defence walls.

“There are three main elements to the project,” says Mott MacDonald Boston Barrier design lead Jim Gelder. “There’s the tidal barrier itself, the entrance to the wet dock and then there is 1.6km of sheet piled or concrete flood walls.

”The ground conditions comprise lots of made ground over alluvium and glacial till.“

The geology is relatively simple but where the geotechnical design gets exciting is where the temporary and permanent works join,” says Gelder.“The BIM model has been essential for sorting out how the two interact at different stages of construction.

“One of the main challenges of the ground conditions was the high potential for buried obstructions. Around half of the flood wall is within the estate of Port of Boston which is over 100 years old so there was a high potential for unexpected obstructions. The desk study had identified an anchored quay wall, a buried former slipway, old railway tracks and other heavy civil engineering infrastructure.

“There was also a concern over reduced levels of glacial till in some areas. The previous ground investigation did not include many boreholes within the river, so one of the first tasks was to plug this data gap,” says Gelder.

Where the geotechnical design gets exciting is where the temporary and permanent works join

The JV commissioned further overwater ground investigation with Harrison Group undertaking boreholes, Lankelma delivering cone penetration tests (CPTs) and Cambridge Insitu undertaking pressure meter testing.

Testing included seismic CPTs and extension and compression triaxial testing for design of the gate.

“The gate is a relatively unusual structure,” says Gelder. “The only comparable structure is the one at Ipswich, which is only just being commissioned.

“The gate sits in a U-shaped box and there is a whole range of load cases that develop in construction and use.”

Other elements of the work have resulted in some geotechnica linnovation too. The reference design called for an anchored wall for the new gate, but this has been replaced with a sheet piled wall that is tied back to a row of 1,400mm diameter tube piles at 4.2m centres.

“The anchored pile wall structure would have been challenging due to existing structures in the ground but the tube piles could be slotted between these obstructions,” says Gelder.

Other changes to the reference design have reduced the need for piling, as well as adding flood resilience to the new facilities.

“The control room for the gate is on the defended side of the wall, but there is still the potential for up to 0.5m of flooding in that area, so placing the plant and machinery on the ground floor of the control room meant that bulk head doors would be needed to protect it,” explains Gelder.

“Flood defences for the building would have limited the size of plant we could place in the building and presented issues with noise too.”

The solution was to place the planton the first floor, which also give the control room a better view of the river itself too. The added benefit of this change meant that the building did not need to be waterproofed and piles were no longer needed to mitigate the effects of buoyancy.

“Piling in the control room location was going to be challenging due to clashes with the anchor beam and existing utilities,” says Gelder.

The use of shallow raft foundations enabled 54 piles to be removed from the design. Despite the additional ground investigation work, there were still concerns about the variable level of the glacial till, so the JV used piling records for the temporary works to undertake some back analysis and deliver more data on the till levels.

“The data gave added confidenceon the till levels and showed that there was not a problem,” says Gelder.

boston flood defences 5

boston flood defences 5

The barrier design calls for lots of interaction between temporary and permanent works

Piling on the right bank is now 50% complete and the cofferdam for the gate construction has just been closed using 200 sheet piles. The sheet piles that have been driven on the landward side of the right bank feature an unusual design to remove the need to access the back of the piles to join the anchors.

“These piles back on to residential properties, so the access was limited,” says Gelder. “The piles at the back have been driven with the anchor head in position with a nuton the inside to allow easy connection of the anchor.”

Easing anchor connections on the tube pile anchored wall on the left bank has also led to an innovative design with a steel stiffener plate within the pile itself so the construction team can stand within the piles to access the anchor connection.

Noise and vibration have been a major concern. “We have been vibrating the piles to get them lined up and then “tapping” them into position with a drop weight,” explains Gelder.

Sequencing the project is complicated says Gelder. “The phasing has to go in the order of right bank, barrier, left bank and then the wetdock will be completed last as we cannot access that until the barrier isc ommissioned,” he says.

Piling on the right bank is expected to be completed next year and the remainder of the piling on the left bank will start early next year.

Geldersays that 70% of the design for the leftbank is currently complete. While large elements of the project have still to be constructed, Gelder seems confident that the contractor/consultant joint venture approach means that buildability has been fully considered during the design stages.

Work on site appears to have run smoothly so far, which supports Gelder’s confidence.

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