To most visitors, Great Yarmouth, on the Norfolk coast, is a seaside resort known for its pleasure beach, promenade and its two piers. But the town is also a working port – and an increasingly busy one, at that.
While the town’s fishing industry all but disappeared during the 20th century, the North Sea oil business provided new opportunities for trade and today ships continue to deliver oil and gas to storage facilities at the port.
More recently, the offshore wind energy industry has boosted Great Yarmouth’s economy enormously – so much so that in 2008 work began on a new outer harbour to extend the port’s capacity.
The contractor commissioned to build this new outer harbour was Edmund Nuttall. And now, in joint venture with Farrans Construction, Nuttall (now called BAM Nuttall) is building a new bridge, the Great Yarmouth Third River Crossing, over the River Yare to ease the growing traffic flows through the town.
The £120m project, designated a “nationally significant infrastructure project” by the Department for Transport, will provide a link from the A47 road at Harfrey’s roundabout on the west bank of the river to the port and the enterprise zone via South Denes Road on the eastern bank.
To allow access for commercial vessels, the new structure (like the two existing bridges over the river in Yarmouth) will be a bascule bridge, or counterbalanced lift bridge, the two halves of which will swing up to give clearance to ships.
The bridge mechanism will be built within two deep bascule pits, one on either side of the river, which are currently under construction. On either side of the new bridge, the approaches are being built on raised embankments of reinforced earth.
“The embankments on both sides of the river are being constructed on historically poor ground conditions so we are using precast concrete driven piles at 2.5m to 3.0m centres under the footprint of the embankments to ensure no ongoing settlement once the construction is finished,” says BAM Farrans construction manager, Richard Hayman.
Piling specialist Roger Bullivant has installed approximately 1,500 precast piles along the approaches. These were then broken back to expose the rebar and insitu pile caps cast on top. The pile caps along the outer edges of the embankments are linked together to provide additional support and to resist lateral forces.
BAM Farrans is building up the reinforced earth embankments with parcels of imported fill layered between geotextile membranes to distribute the load evenly over the concrete piles. The earth used for this is being imported from another project (a new hotel) currently under construction further up the coast.
In order to take traffic from the western embankment onto the bridge over the river, BAM Farrens is building a precast concrete bridge over the existing Southtown Road, running parallel to the river.
This bridge’s western abutment, where the new embankment ends, sits on a series of tubular steel piles, while on the eastern side of Southtown Road, the bridge beams bear on the top of the concrete bascule pit itself.
Once the embankments have reached the desired level, they will be finished with precast concrete edge beams ready for construction and surfacing of the carriageways. The embankment on the western approach, in a mainly residential area, will be turfed over while on the eastern side, which is mostly industrial, the embankment will be faced with concrete bock paving.
The hydraulic mechanism will be built within the two deep bascule pits, one on either side of the river. These are currently nearing completion.
The excavations each measure approximately 19m x 24m x 10m deep and are surrounded by water on three sides. This, together with the extremely poor ground conditions, means that substantial structural support was required around the perimeter of both excavations.
The pits have been dug within robust cofferdams comprising a combination of interlocking steel sheet piles and tubular steel driven piles installed by a rig mounted on a jack-up barge. The line of piles is extended either side of the cofferdams and back-filled to create impact structures, or ‘crumple zones’, to protect the bridge itself from accidental collisions by ships.
Once the cofferdams had been completed, the jack-up barge was off-hired and all subsequent work on the bascule pits has been done from the river bank.
Using a long-reach excavator, the contractor dug down to minus 10m inside the two cofferdams to reach good dense sand and gravel and then dewatered before casting 2m-thick base slabs to support the bridge leaves and their lifting mechanism.
Each slab comprises approximately 900m3 of reinforced concrete, poured in a continuous 17-hour operation requiring 300 lorry-loads.
The bridge span is 50m clear between the abutments and the two leaves are of lightweight steel construction. The project’s consulting engineer, Irish firm Roughan & O’Donovan, brought in US specialist design engineer Hardesty & Hanover to design the bridge itself.
“They are probably the number one movable bridge experts in the world,” says Malcolm McAlister, community engagement officer for the BAM Farrans joint venture.
The two bridge leaves are being fabricated by another specialist in the field of movable bridges, Belgian company Victor Buyck. When completed, these will be brought over to Great Yarmouth by barge and lifted into position with a heavy-lift floating crane.
“All these works are planned to be done in a 72-hour possession of the river,” says Hayman. “We have three possessions available to us under the contract but the current plan is only to use two, with the third as a back-up,” he adds.
The bridge leaves, each of which weighs approximately 1,000 tonnes, will be delivered and installed one at a time. Once they are both settled on their bearings within the bascule pits, they will be locked into the open position to allow the river to reopen to traffic while the team commissions the hydraulic mechanisms that will operate the bridge.
Once the bridge leaves are in place and the hydraulic system operating, BAM Farrans will cast the concrete roof slabs on the bascule pits and complete the approach structure works. The completed bridge is scheduled to open in mid-2023.