Despite seals and satellites hindering the installation of protective concrete fenders for the Tay Road Bridge, the project is being delivered ahead of schedule. David Hayward reports from Dundee.
The risk of ship impact to Scotland’s Tay Road Bridge is about to be considerably reduced later this month following completion of complex piled concrete fenders built in the fast flowing tidal estuary.
For over 45 years motorists have lived with the possibility that sudden closure of the 2.3km crossing, linking Dundee with all routes south, would trigger an 80km road diversion, perhaps for months, via the nearest alternative crossing at Perth. Local authorities in the region claim that any extended severing of such a vital commercial link would translate, into a £700M loss to the regional economy.
The threat has been real. Since the bridge opened in 1966, ships have hit it three times although fortunately they have only incurred minor damage.
The crossing’s three, 76m wide mid-river navigation spans, supported across the Tay Estuary on exposed concrete piers and bases, are no different to the other 39 spans. They all lack external protection against collision impact from large tonnage ships, up to five of which sail between the estuary and Perth’s dockyard upstream each week.
But the risk of collision damage is being reduced substantially. Contractor VolkerStevin Marine will, by late this month, have surrounded the three crucial pier bases with hefty oval shaped free-standing concrete fenders to protect the twin parabolic pier legs and their bases.
Closure of the 2.3km crossing, linking Dundee with all routes south, would trigger an 80km road diversion.
Construction of the £15M protection has involved installing 63m long piles pitched less than 5m away from the bridge. Locating massive precast concrete beams to form the impact barriers under the bridge and on top of the piles, involved crawler and fixed crane jibs, operating a tandem lift from a barge. They were working just 3m from the deck.
“We were not allowed to interrupt traffic on the bridge at any time, or to have any access to the structure at all,” says VolkerStevin Marine project manager Emmet Scanlan. “Working so close to the bridge, in such fast flowing tidal waters has proved one of the most exacting challenges ever faced by our piling crews.”
Add to his list of headaches the fact that GPS pile setting out was not possible because of the bridge’s proximity, plus the need to stop construction if any seals happen to swim by, and Scanlan’s estimate to be off site before December - two months ahead of programme - looks impressive.
A £1M detailed site investigation at the start of VolkerStevin Marine’s two year early contractor involvement contract allowed initial pile lengths, suggested by client the Tay Road Bridge Joint Board, to be shortened by 5m. When bedrock below the seabed was discovered to be 20m lower than expected, the tubular steel piles were redesigned to found in the upper clays and gravels with loading changed from end bearing to friction resisting.
Though the largest three dozen, 980mm diameter piles were shortened to 63m, these 55t monsters still had to be positioned to 100mm accuracy, with some driven just a few metres from the bridge deck.
The contractor’s state of the art Dina M piling barge, located midestuary around the bridge’s navigation piers, had to change position every few hours guided by a network of computer controlled mooring cables attached to eight large sea anchors. Together with half a dozen service boats and pontoons, this £20,000 a day fleet received, pitched and drove the piles using a combination of impact hammers and vibration.
Working in the 4m/s currents, plus a 6m tidal range, the 15-strong crew was on call around the clock. Despite the currents, the required 100mm positioning accuracy was met for all 48 piles, and most were pitched to within 25mm.
The eight piles a week programme was regularly exceeded by 50%, with the entire piling operation completed a month ahead of schedule.
“Working so close to the live bridge meant we had to repeatedly brainstorm a range of ‘what if’ scenarios,” explains Scanlan. “Our original plan to use satellite GPS to set out pile positions was quickly abandoned as the bridge structure itself interfered with the signals.”
To the piling crew, the barge was as much home as work place as they lived on it almost constantly for three months. A four star chef, laundry stewards and a gym helped compensate for the crew’s 24 hour, tide dependent regime.
Six crew members were officially seal-watchers, with always one on duty. No piling was allowed if a seal was spotted within 500m of the barge and the team had to wait an agonising 20 minutes after the last seal had gone before resuming work. All work was banned during the mid summer seal puppy season.
“Working so close to the bridge, in such fast flowing tidal waters has proved one of the most exacting challenges ever faced by our piling crew”
Piling work was completed this summer. The impact protection around each pier consists of hefty 4m tall, 16m wide concrete D-shaped end units, linked by 31m long side beams. All four components, each averaging 300t, are hollow and were precast close to the bridge site, towed out on barges and slotted over their piled supports using shearleg and barge-mounted crawler cranes.
The final task, now underway, is filling the hollow structures with concrete placed in the voids from adjoining barges. This operation will convert each pier protection assembly into an impressive 3,600t barrier.
The new barriers can also resist scour courtesy of geotextile mattresses. Formed of polypropylene membranes, weighted down with precast concrete blocks, the mattresses were lowered to the seabed to resist scour around pier bases.
Designed originally to protect underwater gas pipes, the 30m long, 100t mats were positioned using computer-controlled frames with hydraulic release hooks.
Millimetre accuracy was achieved by using the piles as guides and slotting their then open tops through holes cut in the mattresses. This allowed the mats to slide down through 20m of water to settle on the seabed. Such accuracy meant that the planned use of divers was avoided. The whole operation cost £250,000 less than budgeted and was finished a month quicker than programmed.
The contractor’s anticipated November overall completion would not only be eight weeks ahead of schedule but would deliver to the client a £1.3M cost
“This has been a complicated, high risk project and we are delighted it is being brought in ahead of programme and under budget,” says Engineer to the Board and Dundee City Council city engineer Fergus Wilson. “Our decision to go for early contractor involvement promoted joint teamwork in both design and buildability, giving us a bridge with a now significantly reduced likelihood of damage from ship impact.”