Construction of Bordeaux’s new signature lift bridge has reached the half way stage. NCE reports from south west France.
After more than six years in the planning, Bordeaux’s new signature bridge is now racing ahead. Contractors have spent the last month applying the finishing touches to the first two of four 77m tall towers on the Pont Bacalan-Bastide. Completion of towers marks the half way stage of construction, with the bridge scheduled to open in late 2012.
Conceived by renowned bridge architect Charles Lavigne with input from world renowned bridge designer Michel Virlogeux, the €156M (£135M) vertical lift bridge across the Garonne river is being built by Vinci Construction Grands Projets and its subsidiaries GTM Sud Ouest and Cimolai under a design and build contract. Vinci’s structural engineer is Egis.
Vinci beat off four competitors to win a design competition run by client Communauté Urbaine de Bordeaux in 2008 and began construction in October 2009.
“Working in the Garonne is difficult. The current is strong and it has a large tidal range”
Gilles Vanbremeersch, GTM Sud Ouest
Getting to that stage was success in itself. “There is really only one bridge in central Bordeaux. The city needed another crossing point. But Bordeaux is a port city so the bridge needed to provide sufficient clearance to shipping.
So after a long consultation process we opted for a lift bridge,” explains GTM Sud Ouest project director Gilles Vanbremeersch.
“The process was extremely long,” adds Vanbremeersch. “There were always organisations bugging the hell out of us. We began work in 2004, got our design idea in 2008, then went into 18 months of detailed design to get the design approved. We only began construction in 2009.”
The 433m long bridge will link the Bacalan and Bastide districts on the left and right banks of the river, 2km downstream of the city centre. It will carry four lanes of traffic, two tram lines, cycle lanes and pedestrian walkways. A viewing gallery will top one of the towers.
The central section, 117m long by 53m wide, will lift up to 60 times a year to allow sailing ships up to 53m tall to pass beneath. “It’s the equivalent of raising a football field 60m high,” says Vanbremeersch. Raising the 2,750t section will take just 11 minutes.
When in its lowered position, smaller river traffic will still be able to pass - vital given that further upstream is the offloading point for A380 wings making their way from a fabrication facility in Wales to the Airbus assembly factory in Toulouse. Clearance, depending on tides, is 3m to 8m - enough for the towboats pulling the wings.
That 5m tidal range is presenting the biggest construction challenge. “Working in the Garonne is very difficult,” says Vanbremeersch. “The current is very strong and it has a large tidal range.”
The bridge will have five spans in total, two to each side of the main lifting section. The spans sit on four concrete piers, with the two particularly large central piers supporting the four central towers. These 44m long, 18m wide, 16m deep 5,750t hollow reinforced concrete beasts are protected from ship impacts by four 18m diameter, 9,500t reinforced concrete protection isla
nds. Constructing and installing these concrete giants was a major challenge.
Because of the currents, the two massive central piers and their protective islands were cast 4km upstream and towed into place. These complex operations took place in March and June. “They were extremely difficult to manoeuvre. Two tow boats with 90t pulling power were needed for each unit, with a winching system used to get them into position,” says Vanbremeersch. Once in place the units were clamped to temporary steel caissons before being sunk into position and connected in to the bed rock via 25m deep piles. The central piers demanded 20 piles, each 1.6m in diameter.
“For more than a year a lot of our work was slightly invisible,” says Vanbremeersch. “But for the last six months we have been working on the first two pylons - and we have put down the first two spans on the right bank.”
Getting these two spans to site also proved a challenge. Fabricated and pre-assembled by Italian steel firm Cimolai in Venice, the shipment faced a 5,000km sea voyage by barge to the site. The first one month long voyage was delayed for 10 days by bad weather off the Portuguese coast. The shipment finally arrived in late July.
“The pylons are basically large towers. The only difficulty is achieving the aesthetic qualify”
Gilles Vanbremeersch, GTM Sud Ouest
After this difficult passage, erection of the 96m and 67m long pieces - weighing 1,350t and 750t respectively - was relatively straightforward, with positioning made easier thanks to the construction of a 300m long temporary causeway between the river bank and the central pier. This causeway was the first thing to be built, in November 2009. It has the double benefit of providing access for land-based plant and operatives and also of restricting the water flow, helping water-based plant. Welding the two pieces together was almost complete at the time of NCE’s visit, ahead of placement of precast concrete deck slabs.
In December and January 2012, the same operation will be repeated on the left bank.
Erection of the central towers is, in contrast, a relatively simple job, says Vanbremeersch. “This is the easy part.
The pylons are basically large towers. The only difficulty is achieving the aesthetic quality required.” A slipform shuttering system means the towers go up at a rate of 5m every two days.
Installing the bridge lift mechanism is likely to be more of a challenge, however. The 2,750t deck will be lifted by a system of winches, pulleys and counterweights in each tower. The counterweights will weigh 2,694t in total, leaving 56t of force to be applied by motors in the base of each tower.
The 56t difference between the counterweight’s mass and deck weight is important, as it ensures the bridge deck remains in place in high winds.
Operating the bridge will cost around £860,000 a year, of which £86,000 is for operating the lift mechanism and the remainder being spent on maintenance.
“This is one of my favourite projects, and it will be my last,” muses Vanbremeersch. He retires next year. The Pont Bacalan Bastide is a fine swansong.