|~|120proj200.gif|~|The Ras Al Khor bridge will complete a ‘trilogy’ of crossings across the Creek. Besix has also started work on the new Garhoud bridge, which will built alongside the existing structure. On completion, the old Garhoud bridge will be demolished.|~|In a bid to ease Dubai’s chronic traffic problem, work is progressing on Ras Al Khor bridge — the third Creek crossing that will eventually carry traffic from Dubai Festival City (DFC) towards Interchange One on Sheikh Zayed Road.
Contractor Besix won the US $106 million (AED388 million) project, which consists of three phases: the bridge crossing itself, onshore roads and ramps leading up to the bridge, and an underpass on nearby Al Rabat Road that will carry traffic from Rashadiya straight through to the airport tunnel. Consulting engineer on the project is Parsons.
While the onshore roads, ramps and underpass all involve relatively straightforward construction techniques, building a bridge over the waters of the Creek calls for some specific methodology.
Besix started work on site at the end of March 2005. The sequence of works involved beginning the independent portion of the Creek crossing from the Jadaf side of the Creek, while work simultaneously got underway on the bridge from the DFC side.
“Currently we have four stages of the bridge complete starting from the Jadaf side, and we’re also working on the second stage —the bridge leading up to the Creek crossing from the DFC side,” says project manager, Bernard Patze.
Individual cofferdams are being used to install piling for the six bridge piers that will sit in the water in the middle of the Creek to support the structure. Two main cofferdams are also being constructed on either side of the Creek to support the bridge deck scaffolding.
“We have constructed the main cofferdams up to around 30% of the overall length,” explains Patze. “We build a cofferdam around the location of the pier and backfill with sand to above the seawater level.”
The cofferdam enclosure itself is made from sheet piles, which are vibrated 6m into the creek seabed using a crawler crane and a vibro hammer.
The Creek base is made up of 1m to 2m of soft layer followed by sandstone underneath. When in position, the sheet piles rest about 1m above the high tide water level.
“We then backfill with sand up to seawater level to make a platform on which we can drill the piles as we would do in a normal onshore piling situation.”
According to Patze, the main difference between piling on the land and the sea is the type of drilling fluid that is used: “On the land we use bentonite, but for a project like this on the sea we use a polymer drilling slurry.
“I would say the Creek crossing part of the project is the trickiest,” he adds. “Working over the water means doing sea piling and backfilling, but once you’re on the shore, it’s more straightforward.”
While the first section of the onshore part of the bridge on the DFC side is being constructed using straightforward scaffolding methods, barges are needed to enable construction material supplies and movement of scaffolding supports for the bridge sections that are being built on the water.
Five barges are currently in use — three large and two small. “Large steel flying shutters, which are supported at the bottom, are being moved along from stage to stage of the
bridge by the barges,” explains Patze.
The span of the Ras Al Khor bridge crossing is approximately 400m, with 80m spaces in between the columns. Unlike the existing Maktoum and Garhoud bridges, the new Ras Al Khor crossing will not open. It is therefore being built with a much higher clearance above the water level.
“During construction of the bridge there will be a junction portion in the middle, which we have to keep open for navigation purposes up to 1 October 2006,” says Patze. “After this date we will close the last section and ships will have to go below the bridge.” The clearance for navigation under the structure will then be limited to 15m above the high water level.
The width of the south side of the bridge is around 30m, while the north side is between 33m and 37m. “The north bridge heading towards Festival City carries two additional lanes that are not present on the south bridge,” explains Patze. “We expect the north bridge to carry around seven lanes, and the south bridge five lanes,” he adds.
On the Jadaf side of the Creek, Besix’s contract finishes about 300m beyond the bridge, after which point Bin Hafeez has the contract for the first part of the road section that will eventually join up with Interchange One on Sheikh Zayed Road.
Besix is also progressing with the onshore portion of the project, which includes onshore bridge sections to link the Creek crossing bridges with the existing Rabat Road, as well as providing accesses to DFC.
“The onshore extensions of the main bridge are 700m long with a height varying from 4m to 18m above ground level. About 110 columns are required to support the decks,” explains Patze.
The foundations works are now finished and the bridge decks are about 25% complete in this area. The junction with the existing Rabat Road is due to be completed by January 2007.
According to Patze, one of the main challenges with constructing all of the access roads around Rabat junction is the fact that the construction is taking place at a very busy intersection.
“Programme-wise it’s also very tight — before we could begin construction work on site we had to do the preliminary stages i.e. utilities diversions, which also comes under our remit,”
Besix also had to implement traffic diversions before work could begin on the Al Rabat Road underpass. Constructing the underpass will enable a freer flow of traffic from Garhoud area towards the Beirut tunnel at the airport. Marrakech Street will run above the tunnel, with Rabat Road going underneath it. This part of the project will eventually join up with the interchange currently built in Rashadiya by Dutco Balfour Beatty Group.
In terms of the stage of construction, Patze says the thick bottom slab of the underpass is in place and work is beginning on the covered portion (which is 30m to 35m long).
“The biggest challenge we face with the underpass is the thick concrete pours required for the bottom slab, which is 2.5m at the thickest point,” explains Patze. This can mean single pours of up to 1,500m3 or 2,000m3 in one go.
“There are specific precautions that need to be taken when dealing with such big pours in terms of temperature of the concrete to prevent it from going too high,” adds Patze. To counter this, Besix pours the concrete at varying temperatures to control the overall temperature in the pour, which helps to avoid concrete cracking.
“The bottom portion of the slab is poured with concrete at 32 degrees C, then we continue with the central portion poured at 25 degrees C, and finally the top is again poured at 32 degrees C.
“This helps to keep the difference in temperature between the core and the surface to a minimum,” explains Patze. The first pour for the slab was in December, with the last scheduled for September.
According to Patze a total of 200,000m3 of concrete will be poured for the overall project (the underpass, onshore section and the Creek bridge combined).
This is made up of around 80,000m3 used on the main Creek bridges, 50,000m3 for the underpass, with the balance 70,000m3 for DFC access bridges and ancillary works.
A large amount of concrete pouring is currently taking place on the deck of the road that leads up to the DFC side of the Creek crossing. And with the temperature continuing to rise as summer approaches, this means taking care to prevent the freshly poured concrete from overheating.
“It is important to prevent excessive evaporation of the water from the concrete,” says Patze. “If it dries too quickly it might create some cracks, so we need to keep it wet by fog misting.”
Besix currently has approximately 2,400 workers on site in its bid to meet the January 2007 completion date for its three elements of the project.
In addition to the Ras Al Khor Creek crossing, Besix recently won the contract to build the new 14-lane Garhoud bridge —
as well as demolish the existing structure once the new one is up and running.
As work on these new bridges proceeds, let us hope that it can provide a solution to the traffic chaos that is threatening to bring the city’s roads to a standstill.||**||For all the latest business news from the UAE and Gulf countries, follow us on Twitter and Linkedin, like us on Facebook and subscribe to our YouTube page, which is updated daily.
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