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GREEN LINE

Growth in Dubai seems to be unstoppable as Ed Owen discovers when he visits the Emirate's first public transport project.

Until Crossrail breaks ground, scheduled for 2010, the largest infrastructure project in the world is the Dubai Metro, and consultant Atkins has been on a steep but satisfying learning curve managing this mega-project in a city-state full of mega-projects – Dubai.

"When we joined Crossrail we did so in a joint venture [with Arup] because we were a little short in tunnelling experience. We no longer have this problem," says Atkins' managing director for the Middle East, Tim Askew.

Details of how Atkins wrested the contract to design the Dubai metro from the original designer, Capita, remain vague, but many of the original engineers transferred from Capita to stay with the project and now work for Atkins.

Tunnelled sections of the Dubai scheme are not far off the scale of Crossrail – 11km in Dubai to 16km in London. Crossrail's 6m internal diameter twin-bore tunnels compare with Dubai's 8.5m internal diameter single bores. Both projects are located in a dense urban
environment.

Comparisons with London begin to break down when you realise Dubai does not yet have a mass transit transport system. But its gridlocked roads are unable to cope with the city's astronomical growth. The Metro will be the first part of a greener solution.

The first – Red – line will stretch 47km from Jebel Ali in the west to the existing airport in the east, running through four stations located below ground level. The majority of the remaining line will run on a route parallel to Sheikh Zayed Road, carried on piers.

If the London Underground map was laid across this plan at the same scale, "in length it is equivalent to the distance from Heathrow to Epping," said Atkins project director John Newby.

The second – Green – line is shorter, skirting and then burrowing under Dubai creek, with six stations below ground, and running from Festival City to Rashidiya.

The project has consumed engineers and architects, with more than 1,000 designers in 12 countries working on the project. Newby says Atkins receives bills from 40 cost-centres around the world.

Initial ground investigations yielded few surprises. "We found a lot of sand," says Newby. "But ground water is also an issue for the sub-surface sections."

Because of the wet ground, Atkins opted for three earth pressure balance Mitsubishi tunnel boring machines (TBMs) for the two underground sections, with two interchanges. The slurry mode of boring, which uses bentonite to remove spoil, was rejected because the ground exerts 200kpa of pressure 35m below the surface – 4m below bored piles and a full 8m below the Dubai Creek.

"Bentonite was needed to produce an artificial pressure to launch the TBMs before they reached wet sand," said design manager Kevin Williamson. "A foam is used to maintain a constant pressure on the cutting face in case of unexpected fissures in the sand, and to improve the texture of the spoil for dumping," he said.

Earth-pressure balance machines also keep vibration to a minimum, a major consideration when passing under Dubai's built-up areas. Modelling showed that buildings above the bores would not need to be underpinned. "There were a few hiccups at first on the initial drive, but settlement has been less than 15mm, which was within what the models suggested, and in most places less than 5mm," said Williamson.

In fact, the output of the TBMs has been unexpectedly good, the machines progressing more quickly than anticipated. "Actually, one TBM is waiting underground at Rashidiya for a shaft to be dug to meet it, because it arrived a couple of weeks too early," says Paul Groves, associate director on the project.

Shafts were the first visible sign of progress on the project, at the vast Union Square site, which doubles as the project HQ. The huge station box was built top-down using diaphragm walls up to 1200mm thick extending 21m down. The concrete mix included blast furnace slag, "which makes it durable; a triple blend mix with Portland cement and microsilica," explains Groves.

Because of the ground conditions, the box was made up of a number of cells rather than being a simple four-sided structure. The majority of the walls will be retained in the final station box. "They are basically holding the bottom slab down," says Williamson. Once complete, the vast building site will be transformed into a Metro system that will surpass Moscow's for opulence. Stations will be earthquake-proof, roofs flexing under vibration, and themed on the ancient elements – earth, air, fire and water.

Once out of the tunnels, the Metro line runs mainly on elevated section. The monopile columns began to sprout around the city around a year ago. According to Bruce Maney, Atkins' design and programme director for the Metro, "Pier alignments had to be slightly flexible, as (the location of) utilities are not clear in the ground; 132kV cable can be a problem.

Sometimes it is easier to move the pier than the cable." Additionally Groves explains, "the project has to evolve with Dubai. In some places the alignment appears to rise for no reason, but anticipates new highways, or is chosen to go around new buildings not yet built," said Groves.

Altogether some 30,000 pre-cast deck units have been sourced for the project and launching of the viaduct sections is well underway. Segments are glued with epoxy and then strung together with cables, "to stress them up, so they act as one unit, and are then placed in position. For triple spans, the viaducts are extended in opposite directions from a single pier, segment by segment, to generate a growing, balanced cantilever," said Janardhan Sundaram, design manager for the viaducts.

The viaducts have reached the point where track-laying has begun. The 1.5m gauge continuously-welded track is set into a concrete slab. "If sand gets wet, it can set as hard as concrete in this climate, so concrete slab was specified as it can be cleaned easily," says Groves. The slab houses other essentials such as utility pipes and drainage.

At the Jebel Ali end of the Red line an 11ha site is being transformed into a huge depot for railcar maintenance and storage. The 400m-long, 70m-wide depot buildings will hold up to 52 cars. Two further depots near the airport are each half as large again.

But the Jebel Ali depot is especially interesting because, despite its size, it could be temporary. "The whole building is designed so it can be dismantled and reassembled somewhere else," says Maney. "There is talk that the line could be extended to Abu Dhabi in the future," he explains.

The Dubai municipality predicts that the Metro service will quickly fill a vast gap in the market, capturing up to 12% of journeys when both the Red and Green lines finally open – 9 September next year for the Red line, the Green line following shortly after.

Dubai's Metro system is far from complete, and the expansion of its transport network seems to be unstoppable: A third, Purple, line will go out to tender later this year, and a monorail is also planned which will sweep along the coast and into the Palm Jumeirah.

Who's Who:

Employer and owner is Dubai's Roads and transport Authority (RTA).

The DURL consortium are contractors - a joint venture between JT (Japan/Turkey) heavy civils (Mitsubishi Heavy Industries, Mitsubishi Corporation, Obayashi Corporation, Kajima Corporation and the Turkish company Yapi Merkezi) and Mitsubishi Railway systems, responsible for rolling stock.

JT act as Atkins' clients on the job.

A Systra-Parsons joint venture are engineers, with Serco as operators.

Atkins' job is the detailed design of elements of railway infrastructure, from reinforcement to design.

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