First, again: Dubai's new metro system

Mooted as the largest urban infrastructure project under construction anywhere in the World, Dubai' new metro is also the United Arab Emirates' first public transport system. Richard High reports.

Think construction in the Middle East and the chances are you'll think of Dubai. Rotating cities, islands in the shape of palm trees, or even the world, buildings shaped like chess pieces, super-hero theme parks and the world's tallest building are all either under construction in the tiny Emirate or being touted as the next "big thing".

While cornering the market in record-breaking construction projects is nothing unusual for Dubai, its new metro system is a double-whammy. Possibly the largest urban public transport project on the planet it is also the UAE's first public transport project.

"Where else would you get two lines, over 70 km of track and 40-plus stations all being built at the same time? In any other country it would be Phase One, followed by Phase Two, and so on," said Paul Abbosh, engineering consultant Atkins' director Dubai Metro Project.

One reason Dubai so badly needs a public transport system is the gridlock afflicting its roads from dawn until dusk. At present Dubai has the largest number of vehicles per person anywhere in the world. According to its Road and Transport Authority (RTA), it has 541 vehicles per 1000 residents, compared to 444 in New York and 345 in London.

The rapidly expanding population will only add to the burden placed on the network, with economic implications too. Traffic congestion is estimated to cost the economy about US$ 1.25 billion per year, equivalent to 3.15% of gross domestic product (GDP).

Breaking ground

With this in mind ground works on the metro began in February 2006, and the first two lines - Red and Green - are due for completion in 2010 at an estimated cost of AED 15.5 billion (US$ 4.2 billion). They will run underground in the city centre and on elevated viaducts elsewhere.

Operated by Dubai's RTA, and constructed by the Dubai Rapid Link (DURL) Consortium - Japan's Mitsubishi Heavy Industries, Mitsubishi, Obayashi, Kajima and Turkey's Yapi Merkezi - it will, surprise, surprise, be the longest, fully automated rail system in the world.

The 52.1 km-long Red Line will have 35 stations and run from Jebel Ali Port to the Airport Free Zone. The 17.6 km-long Green Line, with 22 stations, will run from Dubai Festival City to Dubai International Airport Terminals 1 and 3, and to Rashidiya. Current estimates suggest the two lines will carry over 1.2 million passengers per day.

The underground sections include a mix of bored tunnels, cut and cover tunnels, annex shafts and mined adits. These include 10.7 km of 8.5 m internal diameter, twin track, bored tunnel constructed in 10 drives, seven annex shafts for tunnel ventilation and emergency egress. There are 14 short mined adits to the bored tunnels and 1.7 km of cut & cover tunnels in four stretches.

Bob Eves, Atkin's deputy project director, told iC the tunnel works are notable for several things, including being the first major bored tunnelling and tunnel mining works in Dubai and the UAE; tunnel driving in sands and extremely weak sandstone rocks with very high permeability; the use of permanent diaphragm walls and top-down construction methods for the annex shafts and cut and cover tunnels.

The first drive between Union Square and Burjuman Stations passes below the Dubai Creek. It used one of three Mitsubishi earth pressure balance Tunnel Boring Machine (TBM), nick named "Al Bugeisha".

Earth pressure balance TBMs were used, said Mr Eves, because, besides the country's ubiquitous sand, there was a lot of ground water. The alternative method, slurry boring that uses bentonite to remove the spoil, was rejected because the ground pressure 35 m below the surface is 200 kpa.

Using this system also meant keeping vibration to a minimum, useful when the machines pass just 4 m below the deepest bored piles on their route and about 8 m below Dubai's Creek.

"Everyone thinks the tunnel under the Creek is the most difficult part," said Mr Eves, "but its probably the easiest because your not having to deal with old test piles, cemeteries, buildings, wells and sink holes."

Above ground

For the elevated sections most of the bridges comprise pre-cast, post-tensioned segmental decks with a U-shaped or box-girder cross-section. Spans range from 17 to 72 m.

Most of the decks up to the 44 m spans are erected using seven overhead launching girders. For the 47 km of viaduct on the Red Line the precast segments are lifted into place and over the piers. For longer spans - those over 44 m - the precast deck segments are erected using the balanced cantilever method with a specially manufactured pairs of travellers.

The substructure consists mainly of cylindrical piers supported on large diameter bored cast in place mono piles. The pier heads are formed using a precast concrete shell with a complex double-curvature architectural shape and a reinforced concrete infill, pre-stressed using multi stage post tensioning.

The main advantages of having pre-cast, post-tensioned segmental viaducts, according to Mr Eves, is the speed of installation and assembly, as well as the limited impact they have on the surrounding road traffic.

The viaduct segments are pre-cast in a 540000 m2 casting yard located at Jebel Ali, where they are produced and stored before being delivered for use. At the site they are lifted into place, assembled and then glued and tensioned together by temporary and permanent post-tensioning.

Most of the Red Line's elevated track will run parallel to Sheikh Zayed Road from Bur Dubai to Jebel Ali. A shorter section will extend from Dubai Airport to Deira City Centre. The Green Line will have two sections of elevated track on the Deira and Bur Dubai sides of the Dubai Creek.

Network expansion

The next phase of development will see the Blue Line run along the Emirates Road, while the Purple Line will run along Al Khail Road and act as an express route between Dubai International Airport and the new Dubai World Central International Airport (JXB), the largest airport currently under construction in the world.

Construction is now well advanced across the whole of the planned system. And there is talk of linking it to a regional rail system.

A feasibility study into the extension of the Red Line to the Abu Dhabi border is nearing completion. According to Mattar Al Tayer, chairman of the board and executive director of the RTA, the construction contract should be awarded later this year.

"Expansion is inevitable," said Bob Hope, Atkin's director, Northern Gulf. "It will only bring benefits to the region - ease congestion, reduce the number of cars and the amount of freight on the roads, and make moving around easier - all of which will improve people's quality of life."