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Reclaiming the streets

Geotechnics - Sydney's streets will be transformed later this year when the Cross City Motorway tunnels open to cars.Ruby Kitching went down under to see the final stages.

While London chose to introduce congestion charging to combat road-clogging traffic, on the other side of the world Sydney is tackling the same problem by building two new toll roads beneath the city.

Sydney mayor Clover Moore says the £412M Cross City Tunnel scheme will benefit motorists, who will be able to cross the city in just two minutes instead of the 20 it takes at present, and pedestrians, who will have fewer cars to dodge.

Peter Samson, chief executive of tunnel operating company Cross City Motorway (CCM) confirms:

'It's about giving the streets back to the people.' By October this year, two 2.1km long tunnels will have been burrowed east-west under Sydney's central business district between Darling Harbour and Rushcutters Bay. CCM is responsible for design and construction of the link, as well as its operation and maintenance for 30 years. It will eventually be handed over to the New South Wales government. A joint venture of contractors Baulderstone Hornibrook and Bilfinger Berger completed the tunnels - plus a ventilation tunnel - last month using road headers with cut and cover techniques at the tunnel portals.

Need for the Cross City Tunnel stems from the way Sydney has changed over the years. Traffic flow was originally orientated north-south for access to the harbour crossings. But as the central business district developed, Darling Harbour sprouted hotels and restaurants, a new airport was built south west of the city, and traffic patterns changed. Crossing the city centre from east to west became more and more difficult for drivers, and pedestrians were increasingly marginalised.

Samson says that tunnel construction is being undertaken with an eye on the future:

50,000 extra people a year are expected to visit Sydney within the next decade, putting extra strain on the city's transport systems. The tunnels will also help reduce pollution in the city, brought on by traffic jams.

Despite the tunnels' benefits, the people of Sydney started out very sceptical about the project. Traffic disruption and settlement problems were caused by construction of the Eastern Distributor Tunnel six years ago, and locals wanted to make sure they had a greater say on how the project was programmed. With 70% of the project being constructed using cut and cover, and with at least nine construction sites edging to within 2m of buildings, co-ordination was the name of the game. Public consultation on the scheme produced 292 conditions to planning consent.

Concern has focused mostly on noise and vibration from tunnelling. 'If a person rings up complaining about their cutlery rattling, we have to go down and measure the vibration within two hours, ' says BHBB project superintendent Geoff Foster.

'Sometimes people prefer that you work for three days straight and three off instead of working shorter days for a week, ' adds Samson.

An extra ramp to link the tunnels with the Eastern Distributor dual carriageway was built to enable extraction of spoil without clogging up the city's streets. And the road header launch pit has been shrouded by a soundproofed building to reduce noise impact, allowing 24 hour tunnelling to take place.

Tunnelling began at the centre of the route in January 2003 so that cut and cover construction at the portals could proceed at the same time. The main tunnels are being constructed using road headers - nine were in operation at the peak of construction.

They have carved an 8.6m wide by 7.8m high face through the tough sandstone rock underlying Sydney, covering up to 6m a day. The tunnel has a wide, square section. Rock is competent, and the tunnel has been advanced in 600mm lengths, with rock bolts at up to 2m centres and two 50mm thick layers of sprayed, steel fibre reinforced concrete being used to support the walls and face.

These form the final lining.

The roadheaders are guided by on-board computers. 'This has sorted out a big safety issue - you don't have surveyors with spray cans walking around the back face any more, ' says Foster.

At its deepest, the tunnel is 53m below ground level, meaning that settlement risk is minimal. Risk to buildings is further reduced by routing the tunnel over most of its length under William Street, a principal thoroughfare. This solution is not without hitches, however, as William Street is also the main route for utilities.

'The main electricity supply passes through the Druitt Street portal and had to be incorporated into the design, ' says Foster. He adds that some service ducts were so heavy they were originally supported by piles. These have been removed and replaced by steel transfer structures.

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