RUSH-hour motorists in Belfast were forced to take alternative routes today to avoid traffic mayhem following the collapse of one of the main arterial routes in the city.

Traffic dwindled to a fraction of its usual levels around Cromac Street this morning after motorists heeded warnings to avoid the scene of a road collapse.

Motorists had been warned to expect up to a week of serious congestion after an underground air bubble caused tarmac on Cromac Street -- of the city's busiest roads -- to sag, leaving a hole big enough to hold two double decker buses.

A team of more than 20 workers spent the weekend battling round the clock to get the road back into pristine condition.

And this morning, Roads Service sounded a cautious note of optimism, saying that if all goes well Cromac Street could be reopened to traffic by midweek.

Network maintenance manager Colin Brown said work had progressed well thanks to the hard work of staff employed by Roads Service, Northern Ireland Water and the contractor as well as BT and other utilities.

It is believed work on the Belfast Sewer Project had disturbed an air pocket in the substrate underlying the city, causing it to rise to the surface.

"The collapse had breached an old Victorian brick sewer and that was restored yesterday and is now up and running," Mr Brown said.

"The road sub base is being laid and compacted and they are building it up from formation level. They are peeling off the existing surface today.

"The tunnel engineers are carrying out tests today to check for compaction and depending on the outcome it will allow us to progress. It's all hinging on the outcome of today.

"Northern Ireland Work who are carrying out the physical civil work are aiming for a week but depending on progress it could be as early as the middle of the week."

Mr Brown said the problem arose because the road is built on a deposit of material known as Belfast Sleech.

"Periodically we can get voids forming under the roads. Sometimes it's very clear what has caused it, other times it's quite a mystery," he said.

He added he was encouraged by the number of motorists who heeded advice to come in early and avoid the congestion or to choose other routes.

The section between East Bridge Street and May Street has been closed and drivers were warned to expect considerable congestion. Motorists were advised to avoid the area altogether if possible.

Among those passing by was Jim Robinson from Lisburn who said he hadn't noticed any additional traffic problems on his drive in this morning.

However, his daughter Clare had been forced to use the motorway this morning because of problems on her usual route.

The rupture occurred above a storm-water tunnel which is part of the Pounds 120m Belfast Sewers Project. It is thought to have been spotted by a passing bus driver early on Saturday morning.

An underground air pocket is believed to have become disturbed and risen to the surface. The tarmac mostly stayed intact, but a 15sq metre hole emerged underneath.

Bill Gowdy from Northern Ireland Water said the air pocket was probably disturbed a year ago during work on the sewers project.

"The tunnel is being driven under public roads so there was never any risk to buildings in the area," he said.

"It was a very unusual event which occurred after a rare chain of events and is unlikely to happen anywhere else in Belfast.

"We have dug out the hole and hope to have it filled in by the weekend. The work is continuing 24 hours a day and we will do our best to have the road open quickly."

Mr Gowdy said the new storm-water tunnel lies 15 metres beneath the surface and was not damaged.

"The tunnel has not been affected and is on course to begin operating before the end of the year."

CAPTION: Paul Johnston (above) surveys the damage done to Cromac Street__Belfast city centre traffic (below left ) and (below) repairs begin__Colm Lenaghan/KELVIN BOYES/alan lewis

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