Fears over tunnel safety halt Tube work: Cave-in raises questions over construction method. Martin Whitfield reports

WORK ON parts of London Underground's Jubilee Line extension was halted yesterday following the collapse of a tunnel during the construction of the pounds 300m Heathrow to Paddington rail link.

Both projects, the capital's largest transport infrastructure investments, use the same controversial tunnelling methods. The landslip, close to the airport's Terminal 3 car park, happened early yesterday morning and led to the closure of roads and Tube stations. About 25 workers were evacuated from the tunnels after shifts in the earth were detected. No one was injured.

Surveyors were studying the extent of the damage last night. It is known to involve the main concourse area for the link and Terminals 1, 2 and 3 and the platform areas close to the main entrance shaft.

'Our first priority is to make the site safe,' said a spokeswoman for the Heathrow link. Work on the Terminal 4 concourse, which also uses the new tunnel method, has been halted. The Picadilly Line Tube links to terminals 1, 2 and 3 were re-opened but the line to Terminal 4 is expected to be closed until at least Monday. Bus sevices are transporting passengers from Hatton Cross.

Represenatives of Heathrow Express, the company responsible for the link, said it was too early to say what caused the collapse but investigations are likely to concentrate on use of the 'New Austrian Tunnelling Method'. London Undergound, which is using the same system at sites beneath Waterloo and London Bridge stations on the Jubilee Line extension, said work had been suspended.

'We have stopped work until we know the detailed causes of the collapse. We have detected no movement at either site,' a spokesman said.

Doubts about the new method were first raised after a collapse in Munich earlier this year in which two people were killed when a bus fell into a crater left in the road.

Nick Raynsford, Labour MP for Greenwich and opposition spokesman on London, visited the Heathrow tunnel site on Tuesday but said he was told that a landslip on the Munich scale was impossible in a bedrock of London clay. 'It is a matter of great anxiety as the same method is being used in the Jubilee Line,' he said.

Traditional tunnelling methods, whereby concrete linings are placed in position immediately following excavation, are still being used in both the Jubilee Line and the Heathrow link for the main tunnel lengths.

The New Austrian method, in which concrete is 'sprayed' on the the earth walls before linings are placed in position, is used at intersections and cross-overs. It is said to offer a cheaper and less time- consuming construction process.

Passengers using Heathrow were urged to allow extra time to get to the airport and advised to use public transport. Parking is restricted as the Terminal 3 car park remains closed.

Damage to the concourse and tunnel is likely to be extensive, but the entrance shaft remains intact. The Heathrow link is expected to be completed in 1997 and will give passengers a 16-minute journey from the airport to Paddington station in west London.