This weekend, HSE inspectors widened the scope of their inquiry into the Heathrow collapse to take in the controversial New Austrian Tunnel Method (NATM). The technique was used at Heathrow - despite concern over its suitability for soft London clay - because engineering consultants identified a cost saving of up to 25 per cent over the traditional techniques considered suitable for London's soft earth.
NATM involves lining the walls of an excavated tunnel with wire mesh, then spraying them with quick-drying concrete. A second, tougher concrete lining is laid later. This replaces the old method of inserting pre-cast concrete slabs into the tunnel as it is dug.
The HSE's initial brief was to investigate the circumstances of the Heathrow tunnel collapse, which caused an office block to topple and forced staff to evacuate part of the airport last weekend.
However, the HSE has also decided to consider whether NATM is as safe as conventional methods. Balfour Beatty, the contractor, welcomed the inquiry, but refused to comment further. An HSE spokesman said it was too early to say if any prosecutions would follow the investigation.
The collapsed tunnel was part of the pounds 300m Heathrow-to-Paddington rail link. The same tunnelling method is being used beneath London Bridge and Waterloo tube stations for the pounds 1.9bn Jubilee Line Extension. Workmen at Heathrow, London Bridge and Waterloo stopped work pending the inquiry's results.
This weekend, Stephen Norris, the transport minister and minister for London, warned that the hold-ups could add tens of millions of pounds to the cost of the projects.
'There is no doubt that it is a setback,' he said. 'NATM offered us the opportunity to build both Heathrow Express and the Jubilee Line extension in a way which represented good value for money in as short a time as possible.
'Now we have to reconsider whether it is available to us. It is inconceivable that we could press ahead immediately.'
Michael Meacher, the shadow transport spokesman, will use tomorrow's transport debate in the House of Commons to demand assurances that safety will not be compromised.
He said: 'This appears to be once again short-termism - using cheaper methods which aren't fully tried and found acceptable.'
NATM, which was first developed by an Austrian engineer 36 years ago, eliminates the need for using some expensive equipment during excavation. It also enables workmen to cover intricate cavern walls more easily than with the slabs.
However, some engineers believe its suitability diminishes in softer ground, which can subside when excavated. The slip then exerts pressure on the initial and relatively weak tunnel lining. Investigators are checking whether it was this process which led to last weekend's collapse. Until Heathrow Express, NATM had never been used in soft soils on a UK project. It was used on the Chunnel rail link, but in the harder chalk of the Channel bed.
Doubts surfaced earlier this year when the British Tunnelling Society debated the motion 'NATM is not appropriate for use in London clay'. David Fawcett, the engineer whose motion was outvoted, said the event was tongue-in-cheek. However, he said the possibility of ground movement during excavation meant that NATM required greater caution.
While the Heathrow project was being planned, Mott MacDonald, a firm of engineering consultants, produced a report which raised the issue of NATM in London clay. It recommended digging a trial tunnel, and pointed out that NATM could save 20-25 per cent on the cost of station complexes. John Curtis, a director of the firm's tunnels division, said the test showed ground movement of 25mm, well within the prescribed safety limits.
A month ago, the death of three people in an NATM tunnel collapse in Munich raised fears again. However, digging at Heathrow continued until nine days ago and the first signs of movement. All work stopped and staff were evacuated. Since then, engineers have pumped 3,000 cubic metres of concrete into the excavated area to prevent further slides.