NATM back in use on Jubilee project

The controversial New Austrian Tunnelling Method has restarted on the Jubilee Line Extension and could get underway again on the troubled Heathrow Express project next month.

The Health and Safety Executive has given the Jubilee Line Extension Project team the green light to use the method less than three months after a NATM tunnel - being built by Balfour Beatty - collapsed at the Heathrow Express.

But the HSE's all clear excludes complex junctions and larger diameter tunnels - the JLE must still submit plans for these areas.

And the HSE is still concerned about the use of compensation grouting - which strengthens the ground near the tunnels - and has also asked for more information before this technique is resumed.

JLE project director Hugh Doherty confirmed that NATM was restarting on the two affected contracts - contract 102 (Balfour Beatty-Amec) and Contract 104 (Costain-Taylor Woodrow) - and played down concern about compensation grouting.

'We have not used compensation grouting in the 1km of NATM tunnelling completed so far, though in some of the rest we are making preparations in case we do need it,' he said.

Doherty's team also needs to satisfy the HSE over JLE's emergency arrangements in the event of a collapse and to look at its liaison with public utilities in the event of an emergency, particularly the co-ordination with British Gas.

The HSE go-ahead gives contractors two to three months work, provided work keeps clear of areas that need compensation grouting. Doherty said that he expected any delays to be made up in the overall Jubilee programme.

At Heathrow Express, investigators say they are 80% of the way towards establishing the cause of the tunnel collapse on 21 October 1994.

Design teams are working on ways of opening up the problem area next month - all options involve excavating the area affected by the collapse but the project teams say that the preferred solution could well involve NATM.

'We're confident of being back on the site of the collapse in February,' said a spokeswoman for Heathrow Express. 'We are now working to hold the original schedule which involves us completing the project by December 1997.'

But the HSE's investigation at Heathrow is far from over. 'It is months from being complete,' said a spokesman. 'We are still asking Balfour Beatty for information.'

n Problems with a 5000t caisson at Durand's Wharf on Jubilee Line Contract 107 have been resolved according to JLE. Dewatering was not working, so the caisson was sunk to the bottom by compressed air instead. Tunnelling, which was delayed, will get under way later this month.