Tunnels of love, sweat and tears

'There wasn't much float before in the programme, but now there is none.' Marcus Karakashian, senior supervising engineer for the Jubilee Line Extension on contract 104, summed up the challenge facing joint venture contractor Costain/Taylor Woodrow.

The œ76 million London Bridge station contract, part of the œ1.9 billion Jubilee Line Extension project, was set back by three months when tunnelling, using the New Austrian Tunnelling Method (NATM), was suspended at the time of the Heathrow Tunnel collapse.

Work was stopped voluntarily by the contractor, in consultation with the HSE, although one year's work had so far produced no causes for concern. Despite this delay, the contract, described as the most complex of the project, has now been re-programmed to put it firmly back on the rails.

At 208 weeks, this programme is the longest of the project to place the work on London Bridge station right on the critical path.

However, not only does the contractor have to contend with this pressure and an effective three month's cut in the programme period, it is also having to work in the tightest conditions possible - both underground and on the surface.

For the entire tunnelling workload the only point of access is the shafts at Redcross Way - the focus for all the office accommodation, stores, workshops and materials storage. Here there is one large diameter shaft, used for personnel access, services and cranage, and one smaller diameter shaft used for spoil removal. Surrounded by a cramped working area, the two shafts are used for the transfer of all workers, materials and equipment.

Everything radiates from this point - the contractor has to construct a complex interchange between the new stretch of the Jubilee Line and the existing Northern Line station, all squeezed under and around the London Bridge railway station.

This involves driving eastbound and westbound running tunnels for the Jubilee Line, constructing a station concourse, ventilator shafts and a series of escalator shafts, cross passages and escape shafts. At the same time, improvements are being made to the Northern Line (otherwise known as the Misery Line) with a new section of southbound tunnel enabling the old section to be used as a station concourse.

'It's all part of working in London,' said John Wallace, the joint venture's tunnelling manager. From the bottom of the shaft there is a short cross passage to the Jubilee Line running tunnels. There is also a 25m long, 11.3m diameter trial tunnel constructed at the start of the contract.

Heading in the opposite direction towards the existing Northern Line tunnels, a 50m long, 5.3m diameter trial tunnel was also constructed. This was then extended into a 125m long access adit for work on the new southbound Northern Line tunnel. This adit joins the new tunnel close to the point where a step plate junction is being constructed to bring the new and old southbound tunnels together. As the two tunnels begin to meet, one larger diameter tunnel is formed which then progressively steps down in diameter to the point where the two coincide.

Work is currently under way at this point with the tunnellers (or miners) working in extremely cramped, but dry, conditions. Using pneumatic clay spades, the miners have excavated a large eliptical chamber out of the London clay which is progressively lined with spheroidal graphite iron (SGI) segments - claimed to be the first section of eliptical tunnel in the country. The large chamber surrounds the recently exposed Northern Line tunnel.

Looking as good as they must have done in the 1920s, the cast iron segments of the tunnel firmly bolted together are exposed for approximately 30m and look like an enlarged gas main. The 3.6m diameter 'tube' rumbles at regular intervals as trains pass through just a few inches away.

When the step plate junction is complete, the old tunnel will be gradually dismantled leaving the trains running through an enlarged chamber. But for the moment, the cast iron tunnel is supported in concrete at the invert and by a series of steel frames braced against the surrounding chamber. The miners are now working at the narrow section of the step plate junction with very little room for manoeuvre as they excavate around the tunnel. The face is advanced 3m before the next support frame is put in place.

The SGI segments are then located by some awkward manhandling and the help of an air winch. The rate of progress here is just 3m a week.

But work is progressing on other faces at a healthy rate using various tunnelling techniques. 'For anybody who's interested in tunnelling this site is ideal,' remarked Marcus Karakashian. 'We've used nearly all types of tunnelling .'

This is partly due to the suspension of NATM working and the rapid adjustments which had to be made by both client and contractor. There was no way that both parties could wait for definite conclusions from the HSE investigation into the Heathrow collapse, so alternatives to NATM had to be sought and approved.

What actually happened was that some areas, such as the new Northern Line station tunnel reverted to traditional tunnelling methods. Others, including the Jubilee Line running tunnels, adopted a revised NATM process which involves the normal techniques of reinforced shotcrete but has the addition of a secondary lining of in-situ concrete as a safeguard.

'Most of the changes were made with agreement of both sides,' said Karakashian. 'But there were also many design changes.' Coping with these changes, along with the lack of space and restricted access, has, for John Wallace, been the challenge of the contract.

On top of that, maintaining the interface between the joint venture partners has also been demanding. But both contractors and the client are obviously sore at the need to change or abandon their NATM working.

'We had been working for nearly a year without any problems,' said Karakashian. 'NATM is much more flexible and has a far quicker start up time,' agreed Wallace.

But all are apparently happy with current progress and insist that the revised programme will be met. Although, ironically the new NATM method is really no different than the old.

The primary lining of reinforced shotcrete is being left exposed for some considerable time before the secondary in-situ concrete lining is constructed - none of which is in evidence now.

However, as you would expect, careful and thorough monitoring of earth pressure and movement is being carried out with, they claim, no unexpected results to date.

The running tunnels are excavated using a machine with an articulated boom which can accurately achieve the required profile. The face is excavated as heading and bench, with the heading always maintained 1m ahead of the bench.

When 1m of tunnel profile is exposed, a 25-50 seal- ing layer is applied by the 'nozzle man' spraying the shotcrete. The mesh is then placed with a lattice girder of 1m intervals, and the final layer of shotcrete up to 150mm.

The shotcrete itself is a retarded dry mix with water and accelerator added at the point of spraying, and is designed to achieve 10N/mm2 at 24 hours, and 25N/mm2 at 28 days. For testing, panels are cast in the tunnel and cores are taken every 24 hours. In addition a photograph is taken of every joint.

The exposed tunnel face is also sealed with shotcrete every 1m, and at weekends a full primary lining is applied. But full pressure is exerted on the primary lining only when the tunnel's face is two diameters away. The NATM is progressing surely and safely.
WHO'S WHO

Client:

London Underground

Contractor:

Costain/Taylor Woodrow

Technical Contractor:

Mott McDonald

Ground Treatment:

Keller Colcrete