In June Circle Line trains stop running for nine weeks - thanks to repairs to an understrength tunnel roof. Dave Parker reports on the design and planning of the complex repair operation facing London Underground.
Only 500mm beneath the surface of Kelso Place, W8, lurks one of London Underground's biggest headaches. Six years ago the roof over the twin covered ways that carry the District and Circle Lines below road and roadside property alike failed a safety check, and the road has been barred to vehicles weighing more than 13t ever since.
The roof, a typically Victorian structure of cast iron girders and brick jack arches, is little more than 100m long. But bringing it up to modern standards will require the covered ways to be closed while massively complicated and expensive structural surgery takes place.
LUL estimates the total cost of the cure, including the on-costs of diversions, publicity and extra staff, will be close to £15M. The risks and costs involved in failing to finish within the allocated closure are even more daunting. LUL has therefore opted to undertake the project on an alliancing basis - its first ever.
The project team, made up of LUL and Mowlem Southern, got together in January 1998. But LUL has been carrying out feasibility studies almost since the problem was discovered, according to LUL Construction supply chain manager Richard Bliss.
'Really there were only three possibilities - replace the roof, strengthen it, or make it redundant. The first was totally impractical because of the property above. Strengthening would be far from straightforward - and our policy is to make cast iron elements redundant whenever possible.'
The good news was that long-term monitoring by strain gauges and bi- weekly inspections showed no apparent deterioration of the structure, apart from some crudely-repaired WWII bomb damage. Against that, headroom between the tracks and the roof was already below modern optima, especially at the eastern end close to High Street Kensington station.
Another worry was the long-term stability of the brick retaining walls that formed the outer supports for the roof structure. LUL civil infrastructure design manager, Dennis Church, says the first solution considered made the cast iron sections redundant by filling in the recesses between the outer wall piers with structural concrete. This would support a grillage of steel beams below the existing roof structure. 'This would obviously reduce headroom and require the track bed to be lowered to compensate.
'But that would undermine the retaining walls - which meant underpinning them at least.'
The discovery of major sewers close below the trackbeds effectively ruled out that solution. By 1997 a more sophisticated alternative was taking shape.
Specially-fabricated steel beams of 'aerofoil' cross section would sit tight up under the jack arches, supported as before on the infilled recesses. Headroom would still have to be restored to modern standards, but maximum excavation to achieve this would be no more than 800mm, keeping the trackbed well above the sewers. Minipiling beneath both outer walls and the central dividing wall would ensure stability and increase load-bearing capacity.
Church says the only real engineering problem was the fact that the arches were far from uniform. 'An 8,000 point survey and 3-D modelling eventually got us down to only five different beam versions,' he reports. 'But few of the 45 new beams over each covered way will be either level or parallel.'
For the last six months or so the major challenge for the alliance team has been logistics. Mowlem project agent Jim Buckley says the real nub of the problem is getting materials into site.
'There's effectively no road access, and no easy train routes into the site. And the actual possession is so short it's very difficult to fit an engineer's train onto it.'
Handling and positioning the aerofoil beams - which weigh up to 6.5t each - was another problem to be overcome. 'At first we assumed we would use a custom-built mechanical arm mounted on an engineer's train,' Buckley says. 'Now we've gone for much simpler technology - rollers, slides and the like. It's all about buildability, really.'
A similarly pragmatic low technology approach has been adopted to get concrete into the site. Mowlem will simply drill down from Kelso Place above and install a concrete pump line to place the 1,300m3 of concrete needed in each covered way. Permanent glass-fibre cement formwork is another rationalisation introduced by the team, says LUL project manager Peter Madden, who was seconded from EC Harris for the duration.
'Having the contractor on board has enabled us to go through the plans from top to bottom. We've been very successful in reducing the number of processes and components involved - and we're soon to begin prototyping the main operations at a tunnel mock-up off site.'
Enabling works began in January. Much of these are concerned with moving and protecting cables and equipment - particularly the 30 year old high voltage gas-filled cables that supply power to five different Tube lines and will be live throughout the contract. More than 50t of 8mm steel plate is currently being fixed to protect the latter during the main works.
Although carbon fibre-reinforced plastic sections were considered and rejected as a 'much too expensive' alternative to the aerofoil beams very early on, says Bliss, CFRP will be used on the project. 'Because of the disturbance caused by construction we decided it was necessary to strengthen the cast iron girders temporarily with CFRP plates bonded to the underside.'
With a fully developed set of engineering proposals ready to present to the management, the project team also had to finalise the best option for the work schedule. Madden says: 'For a long time our preferred option was to do each covered way over normal engineering hours at night, plus 28 consecutive weekend closures and one week complete closure. But when we looked at this again we decided the risk to the rest of the network was horrendous.'
So the decision was taken to close the Circle Line this summer for nine weeks, during which work would go on 24 hours a day. Next year the District Line will be treated the same way.
This decision was not taken lightly, Madden says. 'Closing the covered way and laying on extra District Line trains to replace Circle Line services doesn't just mean changing all the Tube maps and taking on extra drivers.
'It also constitutes a Category 1 safety case change - which means we have to convince the Railway Inspectorate our plans will work as well.'
LUL also plans to trial a new rubber ballast mat in the covered ways. 'Once you've lifted the track and removed the ballast, the extra cost of the ballast mat is trivial,' says Bliss. 'But it will significantly reduce noise levels in the properties above, which have been gradually increasing over the years as the track bed deteriorated.'
So by late summer 2000 Kelso Place should have its weight restriction removed. Its residents should be enjoying more peaceful nights - and this particular LUL headache should have been cured. Given the complexity of the engineering involved, LUL's decision to give it the status of a Movement for Innovation demonstration project can only be applauded.
The Kelso Place team will be reporting progress at 'How to Rethink Construction' on 10 June. Details from Micol Klippel-Arden at CIC; tel (0171) 637 8692.