By the time contractor Aoki-Soletanche finished tunnelling on Contract 105, it had already overcome many problems when working in the Woolwich and Reading Beds -unpredictable ground charged with high pressure water.
The Kawasaki earth pressure balance machines had been modified, changing the conditioning additive to allow the material to be properly drawn out of the plenum. Two additional TBMs with 25% increased power were brought to site to speed up the progress.
But the challenges did not stop there - the next step was to complete the station enlargements and cross passage construction at Druid Street while working with ground water pressures of up to 2.5bar.
'On the Druid Street shafts experience from working on the Storebaelt project helped us in providing confidence in the method adopted,' says project director David Lloyd Williams. 'We managed to reduce the pressure to around one bar using dewatering wells but we could not guarantee that it could be kept low enough to use compressed air - and there simply was not enough space on site for the full equipment necessary for working above one bar.'
Localised dewatering systems were set up instead, drilling through stuffing boxes in the tunnels,' explained Lloyd Williams. 'It was the most technically challenging part of the job.'
The contract was originally budgeted at £70M and will finish above £100M. 'I think that is a lot of money,' says Lloyd Williams. 'I have no doubt that if it had been started today the form of contract would be different.
'We need to question whether LUL should have used a management contracting route as this would have provided better controls in integrating the works across the line. In addition, the adoption of generic designs across the line would have eased the construction stage.'
Lloyd Williams also believes that Bechtel was not brought on to the project early enough as the linewide designated contractors would have been managed more effectively.
'The shafts at Druid Street were the deepest on the entire job,' says Lloyd Williams. 'We had to make our best interpolation between boreholes and as a contractor you have to take a judgement of what the ground will be like when you open it up.' However, he is adamant that the ground information could have been better as the troublesome outcrop of water filled laminated sands was identified at Druid Street only at a later stage.
'Delays were inevitable but we managed to hand over the station in December 1998,' he says. 'Since then we have been carrying out additional contract work to meet the requirements of outside bodies and acting as the principal contractor under the CDM regulations.'
Lloyd Williams is particularly pleased with the way Soletanche managed to integrate with Aoki on the job despite the very different cultures. 'The Japanese had very little experience of working in the UK and the language problems made it quite hard to form a team,' he explains. 'The Japanese are much tidier and more structured in their approach to construction. Face plays a huge role for them. But I believe we will have all learnt from each other.
'I do not think the public will really appreciate the difficulties that we have had on this job and what we have achieved,' says Lloyd Williams. 'Problems encountered will soon be forgotten and the public will appreciate what has been constructed.'