CONTRACTORS BUILDING a 32km sewer tunnel under the river Thames must invest heavily in exceptionally robust tunnel boring machines to avoid risk of delay, tunnelling experts have said.
Engineers said that the tunnel will be deeper than any tunnelling project undertaken in the UK, making it probably the most challenging.
Deep chalk layers beneath the Thames will contain flints of almost diamond hardness that are expected to take a heavy toll of the TBMs.
The $3.9bn Thames Tideway sewer will be a 7.2m diameter bored tunnel running beneath the river from Hammersmith in west London to Beckton in the east.
It is needed to intercept combined sewer overflows that discharge into the Thames during heavy rain storms.
'Up-front spending on robust and technologically advanced machines to minimise breakdowns is a better way to cope with the risks than setting aside more money on contingencies, ' advised Arup head of tunnelling Bill Grose.
Regulator Ofwat has already expressed fears that the project will cost more than the $3.9bn budget.
The Thames Tideway tunnel will be at up to 80m depth, requiring work at water pressures of nearly eight atmospheres. 'You are putting unprecedented loads on the face, sealing systems and bearings of any machine you use, ' Amec tunnelling director Peter South told NCEI. Amec is a technical adviser to Thames.
The most westerly quarter of the tunnel will be in London Clay, starting at a depth of 40m and running east through Woolwich & Reading Beds. The eastern half of the tunnel will be in the deep chalk layers, reaching 80m depth at Beckton.
Flints are likely to be particularly problematic in the transition zone into the chalk, said Thames Water project co-ordinator John Greenwood.
Their hardness and capacity to damage the TBM means that machines will need frequent cutter head inspection and maintenance but reaching the cutter head will be difficult. 'We aim to fi nd sections of the chalk with fewer fissures where we'll be able to stop the machines, ' said Greenwood.
The chalk is heavily fissured and the water head runs to the surface. If a dry spot can be found it should be possible to work on the cutter head at atmospheric pressure, said Greenwood.
However, if water flow cannot be stemmed, inspection and repair will have to be carried out by divers working under high pressures.
If a dry zone cannot be reached, it would then be necessary to inject grout into the ground around the tunnel, stopping up fissures to allow inspection and repair, he said.
Normally grouting would be carried out by drilling from the surface, but because the tunnel runs under the Thames this will be problematic.
'It'll involve working from a jack up barge which will block river traffic. The Port of London Authority's not going to love that, ' South warned.