The £635m Lee Tunnel will prevent 16 million tonnes of sewage entering the River Lee each year - a result of the capital’s Victorian sewers not being big enough to cope with heavy rainfall.
Named ‘Busy Lizzie’, the huge machine is being lowered underground in pieces, before it is fully reassembled to its final size of 8-metres in diameter and 120 metres long. The 800 tonne cutter head will be the first ‘piece’ of the drill lowered.
Constructing the Lee Tunnel will be a huge engineering challenge, involving tunnelling through high groundwater pressures and passing through four miles of the most abrasive ground from the starting point at Beckton to the destination at Abbey Mills.
The Lee Tunnel is the first of two tunnels, which will collectively capture an average of 39 million tonnes a year of sewage from London's 35 most polluting combined sewer overflows. The Lee Tunnel will tackle discharges from the largest overflow at Abbey Mills in Stratford, which accounts for 40 per cent of the total discharge.
Roger Mitchell, Thames Water’s Lee Tunnel project manager, said:
“Lowering the cutter head is a four-hour operation, requiring a lot of precision. To lift it we’re using one of the biggest cranes in Europe, which is so large it had to be transported in 60 lorry loads.
“We will drive the tunnelling machine through chalk for most of its length. Chalk is a weak rock, and easy to excavate by machine, but the biggest challenge is that the chalk contains many flints - a very hard and abrasive rock, lumps of which can vary in size from that of a melon to a car, and we could encounter them anywhere.”
Custom-designed to suit London’s ground conditions, the machine will blend more than 100 tonnes of excavated chalk with water for every one metre of tunnel, forming a white slurry - a similar consistency to single cream, before transporting it through a pipe the length of the tunnel, so it can be processed above ground.
Work started in September 2010 to build the 80-metre-deep entry shaft at Thames Water’s Beckton sewage works, where tunnelling work will start. Tunnelling is due to begin in January 2012 and is likely to progress at a rate of 17 metres a day, and is expected to finish in late 2013.
The MVB consortium, made up of three of the world’s leading civil engineering contractors - Morgan Sindall, VINCI Construction Grands Projets and Bachy Soletanche - is working with Thames Water to build the Lee Tunnel.
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