Thames Water has celebrated a major milestone on the £711M Lee Tunnel project with the removal of the 125t cutterhead from the tunnel boring machine that has created the project’s 6.9km long tunnel.
The Lee Tunnel is the first of two tunnels which will collectively capture an average of 39M tonnes of sewage a year from the 35 most polluting combined sewer overflows (CSOs), built by the Victorians as part of a sewerage network that still serves London 150 years on.
The Lee Tunnel will tackle discharges from London’s largest CSO at Abbey Mills Pumping Station in Stratford, which accounts for 40% of the total discharge. Instead of being discharged in to the river Lee, the wastewater will now be captured and transferred to Beckton sewage treatment works.
This has meant boring London’s deepest-ever tunnel and involved tunnelling through high groundwater pressures and passing through 6.9km of the most abrasive ground, without any other shafts along the way. At its deepest - the base slab of the pumping shaft at Beckton - you are 86.5m below ground level.
To do this, TBM Busy Lizzie was used. A closed faced slurry tunnel boring machine, it blended over 100t of excavated chalk with water for every one metre of tunnel advance. The bored diameter was just under 9m in order to create the final 7.2m internal diameter transfer tunnel. The finished tunnel will be completely waterproofed with the inital 350mm thick primary lining supplemented with a 300mm thick secondary lining.
Secondary lining now more than 50% complete
Work to install the secondary lining is now 55% complete, and is using a method untried in the UK. Instead of using traditional reinforcement and a sprayed concrete, the Lee Tunnel’s secondary lining uses a patented fibre reinforced concrete, placed using a travelling formwork in 30m sections.
The concrete mix features 60mm long double hooked fibres and a C50 concrete. The formwork has been purpose built by Kern of Switzerland, with two 30m long shutters able to pour an entire 300mm ring section at a time. Four pours a week are allowing work to progress at 240m a week. But it is technically challenging.
“No-one has secondary-lined a tunnel like this, with a full-round formwork,” said Thames Water construction manager Andy Sefton. “The 30m of shuttering is quite a lot. And the batching is a big challenge.”
Concrete is batched on site at ground level and transported via “bullet” wagons on the temporary railway that is used to service the work site deep under east London. Such is the logistics of this operation that concrete batches can be up to three hours old before being placed and a delicate balance is being struck in ensuring it remains workable without the fibres segregating from the concrete.
Lessons are being learned along the way, explained Sefton, with regular feedback offered to the Thames Tideway Tunnel team, the £4.2bn tunnel that will constitute the second phase of the Tideway Improvement programme.
Work is on schedule.
Lee Tunnel: fast facts
The tunnel is 6.9km long, up to 75m deep and when complete will have an internal diameter of 7.2m.
To excavate it a Slurry mix-shield tunnel boring machine was used, with a bored diameter of 8.88m.
The Herrenknecht TBM cost £15M, was 100m long and weighed 1,241t. The cutterhead alone weighed 125t.
Tunnelling progressed at an average rate of 17m/day using a crew of 10.
Spoil excavated by the TBM was 450,000m³.
The tunnel has five shafts, with the largest - the pumping shaft at Beckton - 38m in diameter and 86.5m deep.
Thames Water is client, with CH2M Hill acting as project manager, Design and build contractor is the Morgan Sindall/Vinci/Bachy Solentache joint venture.