Back in January myself and John LeSurf were lucky enough to be invited for a visit down the Lee Tunnel construction site at Beckton. This is the first stage of the long awaited Thames Tideway Tunnel (or "Supersewer") which will capture the estimate 39m tonnes of sewage which overflow into our river every year.
I started writing this up straight after the visit, but then got bogged down and never finished it. So apologies for the rather open-ended article; I'd prefer not to waste what I did write and hopefully what's here is interesting enough anyway.
Visit to the Lee Tunnel
Wednesday 21st January 2015
TAC were recently offered an opportunity to visit the construction site of the Lee Tunnel out at Beckton sewage works. Fellow TAC member John LeSurf was also invited, as fisheries officer of the Billericay Dickies AC.
Arrival
We arrived at Gallions Reach DLR station, a remote and desolate concrete outpost with no parking located in the middle of nowhere, not even within walking distance of its namesake shopping mall. There we met the other visitors for the day; Matthew Frith (London Wildlife Trust); Geoff Loader (Thames Tideway Tunnel communications) and his daughter (writing a dissertation on flooding); Lisa Horman (councillor from London Borough Hammersmith & Fulham) before being collected by our hosts, Thames Water, in a Ford Galaxy, and taken to Beckton sewage treatment works.
Security at the works is pretty tight, we all had to bring a photo id and queue up to have our picture taken and visitor passes issued for the day. This is probably as much for health-and-safety reasons as general security, most areas of the works, especially deep bubble tanks, are pretty dangerous.
Introduction
We were taken to the site training offices where we met Richard Aylard (director of external affairs) , Andy Sefton (construction manager) and one of the tunneling engineers.
Thames Water has 14m customers, 350 sewage works and 66,500 miles of sewers. The Lee Tunnel is part of a £5bn investment by Thames over the current 5-year period.
Beckton is the largest sewage treatment works in the country, with the recent increases in capacity it is now the largest works in Europe. It serves around 3.5m customers. The Lee tunnel, works capacity expansion and saltwater desalination plant comprise a £1bn investment. The tunnel will capture discharge from the largest CSO (Combined Sewer Overflow), Abbey Mills, at 16m tonnes this is approximately 40% of the total Thames discharge.
Desalination Plant
Beckton is home to the UK’s first saltwater desalination plant, commissioned in 2010. It is capable of supplying between 50-250 Ml/d (million litres per day) of fresh drinking water. This water is 10x more expensive than usual rain/reservoir water so it is only used when needed to keep the reservoirs topped up in times of drought and not used continuously. The feed is taken from the river (upstream of the sewage works outlet) via a lagoon which is replenished during the last 3 hours of the ebb tide. At this stage of the tide the water used is less salty and hence less expensive to desalinate . London as a whole uses 2,000Ml/d so the plant can supply about an extra 7% capacity.
Safety Preparation for Tunnel visit
Before visiting the tunnel, we were given a safety briefing. Everything on site is very safety oriented.
We were each issued with personal breathing apparatus which would ensure a supply of oxygen in the highly unlikely event of fire, air contamination or other mishap underground. This is an MSA SavOx chemical rebreather, giving about 30 minutes of emergency oxygen supply (if you’re moving) and up to 180 minutes if you’re at rest. It contains potassium superoxide, which is interesting as it reacts with moisture from your breath to give off oxygen, then further reacts with the carbon dioxide you breathe out producing stable potassium carbonate. These rebreathers are therefore compact self contained devices and there is no need for an air tank. You breathe through a snorkel-type mouthpiece. There are also goggles in the container for emergency eye protection.
After the briefing we suited-up in high-visibility orange jackets and overtrousers, safety boots, hard hats and safety glasses. We were ready to go!
Down the Shaft
The shaft we would be going down was a short walk from the training centre. When complete, this shaft will be the overflow discharge from the tunnel when it becomes full. Therefore it’s the final construction at the far end of the system, one of the deepest points at 75m. The deepest shaft of all is the one used for pumping sewage out up to the works, it is 85m deep and constructed in a slightly further “upstream” part of the system.
First we were taken to an overhanging observation area where we could look directly down into the shaft. The scale is impressive, a hole Xm in diameter, going 75m straight down. To start with they basically dug small sections, sealing off the sides with concrete as they progressed. Finally a smooth lining was applied over the rough wall to seal it. There are a few places where the wall is not perfectly sealed due to adverse weather conditions having affected the setting during installation. These will be fixed by injecting a sealant into the cracks at high pressure. This is the only shaft that has suffered in this way.
We then descended the shaft in a cage lift. Various services and fittings all go down the shaft: an air duct, to push clean air to the workers further up the tunnel; Concrete pump pipes; A set of ladders, in case the lift fails, and all the usual electrical services and so on. At the bottom is a railway, with carriages for workers and rotating concrete trucks. The locomotives are diesel, but there is sufficient volume and movement to keep the air clean.
One difficulty at this depth is the ground-water pressure, about 4 bar. Water is constantly trying to force its way into the tunnel. At a few joints in the wall you could see evidence of this, but mostly it is sealed tight.
The tunnel wall as we saw it is unfinished, consisting of interlocking prefabricated concrete rings. In the unfinished tunnel these rings are bolted together, also various services are also attached to them. As the tunnel is finished, the bolts and other items are all removed and a smooth concrete coating applied, about 30cm thick, this will give a perfect smooth seal.
The positioning accuracy is phenomenal when you consider that they are essentially digging a hole “blind” underground. Sewer construction theory has found that a gradient of 1 in 700 gives a self-scouring effect without excessive turbulence. Keeping close to this magic value ensures that minimum maintenance will be required. The tunnel is therefore constructed to within 80mm horizontally and 30mm vertically of the planned position.
As the boring machine digs along it assembles and locates rings of prefabricated concrete, locking each against the previous ring. Each ring section has a slight overall taper (about 30mm) to one side which is used to steer the direction of the tunnel by fitting the ring in the direction they want the tunnel to go.
Lee Tunnel Facts and Figures
Q & A with Aylard
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