This tunnel right here is the latest addition to London’s subterranean landscape. It’s 25
kilometres long, 7 metres wide and cost more than USD $5BN which is about GBP £4.5BN.
But this isn’t a new rail tunnel and trust me,
when it’s up and running, you definitely will not want to go for a ride in it.
But it will relieve London of one of its most urgent problems.
Every year, 40M tonnes of raw sewage is dumped
into the Thames. That’s enough to fill Wembley stadium 40 times over.
This once in a century project is designed to put a stop to that but there’s just one thing …
The land under London is a maze of tube lines, mail rail, pipes and potentially unexploded bombs.
So how on earth do you dig an enormous new tunnel through
all of this, without bumping into something?
The UK has a sewage problem.
England’s rivers have a chemical cocktail of sewage, agricultural waste and plastic.
Since 2020 England’s water firms pumped sewage into
our rivers and seas for more than 7 million hours
A rainwater and sewage mix for more than 9 months of the year
Across the country, sewers are bursting at the seams and their contents are ending up in rivers.
It’s turning this green and pleasant land into the colour of the BBC weather map.
The Thames is no exception. In 2022,
14.3BN litres of sewage ended up in the capital’s main watercourse.
London’s existing plumbing dates back to the Victorian era, when engineer Joseph
Bazalgette was commissioned to create the city’s first comprehensive sewer system.
When it opened in 1865, Bazalgette’s sewer was a triumph of engineering. But
the population of the city it was built to serve has almost tripled since then.
So in 2016 work began on this, the Thames Tideway Tunnel.
Roger, for people that don't know what is Thames Tideway, what's this project about?
So the Thames Tideway tunnel is a 25 kilometre, 7.2m internal diameter tunnel
from 30m deep in the west to about 70m deep in the east, to capture the flows of
sewage that currently overflows into the River Thames from the old Victorian sewer network.
Currently, when it rains in London, or if there is any other kind of surge, overflow
pipes funnel the excess waste into the river. The new super sewer works by intercepting the
waste and plopping it down a deep shaft into the new tunnel, where it floats away for processing.
We’ve tried to make that sound as nice as we can.
Essentially, it’s a 21st century backup for this centuries-old sewage system.
For the tunnel to work, it has to stay close to Bazalgette’s original system,
running alongside the Thames for most of its route.
It also needs to fall consistently by 40 metres
over its length to allow gravity to facilitate the flow of feculent.
And that’s the catch. The ground underneath London is almost as congested as the state above it.
Where do you start with digging a tunnel that big under London? There's
so much happening under our feet. What kind of stuff you have to think about?
Well, the planning that goes into a project like this is quite enormous. So from 2007 to 2014,
seven years of design and planning. We transit something like 43, 22 or 43 tunnels as we've
come our 25km. Two of them, below us in the west. All the rest of them are above us.
Once the river was mapped, there was just the small issue of avoiding everything else.
This is the Victoria Embankment. It runs for two kilometres along the north side
of the Thames and was created by Bazalgette to house his sewer. Before this was built,
all this would have been the muddy bank of the Thames.
But it’s not just the sewer that’s underneath here. This stretch is also shared by the Circle
and District underground lines. There’s also another 10 tube lines which intersect
the path of the Tideway Tunnel as they cross underneath the Thames,
as well as a multitude of road tunnels, utility pipes, and mail rail to name just a few.
So how do you avoid that?
So you really like threading a needle the whole way through London.
We were getting underneath it really … like here … our equipment is about five metres
away from a couple of tube lines. We've very close to bridges, so we go under,
some of the most iconic bridges in London… amount of instrumentation assessment of
those structures to make sure that when the tunnel boring machine passes underneath,
typically 50 metres below the surface of the river, that nothing untoward is going to happen…
In 2018 with the tunnel route set, it was over to the tunnel boring machines to start digging.
Because of the mix of chalk and clay under London, six different TBMs were used.
So, a 25-kilometre tunnel, under one of the busiest cities in the world with multiple
machines to align … that’s a lot to keep track of. And the team did it by using this: Esri ArcGIS.
We needed to be able to generate an understanding of where things were in the ground today,
the geology as well as the physical built environments. And we needed a tool that
would enable us to see that in a way that engineers and planners could interact with.
ArcGIS enabled the team to compile hundreds of years of data ranging
from the most up to date CAD models to Bazalgette’s original blueprints.
This let the team create a 3D model of the entire project with millimetre
accuracy. This map is then used by everyone from engineers working on specific sections,
to planners looking at the overall project.
That huge source of data is also used to create specific applications for the team on the ground.
If we were out today, worrying about where all the doors were and we wanted to map those,
that's a really easy thing for us to spin out and do. We capture the data in the
field.... Modern telecoms communications allow us to beam that straight back into
our databases and push it straight back out through our web web mapping front
ends. So effectively… it's almost real time data from field to desk
Having the ability to feedback data in real time
was especially valuable in some of the project’s most sensitive areas.
Under my feet is where the river fleet flows out into the Thames where it will
soon be diverted into the new Thames Tideway Tunnel.
But there’s just one problem, it’s directly under Blackfriars bridge.
To get around this, the culvert, which would divert the flow into the new sewer,
was constructed in a cofferdam next to the bridge.
But once again, London’s underground got in the way. The ideal place to build the
cofferdam is exactly where the Waterloo & City line runs under the Thames and up into Bank.
This made it impossible to build the foundations needed to keep the
cofferdam stable, meaning it had to be built even further away from the bridge.
Once the culvert was constructed, it then had to be floated into place,
before being rested on a concrete slab on the river bed.
Tunnel works was completed in 2022 and the team is
now testing the tunnel before it gets commissioned later in 2024.
This might not be the most glamorous project, but it’s
a perfect example of how infrastructure is the unsung hero of our modern lives.
When it’s up and running, this will contain all but the most
extreme overflows into the Thames. And play a vital role in making this river
clean and safe for the people and wildlife that rely on it.
You can learn more about the Thames Tideway Tunnel and the other topics on our channel
over on The World’s Best Construction Podcast – available now wherever you
get your podcasts. And as always guys, if you enjoyed this video and you want
to get more from the definitive video channel for construction,
from the channel that takes you inside some of the world’s biggest sewers, hit that subscribe button.