I’m on the Charles River in the City of Boston, despite its tranquil appearance this is actually
one of the busiest stretches of water in America. Not above the waves but below them; there are
more underwater tunnels here than at almost anywhere else on the planet.
Infact, if you were to connect end to end you could start to form an underwater link
between the United States and Europe. Today we’re going on an adventure through
the history of Boston to discover just how to make that transcontinental link.
Completed in 1904 the East Boston tunnel is the oldest underwater rail link in America.
It connects the eponymous East Boston - including a major international airport - to the main city,
carrying about 70,000 passengers per day.
Counterintuitively, it’s much harder to dig through soft soil than it is through solid rock.
That’s because when you have a firmer material you can dig the tunnel now
and just add the supports in later. Whereas when you’re digging under
the softer sand like underneath a river, then the whole thing just collapses in on itself.
A telling statistic is that although we’ve been digging tunnels for more than 4000 years,
it’s only been in the last 200 that we’ve been able to build them underwater.
That’s all due to an invention known as a tunneling shield.
Here’s what it looks like in the workshop - a rivet steel semicircle
10 metres in diameter and about 3 metres thick. With this interior view you can really appreciate
the insane loads it was designed to carry. Two such shields were constructed and
then lowered into massive pre-dug shafts on either side of the river.
The actual excavation works in a three step repeating process.
First the shield is pushed into the soft soil using some hydraulic jacks.
Next, protected by the shield, workers then remove the material which gets carted up to the surface.
A significant portion of that international airport
I mentioned earlier is built on the soil extracted from this very tunnel.
At this point if we were to then remove the shield then the roof would cave in so to stop that from
happening a strong internal wall of concrete and steel is fabricated directly beneath the shield.
Once the sides are ready to take the load themselves the jacks are reactivated and
the shield slides onwards into the dark. 30 years later an almost identical technique was
used to build the Sumner Tunnel but this time to service Boston’s growing love of the automobile.
What's weird about this tunnel is that while the cross section that was dug
is very clearly a circle, if you look at it today it’s much more rectangular.
That is a lot of extra dug material that doesn’t really go to any use.
So what have they tried to put a square tunnel in a round hole?
At the deepest point of the crossing there are about 250 tonnes of sand, soil, water, and ducks
pressing down on each metre section of length. That is a huge amount of force so to see if
withstanding that force has anything to do with the shape that your tunnel is I’ve
built my own test using some folded up pieces of paper and a fish tank.
We’ve got a circular section - the type that was cut in the tunnel - a square - which would be most
efficient - and finally a triangle because I’ve heard that triangles are the strongest shape.
To see how they withstand some load I’m going to be pouring in a bit of sand
and a couple of bricks to see which one breaks first.
The triangle crumbles first, followed by the rectangle,
and finally the circle is last to fall.
Given the extreme deformation we saw I wouldn’t feel particularly safe under any of our tunnels.
Least of all because they’re made of paper.
However it was very clear that circles worked the best.
That’s because their smooth sides and lack of corners meant that there were no stress
concentration regions, allowing stress to be evenly dispersed and the structure to last longer.
This means that if we’re going to be building a Transatlantic tunnel, or indeed
any tunnel under a high load situation, then circles are the best shape to pick,
Our tunnel bore will take us about 400 km off the coast although as we descend
past the continental shelf even our cylindrical shell would be crushed by the extreme pressure.
On top of that, you’ll eventually run into the mid-Atlantic ridge
which has weekly magnitude 5 earthquakes and actual lava.
We need a tunnel that is above the sea floor so that it doesn’t get crushed by the pressure,
but is still below the surface so that it won’t get destroyed by hurricanes and pirates:
a special sort of tunnel pioneered within this very parking lot.
Now I could proceed to explain how it works within said parking lot, but I think I found somewhere a
lot more interesting just up ahead. Welcome aboard the Beaver.
A replica of the 1773 trading ship which was the ground zero for the Boston Tea Party.
Late at night a group of around 100 colonists stormed aboard
intent of dumping it’s cargo of taxed British tea directly into what is not the Fort Point Channel.
About 200 years later it was ground zero for another revolution but this
time instead of the American Revolution for Independence instead it was the Boston Big
Dig Revolution for Better Transportation. As it turns out, the properties that make it
such an attractive place to put ships in - namely its central location and massive size - have also
made it a nightmare for Boston’s road and rail. The Big Dig was a megaproject constructed from
1991 to 2007 with a goal of opening up the city and cutting back on traffic.
This had actually been attempted before in the most 1950s American way possible with a massive
elevated motorway right through the CBD. The Big Dig took a different approach
by replacing these motorways and adding in some new ones but all underground.
Most relevant for this episode, the new Ted Williams tunnel beneath the Charles
River needed to be connected to the Massachusetts Turnpike and Highway One.
A route that passes directly through the very large and very central Fort Point Channel.
The problem with this proposal is that space was incredibly tight.
On that side is the US Postal Service and this the global headquarters for the Gillette company.
And below me is the Red Line which is the subway system
which is the busiest past of the entire network.
So, we can’t go over the river, there's no room and these companies weren’t going to move.
And we can’t go under, there's a really old trainline in the way.
The only solution was to go through the Fort Point Channel.
Building underwater is incredibly difficult. As you can imagine, if I were trying to assemble
this box then the duct tape wouldn’t stick right, the cardboard would disintegrate, and if I were
wearing scuba gear I’d have lots of trouble just trying to fold it up in the first place.
I did actually try to rent some scuba geat to demonstrate just how difficult it would be,
but it’s actually illigal to swim in the Charles due to some
relatively dangerous levels of blue green algae.
There are workarounds for each of these situations like using trained divers
with specialist underwater equipment and concrete designed to actually work in this situation.
However, they are incredibly expensive and best left to one-off tasks.
Ironically enough, the first step in building underwater should be to get rid of the water.
With the Fort Point Channel crossing engineers took advantage of what's called a casting basin.
This is where a temporary dam is constructed facing onto the
channel and behind which a pit gets dug. Because we’re really good at making things
on dry land this gives us the perfect opportunity to do some optimising.
With a regular tunnel you can have a maximum of two teams working at any time - one on one
side of the water body and one on the other tunneling to meet each other in the middle.
However now that we have a casting basin and all of the manufacturing benefits
that entails we can engage in a little bit of parallelisation.
You see, if I had to build one massive tea chest all by myself then it would take some time,
but if we split it into three smaller sections and have the Sons of Liberty helping me out
then we can do the entire thing much faster. In the same way, rather than taking 100s of
years to construct our channel crossing, we can do the entire thing in just a
few months by converting all available beachfront property into casting basins.
Looks like we’re all pretty much done, let’s grab some of the actual tea chests
and show King George just what we think of him. - These sir at the tools that King George and his
Parliament will use to rob you of your natural British rights. Will you pay this tax on tea?
- No - Ready. One! Two! Three!
- HUZZAH!!!!! - Well done
Once manufacturing of the actual tunnels was complete, the basin was
flooded and sections floated out into position. To stop the Red Line from being crushed, pylons
were installed, extending deep into the bedrock. When the signal was given the ballast tanks
flooded and tunnel sections sunk to come to rest on these supports.
Using these techniques we can build a tunnel all the way through the oceans,
kept in place with some massive pylons. Alternatively rather than filling
the ballast tanks all the way up we could carefully control their volume
allowing us to remain neutrally buoyant like a massive submarine.
We could keep it in place using some massive tether lines or carefully controlled thrusters.
lives of all those who live within it. With 100 more years of innovation,
who knows what we’ll create. This has been James Dingley from
the Atomic Frontier. Keep Looking Up.