Foundations of Engineering | Leaning Tower of Pisa, Italy

Foundations of Engineering | Leaning Tower of Pisa, Italy

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The world was built by engineers, so why
try learn engineering just through a textbook. In this series we’re exploring
some of Europe’s landmarks to find out some of the theory and practice of
engineering. Today we begin with one such monument, the Leaning Tower of Pisa. If
800 years of trying to stop this structure from falling over has taught
us one thing it’s that you can’t skip on the foundations This is the Arno river and a key source
of Pisa’s soft soil. Even before it was built people noticed that the Tower of
Pisa was trying to sink into this soft ground. Today land prices are becoming
more expensive, making reclaimed land more popular, however this suffers from the
same problem as the soil here at Pisa. We need to find a way of building on soft
soil. Welcome to my laboratory, well technically it belongs to the University
of Leeds, but they’ve been kind enough to allow us to borrow it to help make this
series. It has everything that we need in order to demonstrate European
engineering. Most importantly for us today it also has a supply of liquid
nitrogen, but we’ll get to that later. This tub of water is going to help us
simulate land reclamation. We’ll begin by pouring in some of the sand. The sand
looks relatively dry but dig just a little bit below the surface and we find
it wet and waterlogged. This is just like the soils around Pisa,
or freshly reclaimed land. We’re going to finish off our scene by adding in our
own Tower of Pisa. Looks okay for now but give it a little bit of time. [Splat!] Soft soil
is terrible, in fact it’s so bad that the best thing to do is just rip it up and
replace it. That’s what we call foundations. The Leaning Tower it does
have some of these but at only 3 metres thick they are not nearly enough to
support this 14,000 tonnes tower. For buildings as large as the Tower of Pisa
on timescales of decades or even centuries, soft soil may as well be
really slow moving water. How do you build on water? Well, you can’t but what
we can do is use a raft. Today we’ll be using these two bricks to simulate
something appropriately called raft foundations. As you can see this is much
more stable. Laying massive slabs of concrete isn’t the most cost efficient
way of creating foundations, so instead we often turn to pylons. Today we’re
sinking these PVC pipes through the soil. The goal is to find bedrock or in this
case the bottom of the container. As you can see that’s just a stable and a lot
easier to create. Of course these two methods only work if you put the
foundations in at the beginning of the project. Unfortunately for the Leaning
Tower of Pisa that just wasn’t the case. The only reason that this tower can
exist today is because of a hundred-year break between building the first two
storeys and then the upper levels. This time and pressure squeezed on the soil
taking out some of the excess water making sure the soil was at least and
relatively stable. Today it’s now standard practice to wait a couple of
years before building on reclaimed land but what if you don’t have the time? That
was the case in 1995; engineers working on the tower needed a way to make sure
the ground was going to be super stable while they were adding in some extra
supports. The problem isn’t that there is water in the soil, it’s that this water
is allowed to move. Their solution: freeze it in place by using liquid nitrogen.
Let’s head to the lab to see if that would actually work.
Super-cold nitrogen from this tank passes through a plastic pipe and into
this copper tube. The plate is here to help with the cooling process. It’s buried
deep enough not to provide any stability. The now gaseous nitrogen passes through
this pipe and out of the lab, to be harmless and dissipated into the
environment. It’s been about 40 minutes and in that time everything has cooled
down. The surface is now solid, it’s basically a sheet of ice. Let’s see if
that will affect our tower. Putting it in now…. yes much more solid,
that’s not going anywhere. By freezing the ground using liquid nitrogen, Pisa’s
engineers were able to keep the tower steady until a more permanent solution
could be found. Melting the ice came with a whole new set of challenges, but in
principle it works just fine. I think there’s a lesson we can learn today: if
you can build solid, good foundations but if you can’t, a bit of ingenuity goes a
long way. Huge thank you to everyone who helped
make this video and in particular John Turton for help with the demonstrations.
We’re releasing new episodes of Engineering Europe every two weeks. Next
up we visit the Catacombs of Paris. Until then this has been James Dingley from
the Atomic Frontier. Keep Looking Up

5 comments

  1. This is the one and only perfect demonstration/educative video i have ever been recommended. Good job fellow INTJ.

  2. I understand that freezing the ground will stop the tower from moving, but for how can you keep this up? It seems like a very expensive way of problem solving.

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