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