Tilted towers: Why do buildings lean?

From the Tower of Pisa in Italy to Britain's Big Ben, some of the world's most famous buildings have a distinct list.

However, architectural leanings are not confined to history.

A few modern buildings -- like the 541 foot (165 meter) Montreal Tower, whose 45-degree lean makes it the tallest tilted building in the world -- are intentionally inclined, but those that lean without design are a property developer's nightmare.

As leading civil engineer John Burland explains to CNN, although most slanted skyscrapers are safe, people don't want to live in them.

"If residents know their building is leaning, they're not going to be very happy -- unless of course it's leaning so much that they can charge people to go up it," says Burland, an emeritus professor in engineering at London's Imperial College. Burland designed the solutions that stabilized the Tower of Pisa and Big Ben.

The reason why some buildings develop a lean, and how such problems can be corrected, is a complex area of geology and engineering called geotechnics. CNN spoke to Burland to unravel the science of foundations and find out why he likens Pisa's lean to children building blocks on carpet.

CNN: Given our modern understanding of the earth and access to technology, why do buildings still end up leaning?

Burland: "We are very sophisticated in our analysis now, but we still have to understand how mother nature works.

"Mother nature lays down ground in all sorts of variable ways and unless you actually spend the money and the time investigating the ground properly -- and employ people with good knowledge and experience -- then you can run into big problems."

CNN: Are leaning towers a common problem and are they safe?

Burland: "They're relatively rare but I think when you are dealing with towers you do need to be extra careful because they reveal themselves in a way that a lower building wouldn't.

A low building may settle a bit more at one end than the other, but most of that can be taken up within the building's structure.

Most of the leaning buildings I'm involved with are not unsafe but selling space in a building that has been known to be leaning is very difficult because often the leaning is caused by some defect in construction or design.

The trouble with a tall building is that you can see it leaning, which people don't like. It might be safe, but the owners of the building are not going to be at all happy."

CNN: What are some of the foundation issues that can cause a tower to tilt?

Burland: "The ground may have softer or weaker spots which you haven't detected. There can be unexpected geological faults, or you may not have gone deep enough with your ground investigation."

There can also be design issues and, quite often, depending on the type of foundation, there's construction control issues, for example on the installation of piles. So there are many causes of problems that range from the ground through to design right through to construction control and supervision."

CNN: Looking at modern buildings, are the geotechnical problems that occur more the fault of mother nature or human error?

Burland:"If it's mother nature then it's human error because they haven't investigated it properly.

There's this clause in most contracts about unforeseen ground conditions, which is designed to protect contractors. But if it's unforeseen then the ground investigation, or its interpretation, has not been adequate."

CNN: How do you stabilize or center a leaning tower?

Burland: "In principle its simple -- and that's the essence of genius isn't it, simplicity -- but actually it requires a lot of expertise.

You've got to find out why it's leaning, so you've got to spend a lot of money investigating the ground around the tower (and) what's happening over time.

The classic solutions involve what's called compensation grouting or injecting grouting into the ground to try and lift one side of the building compared to the other. It has to be carefully controlled and it uses very specialized equipment.

Or you can cut the building at its foundation level to insert jacks and jack the building straight, that has been done many times.

At Pisa, because the ground was so incredibly soft, we did the opposite and took some ground out.

But it's not something you rush into...you can get it terribly wrong if you haven't got the techniques to do it properly.

CNN: Why was stabilizing the Tower of Pisa so difficult?

Burland: "Pisa is rather special and it becomes highly technical but the best way is to give an analogy.

The ground Pisa is built on is very compressible indeed, and the reason for its leaning can be explained by the way most children will try and build towers on the floor with wooden bricks.

They will know that if you are on a solid wooden floor you can build a tower that's quite high, but if you start building it on a carpet, you can only get to a certain height and then it starts to lean. However hard you try you can never get higher.

That's because once the tower begins to move, even a little bit, the center of gravity moves and it drives the tower even more towards the direction that its moving.

And that's the reason why Pisa is leaning -- it's a bit like trying to build a tower on a soft carpet."

CNN: What building do you consider to be the world's worst titled tower?

Burland: "There is a kind of joy in having the biggest leaning tower in the world...a kind of competition about who's got the world's biggest lean, and I think I have a wry smile about it all.

Really, the fact that Pisa Tower is so magnificent and so breathtaking in its architecture -- it would have been a world heritage monument even if it wasn't leaning -- I think that's really what makes it so famous really.

So it's fun, but none of them are as beautiful as the Pisa Tower."

This interview was edited for length and clarity.

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