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The 21st century Leaning Tower of Pisa

You wouldn’t think it possible that anyone could build the modern day equivalent of the leaning tower of Pisa.

Surely with all of the engineering knowledge we have today, this couldn’t happen?

And yet some unlucky San Francisco residents have seen their prestigious apartments become something of a joke. Their 58-storey building now leans by 14 inches at the roof, and has sunk 17 inches since construction started in 2005. And it is still moving, though not at a rapid pace.

When you’ve paid between $1.6 million and $10 million for your apartment, it’s probably not an amusing situation to be in.

Interestingly, the proposed solution to the problem – to drill 50-100 new foundation piles into the bedrock 200 feet below – was discussed before construction began, but the developer decided it wasn’t necessary.

There are several buildings in the San Francisco Bay area that are built like the Millennium Tower, with hundreds of piles into sand, and the friction means that they are stable. But in this case, whether it is due to local construction work – the pumping of water from a neighbouring construction site – or other reasons, this building isn’t as stable as it should be.

It’s estimated that the mistake will cost between $100 million and $150 million to put right, and an ongoing lawsuit will decide who foots the bill.

Although the soil conditions are different, it’s not unlike the problem suffered by the infamous landmark, the Leaning Tower of Pisa.

Except with the Italian tower, locals have always been keen to keep the “lean” while making sure the tower doesn’t collapse.

After all, the “No Longer Leaning Tower of Pisa” isn’t much of a tourist attraction.

With the Millennium Tower, the problem appears to be fairly straightforward, if expensive to solve.

Whereas with the Leaning Tower of Pisa, it has taken engineers several centuries to figure out what the problem is and to remedy it. It was John Burland, a professor at Imperial College London, who finally found the solution.

He solved the 800-year-old mystery when he discovered that the tilt was caused by a fluctuating water table, which would perch higher on the north side of the tower, causing its tilt to the south.

At the time of his discovery, the tower had been closed due to fear of collapse – it was leaning at 5.5 degrees – and its future was uncertain. The preservation team decided to tackle the problem by extracting soil from underneath the tower, removing around 77 tonnes of soil and straightening the tower by 44cm. This put it back to its 1838 inclination.

Today, the Leaning Tower of Pisa leans at 3.97 degrees, and it’s expected to be safe for the next 200 to 300 years.

The underlying soil is an important element of any building project. If you need the input of a structural engineer on your next project, please don’t hesitate to get in touch.

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