Why Does The Leaning Tower of Pisa Lean?

Alice Roberts

3 mins

30 Oct 2019

In the 12th century, Pisa cathedral was a beacon of power and piety. Its white stones, which shone brightly in the hot Tuscan sun, were embellished by Romanesque details and multi-coloured marble, mosaics and bronze – the spoils of war.

When the patrons sought to compliment this with a campanile, a bell tower, they would never have guessed it would become one of the most famous engineering puzzles of history, with experts still working on it almost 900 years later.

Image source: Saffron Blaze / CC BY-SA 3.0.

Construction of the campanile

The construction of the tower was recorded by Vasari:

‘Guglielmo, according to what is being said, in the year 1174, together with sculptor Bonanno, laid the foundations of the bell tower of the cathedral in Pisa’.

Indeed, the design of Guglielmo and Bonanno Pisano was a free-standing structure resembling a column. Despite this beautiful design, it soon became clear that the foundations were too weak for the unstable sub-soil below.

At the construction of the third story, the tower was already leaning, and work was immediately halted – Pisano couldn’t risk the ruin of his reputation.

The entrance door to the tower is at a large tilt. Image source: Lonewolf1976 / CC BY-SA 3.0.

It was almost a century later, in 1272, that Pisano’s disastrous project was picked up by the successful architect Giovanni di Simone. He built four more storeys at a straight angle to try and compensate the lean.

Not only did this look totally bizarre, but the extra weight caused the foundations to sink further into the soft earth. By 1360, the tilt was at 1.63 metres.

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200 years after the first stone was laid, the ‘Torre Pendente di Pisa’ was finally completed when the bell chamber was added in 1372, containing seven bells. The tower now stood at 56 metre tall and had a diameter of 7.4 metres at the base.

From bad to worse

The gradual increase in tilt seemed to slow over the following centuries. It seems that the weight of the tower had compressed the sub-soil, giving it a degree of stability. This was soon compromised in 1838, when the architect Alessandro Gherardesca made an attempt at restoring a straight angle.

After digging a trench around the foundations, the soft earth was removed and replaced with a marble basin. Like other efforts before this, it only worsened the problem. The base flooded regularly, and by 1918 it was 5.1 metres off the vertical, increasing at about 1 mm per year. Evasive action was needed.

The Pisa Baptistery, the Pisa Cathedral and the Leaning Tower of Pisa. Image source: Florian Hirzinger / CC BY-SA 4.0.

In 1990, the Italian government closed the tower to the public to carry out essential stabilising measures. In 1994, 690 tons of lead bars were attached to the north side to act as a counterbalance.

In 1998, the north side was tied down with two steel cables, each one weighing 4 tons. Other measures, such as injecting concrete into the foundations, proved ineffective.

Setting the record straight

At the turn of the century, new approaches were still being applied. Vacuum tubes eased under the north wall removed some 30 tons of earth, enough to allow the tower to be straightened by 50 cm. It seems to have worked, and the current tower stands at the same angle as it was about 250 years ago. This work enabled its reopening in 2001.

Lead counterweights from 1998. Image source: Rolf Gebhardt / CC BY-SA 3.0.

As well as being enjoyed by tourists, the tower was used between 1589 and 1592 by Galileo Galilei. He dropped cannonballs of different masses to demonstrate that their speed of descent was independent of their mass.

During the Second World War, the Allies suspected it was being used as a German observation post. Although an artillery strike was planned, a U.S. Army sergeant was so impressed by its beauty that it was spared from destruction.