The secret of the Leaning Tower of Pisa: Study finds unique mix of soil helped it stay upright during four earthquakes - Daily Mail

May 10, 2018

The secret of the Leaning Tower of Pisa: Study finds unique mix of soil helped it stay upright during four earthquakes
 58-metre tall tower leans precariously at a five-degree angle
This leads to an offset at the top of over five metres
Survived at least four strong earthquakes that have hit the region since 1280
By Mark Prigg For Dailymail.com

PUBLISHED: 03:29 AEST, 10 May 2018 | UPDATED: 03:44 AEST, 10 May 2018

The mystery of how the Leaning Tower of Pisa stays intact has long baffled scientists.

Despite leaning precariously at a five-degree angle, leading to an offset at the top of over five metres, the 58-metre tall tower has managed to survive, undamaged, at least four strong earthquakes that have hit the region since 1280.

Now, researchers they believe they have the answer - soil.

Despite leaning precariously at a five-degree angle, leading to an offset at the top of over five metres, the 58-metre tall Tower has managed to survive, undamaged, at least four strong earthquakes that have hit the region since 1280. +3
Despite leaning precariously at a five-degree angle, leading to an offset at the top of over five metres, the 58-metre tall Tower has managed to survive, undamaged, at least four strong earthquakes that have hit the region since 1280.

The research team concluded that the survival of the Tower can be attributed to a phenomenon known as dynamic soil-structure interaction (DSSI).

 'Ironically, the very same soil that caused the leaning instability and brought the Tower to the verge of collapse, can be credited for helping it survive these seismic events,' said Professor Mylonakis, of the Department of Civil Engineering at the University of Bristol, who led the study. 

The research team concluded that the survival of the Tower can be attributed to a phenomenon known as dynamic soil-structure interaction (DSSI). +3
The research team concluded that the survival of the Tower can be attributed to a phenomenon known as dynamic soil-structure interaction (DSSI).

The team say the considerable height and stiffness of the Tower combined with the softness of the foundation soil, causes the vibrational characteristics of the structure to be modified substantially, in such a way that the Tower does not resonate with earthquake ground motion.

This, they say, has been the key to its survival.

The unique combination of these characteristics gives the Tower of Pisa the world record in DSSI effects.

Professor Mylonakis, from Bristol's Department of Civil Engineering, was invited to join a 16-member research team, led by Professor Camillo Nuti at Roma Tre University, to explore this Leaning Tower of Pisa mystery that has puzzled engineers for many years.

Given the vulnerability of the structure, which barely manages to stand vertically, it was expected to sustain serious damage or even collapse because of moderate seismic activity.

THE BATTLE TO PRESERVE THE LEANING TOWER OF PISA
In 1987 the Tower of Pisa was declared a World Heritage Site by the United Nation's cultural organisation Unesco, but as fears grew that it would topple over it was closed to the public in 1990 and engineers worked to stabilise it for the next 11 years.

'The tower was on the verge of collapse, but we managed to stop the tilt and secure it,' said Giuseppe Bentivoglio, from the Opera Primaziale organisation that preserves the tower.

The tower was reopened to the public in 2001 and remained open throughout a restoration costing almost £6million - partly to keep tourists happy, but partly because the revenue from ticket sales helped pay for the upkeep. The tower attracts over one million visitors a year.

In 2010 restorers made the tower even more stable by removing soil from beneath one side of its foundations. Its angle was previously 5.5 degrees from the perpendicular, but is now only 3.99 degrees off straight.

Experts say the seven-storey bell tower should now be safe from further intervention for at least the next 200 years.

But how did the tower achieve its lean? The most respected theory suggests the tower began to sink after construction - which began in 1173 - had progressed to the third floor after five years.

The cause was a flawed design - it had a foundation that was only three-metres deep set in weak, unstable subsoil.

Landmark: In 1987 the Tower of Pisa was declared a World Heritage Site by the United Nation's cultural organisation Unesco, but as fears grew that it would topple over it was closed to the public in 1990 +3
Landmark: In 1987 the Tower of Pisa was declared a World Heritage Site by the United Nation's cultural organisation Unesco, but as fears grew that it would topple over it was closed to the public in 1990

Back then that area of Italy was very belligerent, with various local land-grabbing factions jostling for position.

Because of the battles between Pisa and nearby Genoa, Lucca and Florence, the construction of the tower was put on hold for almost a century.

Thankfully this allowed enough time for the soil to settle - had there not been that length of break, many believe the tower would have toppled over centuries ago.

When tools were picked up once more, under architect Giovanni di Simone (who had built the Camposanto Monumentale, the fourth and last building to be erected in Cathedral Square) in 1272, the engineers built upper floors with one side taller than the other, in an effort to compensate for the tilt.

Because of this, the tower is actually curved. Construction was halted again in 1284, when the Pisans were defeated by the Genoans in the Battle of Meloria, and the seventh floor was not completed until 1319. Its stewardship at that point had passed to Tommaso di Andrea Pisano.

Just seven miles from the Mediterranean Sea on the west coast of Italy, the tower, which weighs some 14,500 metric tonnes, is frequently battered by storms that have eroded and discoloured it.

The distinctive, yellowish stone came from the quarries of San Giuliano, visible from the top of the tower on the green hills behind Pisa.

Surprisingly this hasn't happened and until now this has mystified engineers for a long time.

Results from the study have been presented to international workshops and will be formally announced at the 16th European Conference in Earthquake Engineering taking place in Thessaloniki, Greece next month

Read more: http://www.dailymail.co.uk/sciencetech/article-5710455/The-secret-Leaning-Tower-Pisa.html#ixzz5F5hxcUsD
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