The Not So Leaning Tower of Pisa! Famous structure has crept upright by 1.6 INCHES over the last 20 years thanks to stabilisation works, study finds
- The Leaning Tower of Pisa was built upon soft soil which was weaker on one side
- It became increasingly tilted over the years, until in 1990 it was deemed unsafe
- An 11-year stabilisation project was started that reduced its lean by 15 inches
- In the 20 years since, the tower has straightened itself by another 1.6 inches
Thousands of tourists visit the Leaning Tower of Pisa every year to pose as if they’re resting up against it or holding it up.
While this photo op is a classic, the tower’s tilt has been a cause for concern for engineers and historians for decades.
Fortunately, a recent survey has revealed that the bell tower in Italy is steadily moving upright thanks to stabilisation works.
An 11-year stabilisation project reduced its lean by 15 inches by 2001, and in the 21 years since, the tower has straightened itself by another 1.6 inches (4cm).
A recent survey has revealed that the Leaning Tower of Pisa (pictured) in Italy has moved upright by 1.6 inches (4 cm) since its last round of stabilisation work
Theories suggest that the 57 metre (186-feet) tower, which has been a mystery to experts for hundreds of years, began to sink after construction – which began in 1173.
The cause was due to a flawed design which meant it had a foundation that was only three-metres deep and was set in weak, unstable subsoil.
This base was also softer on the southern side, resulting in the eponymous lean, so builders tried to compensate for it while they built its eight storeys upwards.
They did this by making the floors shorter on one side than the other, making the structure curved as well as tilting,
The recent study was funded by the preservation organisation Opera Primaziale della Pisana (OPA) to see if their predictions were correct.
While the tilt has been reduced, it still sways at an average of about 0.02 inches (0.5 mm) a year, according to professor of geotechnics Nunziante Squeglia from the the University of Pisa.
‘Although what counts the most is the stability of the bell tower, which is better than expected,’ he told Italy’s National Associated Press Agency (ANSA).
An OPA spokesperson also told ANSA: ‘Considering it is an 850-year-old patient with a tilt of around five meters and a subsidence of over three meters, the state of health of the Leaning Tower of Pisa is excellent.’
The medieval bell tower, a symbol of the power of the maritime republic of Pisa in the Middle Ages, has managed to survive, undamaged, at least four strong earthquakes that have hit the region since 1280.
A 2018 study found that it was the softness of the foundation soil, which was originally responsible for its lean, that protected it from the violent tremors.
Theories suggest that the 57 metre (186-feet) tower began to sink after construction, which started in 1173.
The cause was due to a flawed design which meant it had a foundation that was only three-metres deep and was set in weak, unstable subsoil.
This base was softer on the southern side, resulting in the eponymous lean, so builders tried to compensate for it while they built the eight storeys upwards.
They did this by making the floors shorter on one side than the other, causing the structure to curve as well as tilt.
The build wasn’t completed until 1319 due to various battles between Pisa and Genoa, Lucca and Florence, but these pauses did give the underlying soil time to settle.
Pisa’s leaning tower before the stabilisation works in 1992 (left) and at the end of the works in 2010 (right). Since the works, the tower’s tilt has decreased even more
The tower’s stabilising involved years of work, including emergency temporary steel cables, excavation of soil and digging wells to drain water
HOW DID THEY STRAIGHTEN THE TOWER IN 2001?
In 1990, the Leaning Tower of Pisa was still tilting 5.5° southwards, and it was closed to the public in fear it would crumble.
After applying a few different failed strategies to attempt to straighten the structure, engineers decided to try out ‘soil extraction’.
This involved digging out two lorry loads of earth from under the tower’s north side before using steel cables to pull it upright.
While this was effective, the team found that the building leaned more during the winter rains, as the water table underneath the north side was higher.
They therefore also had to dig drains so that the water could flow into nearby wells and keep the foundation stable.
Fast forward to 1990, the tower was still tilting 5.5° southwards, and was closed to the public in fear it would crumble.
After applying a few different failed strategies to straighten the structure, engineers decided to try out ‘soil extraction’.
This involved digging out two lorry loads of earth from under the tower’s north side before using steel cables to pull it upright.
While this was effective, the team found that the building leaned more during the rains in winter, as the water table underneath its north side was higher.
They therefore also had to dig drains so that the water could flow into wells and keep the foundation stable.
The project was completed in 2001 after straightening the tower by 15 inches (38 cm), or 0.5°, and it has continued to correct itself ever since.
Experts pronounced it would be safe for the next 300 years during a health check in 2005, according to ANSA.
They also said that they thinks that advances in technology will mean the tower will one day straighten up completely.
In 2013, another check-up revealed that the Italian landmark had straightened up vertically by about an inch (2.5 cm) since 2001.
That year, an organisation called CyArk created a 3D reconstruction of the Leaning Tower of Pisa as well as some 500 other landmarks.
The last round of stabilisation works was a 11-year long project that concluded in 2001. Pictured: Tourists visit the Leaning Tower of Pisa, free of scaffoldings for the first time after 20 years of stabilization and restoration works in 2011
In 2013, an organisation called CyArk created a 3D reconstruction of the Leaning Tower of Pisa capturing details that could be damaged by weathering. The resulting reconstructions could be used if the tower ever needs repairing
The project used laser scanning and other reality-capture technologies to create digital models of the sites that are accurate to just two to six millimetres
This enabled details to be captured that could otherwise be damaged by weathering, and the resulting reconstructions could be used if the tower ever needs repairing.
In 2018, another survey was conducted by OPA to measure how far the tower had straightened since 2001, and found that it had by 1.6 inches (4 cm).
While this hasn’t been improved upon since then, its angle is only 3.99° off straight, but this is twice as large as the tilt it started with in 1350.
In 2018, another survey was conducted by OPA to measure how far the tower had straightened since 2001, and found that it had by 1.6 inches (4 cm). While this hasn’t been improved upon since then, its angle is now only 3.99° off straight
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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.
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.
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