The Leaning Tower of Pisa has straightened by around 18 inches (45cm) since restoration work began 28 years ago, say engineers.
Engineers say it would take many years for the famous monument in Italy to straighten due t the extreme care needed with sch a fragile monument.
But to the relief of tourists who are yet to see it – the famous landmark will likely never be completely straight.
Cautious drilling and tubes were installed underground since the tower was closed to the public in January 1990 for 11 years over safety fears.
The Leaning Tower of Pisa has been straightened by half a degree by cautious drilling and tubes that have been installed underground, say engineers
‘Thanks to this system, we recovered half a degree of lean’, said Robert Cela, technical director at the Opera Primaziale (OPA), the organisation who have been working to preserve it.
‘It’s still straightening… And many years will have to pass before it stops’, he said.
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.
Three years after it was declared a World Heritage Site by the United Nation’s cultural organisation UNESCO in 1987, the tower was closed to the public in fear it would crumble.
Engineers began work to stabilise and rescue the tower for the next 11 years, and recordings show that it is leaning less now than it was before.
Engineering lecturer Nunziante Squeglia of Pisa University, who works with the Surveillance Group that was set up after the rescue work, has been studying and measuring the tower for 25 years.
He says that the tower straightened by 41 centimetres (16 inches) until 2001, and another four centimetres since then.
To understand how the 14,500-tonne building is moving, measurements are made as often as once an hour, some automatically using pendulums, some manually using a surveyor’s optical level.
The tower will never be fully straight, said Robert Cela, technical director at the Opera Primaziale (OPA) organisation who began preservation work in 1990
Measurements are taken of the famous monument as much as once an hour. Pictured, Mr Cela looking through an optical level inside the tower
‘The tower tends to deform and reduce its lean in the summer, when it’s hot, because the tower leans to the south, so its southern side is warmed, and the stone expands. And by expanding, the tower straightens,’ said Dr Squeglia.
He explains that there are three pendulums, one dating back to 1935, when systematic measurements began, although annual measurements began as far back as 1911.
‘The tower was much more mysterious when I arrived, it wasn’t clear why it was leaning, and increasingly leaning,’ said Dr Squeglia.
‘It is a building that has been extensively studied for over 100 years but there are still so many things to know,’ he said, including the remains of what looks like a domed roof inside the tower that is still unexplained.
Mr Cela predicted that the tower ‘will never be completely straight’, to the delight of tourists yet to see it.
‘When they were building it, there were attempts to straighten it (by adding stone on one side), so it has a slight banana shape’, he said.
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.
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
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.