The slant of the Leaning Tower of Pisa is reducing. The lean of the structure has fallen back into line by 40mm the past two decades according to a new report from the engineers monitoring the structure.
In 1990 the top of the tower was 4.5m off its vertical alignment and the structure was the subject of a huge remedial project which reduced the displacement by around 450mm.
According to media reports from Italian news agency ANSA, Nunziante Squeglia, a professor of geotechnics at the University of Pisa who works with the surveillance team, said its latest measurements have found it has straightened by 40mm in the last two decades: “Oscillations now varying at the average of 0.5mm a year, although what counts the most is the stability of the bell tower, which is better than expected”.
Professor John Burland of Imperial College London’s soil mechanics department was instrumental in the 1999 project to save the tower, which had been at high risk of structural failure.
Although there had been ideas to reduce the slant through underpinning, these were considered too risky, he said. Instead, the project team used soil extraction under the higher side of the tower.
Burland told New Civil Engineer: “We reduced the out-of-plumbness by about 450mm and that’s what we did by the soil extraction. It wasn’t enough to be immediately visible, if you lined it up yes, but anyone coming to the site would still see its leaning dramatically, so it didn’t alter its character in any way.”
Originally completed in 1350, the leaning tower of Pisa was built in stages over 177 years.
The tower sits on soft clayey and sandy silt over a thicker layer of highly compressible Pancone Clay at a depth of 10m to 25m. This in turn rests on a layer of denser sand.
Compression of the ground and fluctuations in the water table are thought to be the main causes of settlement which caused the tower to lean.
The structure’s distinctive tilt began during the construction phase but gradually worsened, and in 1990 it was closed to the public because of safety fears. It reopened in 2001.
Burland visited the tower three weeks ago, meeting with the team monitoring the site. He said that although the tower is still moving, it is at a very slow rate which is reducing. “It won’t be long before it is stationary,” he said.
But what can engineers learn from the project, whose success is still being heralded?
“I think the lessons we learnt were more to do with the methodology of measuring and controlling the process,” said Burland.
“When it came to this technique it was very closely monitored, I took responsibility. The techniques of controlling and having a clear line of responsibility are very important. There are so many ground operations now, things like controlling the subsidence of buildings above tunnels, as on Crossrail and so on. All that depends on observations and proper control and the techniques we developed at doing that were certainly influential in developing the up-to-date message of identifying responsibility, the processes observing and reacting to it and chains of command.”
He added: “Those are the really big lessons. It has been hugely successful, but it could have gone desperately wrong if it had not been properly controlled and there had not been clear lines of communication and responsibility.”
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