Professor John Burland is the only British scientist on the tower's 14-strong restoration committee, which has spent months trying to coax the tower to a standstill in a complex civil engineering operation. He said yesterday: 'In the past few days we had confirmation . . . we have stabilised the tower.'
Work began on building the tower, the campanile or bell tower of Pisa Cathedral and the world's most famous lopsided monument, in 1173 and took 200 years to complete. It has been moving from the start, but an increase in its rate of tilt in recent years led to its closure to the public in 1989.
Professor Burland, professor of soil mechanics at Imperial College London, is elated by his success and confident it is now safe enough for the authorities to allow tourists access. 'We have stabilised the tower, so it would be entirely safe and in my personal view politically a very good thing to allow people back,' he said. 'The risk of imminent toppling over has been removed. This is the first good news for the tower for 800 years.'
The tower is 58.4 metres high and weighs 14,700 tonnes. 'It rests on a very soft foundation and has been moving around from the word go,' Professor Burland said. The tower has moved downwards by about three metres since it was built. It was closed after its tilt, currently standing at five and a half degrees, began increasing by 1/600th of a degree annually.
The first part of the operation to stabilise it was completed a year ago, with the tower circled by lightly pre-stressed cables to help prevent the masonry cracking.
Then in July, the restoration team began laying 600 tonnes of lead ingots on top of the raised foundations to the north of the tower, opposite to the side that is leaning.
The last ingot was laid 10 days ago and according to Professor Burland, not only has the tower stabilised but it has even moved back by one centimetre at the top.
'Our philosophy has always been to increase the stability of the tower in the short term to give us time to develop a permanent solution without the constant fear of the damn thing coming down on us,' he said.
'The leaning side is highly stressed and we were very concerned about that exploding. Our calculations showed it was accelerating and that the stability was marginal.'
In civil engineering terms, Professor Burland explained that his temporary measures as having increased the chances of the tower not overturning by 15 per cent.
He will now begin trials on a permanent solution. There are two possible approaches, both of which rely on inducing subsidence on the north side of the tower. The aim is to decrease the inclination by between half and one degree - 'enough to make the tower stable for hundreds of years'. This could take another four to five years.
'The situation in Italy has never been easy,' the professor said. 'The bureaucratic difficulties are immense. But the situation now is much more positive than it has ever been.'
The restoration committee meets at the end of March, when it is expected to discuss the issue of public access. The tower is packed with highly sensitive monitoring equipment, so scientists would have very early warning should the traffic of people threaten to cause damage to the tower.Reuse content