The engineer who helped save the leaning tower of Pisa tells the story to an Irish audience, writes Anna Nolan
The Leaning Tower of Pisa doesn't lean quite so much any more. Nor is it in danger of falling over altogether. Much of the change has to do with an English engineer who was instrumental in finding a method to protect the wonderful structure.
Prof John Burland sent instructions and received progress reports over the fax as the rescue attempt proceeded. He gave a lecture on how the tower was saved for posterity at a lecture earlier this week at the University of Limerick.
Appointed to the chair of Soil Mechanics at Imperial College London in 1980, he is now emeritus professor of soil mechanics and a senior research investigator.
The tower had reached a parlous state by 1990 when the Italian government decided to set up a commission to find a means to prevent it from collapsing entirely. Its incline has been a steady tourist draw for Pisa over the centuries, and many people worried that it would lose its charm - or fall down as a result of remedial work to save it.
"The tower was leaning dramatically, standing on very soft ground, with its inclination accelerating and it was about to explode," Prof Burland told The Irish Times.
Today, the remedial work is over and the tower still stands. To the untrained eye, it leans as much as ever, but in fact its tilt has been lessened by about 10 per cent.
The construction of the tower started in 1173, but after a time the work halted for about 100 years. Further building took place, followed by another break of around 90 years, and finally the bell tower was added.
Although the history of the lean was not recorded, the commission did some detective work and found that after an initial lean north the tower began moving south, and by 1278 was leaning substantially. When the bell tower was added, there was a drastic increase in the incline.
Over the years, attempts to correct the lean had only made it worse. "When we started, the tower was leaning at 5.5 degrees, the overhang of the bell tower was 4.5 metres, and in between the cladding there was nothing but rubble with cracks and voids in it," Prof Burland says.
It was thought that because of the soft ground, the south part was settling more than the north, but the team found that in fact the foundations were lifting on one side. They established that if they could correct the tilt by just 10 per cent, that would be sufficient to stabilise it.
A permanent solution for the Tower of Pisa in the end couldn't have been more straightforward, the cautious removal of soil from under one side of the building, making it tip back into place. A church tower in England had been saved from tilting many years previously using this method, which was refined and developed by Prof Burland's team.
One of his MSc students, Helen Edmonds, worked on a model of the tower sitting on a sand foundation. She developed a way of gently inserting tubes and removing small quantities of sand. After much computer modelling and a practical test in the piazza near the tower, work began on the tower itself.
A corkscrew drill was inserted down a series of tubes one by one. The drill extracted a small amount of soil creating a void and this would then collapse. The drill would then move onto the next tube and repeat the process.
The team took the expedient of suspending 900 tonnes of lead weights off the high side of the building to help counterbalance the tilt - just in case. This drilling process was repeated time after time, and when sufficient soil had been extracted to allow the tower to begin shifting back, some of the lead weights were removed.
This stage of the work was complete by 2001, but the effort to protect the tower has continued. Since then improvements have been made to manage the water table underneath the building, a contributor to the initial problem, and the foundations have been enlarged.
"We won't know for another 20 or 30 years if the tower will develop a trend to the south, but I'm confident that if it does, it will be a slow move," Prof Burland says.
"And I'm confident that if remedial work like ours needs to be done in another 300 years, our work can be successfully repeated, as it is all thoroughly documented."