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Saturday, 15 December, 2001, 15:42 GMT
Saving the Leaning Tower
Graphic, BBC
By BBC News Online's Stephen Mulvey

After 800 years in which the Leaning Tower of Pisa tilted steadily to the south, engineers have finally moved it back 45 centimetres (17 inches) and got it stable.

Ten years ago, the tower's movement, of roughly 1 millimetre per year, had brought it to the brink of collapse and made it unsafe for tourists.

The landmark has been turned into a building site, BBC
The landmark was turned into a building site
Indeed, the British engineer who has overseen the work to stabilise the tower, Professor John Burland, says it is hard to explain why the tower did not fall.

Computer models suggest that it should have toppled once it reached a tilt of 5.44 degrees - but by 1990 it was leaning by 5.5 degrees, with the seventh level, just below the belltower, overhanging the ground by 4.5 metres (15 feet).

Lead weights

The engineers solved the problem - at least for a couple of hundred years - by placing lead weights on the north side of the tower, and removing tonnes of soil from underneath the building using corkscrew drills.

Tower facts
Height: 60m
Girth: 19.6m
Mass: 14,500t
Tilt: 5.5 degrees
Commenced: 1173
Subsidence 1.86m (north) and 3.75m (south)
The tower slowly sank into the cavity created. Steel cables were attached to the tower, not to pull it upright, but to hold it up in case anything went wrong.

Professor Burland directed the drilling work from London, receiving and sending faxes every day. "It really was like riding a bicycle by fax," he said recently.

The first drilling programme began in 1999 and for the first few tense weeks nothing happened. Then, finally, it started to bring results.

In February 2000, the full extraction programme began, and more lead was placed on the north side of the tower.

By June 2000, the tilt had been corrected by 16 cm (six inches) returning the tower to the angle it was at in 1870.

By the time work finished on 6 June, 2001, the tower had been returned to the position it was in 1838, before work to dig a walkway round the base of the tower caused it to lurch rapidly further south.

Banana shape

The tower is built on very soft sediments, which explains why it is so unstable.

Crane and the tower, BBC
A steel corset was placed around the tower
Professor Burland likens it to a tower of bricks built on a soft carpet - after it reaches a certain height it becomes shaky, and it will fall if taken any higher.

Before beginning to stabilise the tower, the team analysed the building's history, judging how far it had begun to lean by noting the mason's attempts to correct it.

As Pisans say, the tower is "banana-shaped", with the belltower 1.5 degrees closer to the vertical than the base.

The first four storeys were built in the late 12th Century, the next three in the 13th, and the belltower was added on in the 14th - there are four steps up to it on one side, and six on the other.

Professor Burland says the tower may at first have leaned to the north, leading the builders to over-correct, and send it inexorably moving south.

With each new stage of construction, another major lurch occurred.

Optimism and pessimism

The stabilisation work was complicated by the fact that the commission overseeing it was set up by an Italian Government decree that needed to be ratified at regular intervals by the Italian parliament.

Frequently, the ratification process got delayed, and work suddenly had to stop, sometimes for months on end.

Professor Burland has an optimistic scenario, and a pessimistic one.

The first says the tower has been stabilised for good. The second says that the leaning process will start again - and that in 200 or 300 years, the tower will be back where it was in 1990, on the brink of collapse.

 WATCH/LISTEN
 ON THIS STORY
The BBC's Daniel Boettcher reports
"A team of modern engineers is succeeding where others have failed"
See also:

16 Jun 00 | Europe
Leaning tower safe to climb
23 May 99 | Europe
Tower of Pisa leans less
12 Dec 98 | Europe
Leaning Tower to straighten up
11 Dec 98 | Europe
Bid to save Leaning Tower of Pisa
13 Aug 98 | Europe
Eastern promise for leaning tower
07 Sep 00 | Festival of science
Pisa tower moving to safety
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