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Tale of two towers

ICE News

IN 1999 the world watched as the Leaning Tower of Pisa was partly righted by professor of civil engineering at Imperial College London, John Burland.

Last month Burland laid bare the secrets behind solving 20th century civil engineering's most famous problem in a lecture, 'A tale of two towers - Pisa and Big Ben' held at ICE.

Far from a simple underpinning operation, the centuries old tower called on techniques that had never been used on anything like this scale before.

The need to correct the tower's lean became clear in 1989 when the 13th century Civic Tower of Pavia collapsed without warning. It was promptly closed to visitors and in January 1990 an international committee was set up to safeguard and stabilise the tower.

'At Pisa the tower was on the verge of immediate collapse. It really was that close, ' says Burland.

Throughout the 1990s much time and money was invested to understand the tower's structure, with the final solution hinging on soil composition.

If the soil was clay as originally thought, soil extraction below the skyward facing side of the tower would be almost impossible. Ground movements to fill the voids created would be unpredictable. But once it was realised that the ground was granular, the engineering team was confident the high side would sink controllably.

Burland, who is now ICE vice president, also described his second great tower stabilisation project - Big Ben clock tower.

Big Ben has been threatened by a number of construction projects in recent times. These include the underground car park beneath New Palace Yard in the 1970s and, more recently, London Underground's Jubilee Line Extension tunnels and the new Westminster Station.

At the time of the car park, Burland's model of the Palace of Westminster correctly predicted the movement of Big Ben's tower - but in completely the opposite direction.

'But it was this work and the back analysis of the results which saw the beginning of a whole new age of soil mechanics. The knowledge gleaned was invaluable when we returned to build the JLE, ' he adds.

This time the problem was to improve the tower's support as well as straighten its crooked stance. A novel method of injecting grout beneath the low side of the foundation was used.

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