Wonky wonder worrying you?

Two scientists who save moving monuments talk to Jennifer Currie

Saving historical monuments from collapse is part of the work of Robert Mair, professor of geotechnical engineering at Cambridge University, and John Burland, professor of soil mechanics at Imperial College, London.

As specialist advisers to London Underground's Jubilee Line extension, they made sure that the Big Ben clocktower was not affected by the tunnelling taking place only metres away from the shallow foundations of London's most famous face.

Big Ben started to lean northwards soon after it was built in 1858. Experts predicted that this tilt would be exacerbated by the construction of a station and tunnel 40m below Westminster. "We calculated that without treatment, the movement of Big Ben would haveI caused unacceptable damage to the tower itself and to the Palace of Westminster," Mair recalls.

Mair and Burland put their trust in compensation grouting, a process they first used on a tunnelling project at Waterloo Station. This involves placing a network of horizontal tubes between the tunnels and the surface, then squirting liquid cement into the earth through tiny, one-way rubber valves. The cement hardens to prevent movement during the excavation.

While London Underground maintained that work on the extension was running smoothly, newspaper articles claimed that a crack in the foot tunnel that links Westminster Station to the clock tower indicated that the ancient masonry could not withstand the strain of modern engineering techniques.

Yet 28 months and 24 grouting episodes later, the engineers were pleased with what is now regarded as a world-class achievement. Burland says: "This was the first time that compensation grouting has been used to control a delicate historical building."

Burland, a specialist in landslides and ground behaviour, came to the project while advising Italy's 16th commission to stabilise the Tower of Pisa. The collapse of a civic tower in Pavia in 1989 prompted the government to address the problem of the Tower of Pisa - the top of which overhangs the bottom by 4.5m. "Although we were commissioned to reduce the inclination by 4m, we were not to stop the tower from leaning altogether. Obviously - since it is the tower's vertical lean that has made it one of the seven modern wonders of the world," Burland says. "After ten years, we have moved the top of the tower by 25cm."

Burland calls the project "the ultimate civil engineering challenge" because the tower stands on ground that has the "consistency of jelly or foam rubber". The commission opted to extract small quantities of soil from beneath the high side of the foundations in the hope that this would decrease the tower's tilt. "The tension was great when the first soil excavation took place," Burland says. "There was no discernible response from the tower in the first week, but it began to gradually rotate northwards. The method has now been approved for permanent stabilisation. We have taken the first positive steps - but there is still a long journey ahead."