He was a wise choice: Professor John Burland, of Imperial College, had already saved the Pisa landmark from imminent collapse. Until he stepped in, it looked as though Italy's towering tourist attraction would be brought down by the next moderate storm.
Big Ben's legendary tower and the Palace of Westminster came under threat during the digging of the Jubilee line extension. The dramatic story was revealed by Mr Burland today during the British Association Annual Science Festival at Imperial College, where he is professor of civil engineering. He said Big Ben's clock tower started to lean as soon as the westbound tunnel of the Jubilee line - the deepest excavation ever undertaken in central London - was bored within metres of its foundation.
Without immediate action it would have continued to move, causing major cracking to the structure that links it to the main part of the Palace of Westminster - becoming, itself, a visibly leaning tower. As it is, it now leans by almost exactly an inch, barely discernable to a passer-by, and waggles by a further 9mm according to the season.
Professor Burland said the Palace of Westminster was saved by injecting more than 300 tonnes of grout made of cement, sand and plasticiser, through any one of hundreds of precisely-placed holes in a fanshaped array of 20 steel pipes radiating from a central vertical shaft sunk in Bridge Street, just north-east of the tower.
"The Jubilee line comes very close to the Palace of Westminster clock tower," he added. "Our modelling showed it was very close to the bone. We believed that if everything went absolutely perfectly then the movement would be acceptable, but if it even went a bit awry, there was potential for serious damage.
"We devised a safety strategy, and when the tower started moving, we put it into effect immediately, injecting 125 cubic metres - about 300 tonnes - of grout. It was entirely successful, I am glad to say. We are still carefully monitoring the situation with a number of sensitive instruments, including a digital plumb line hanging inside it that automatically gives average readings every 30 seconds, which are checked every day."
In Pisa, the tower has been saved by removing from beneath its higher side just 30 tonnes of the alluvial silt - in effect, bog - on which it sits, causing it to move back towards the vertical. This has been done through 41 drill holes under it, using a specially invented drill which can operate without causing any disturbance at all to the delicately poised structure.
Figures from Italy today show that the 14,500-tonne tower, which had faced being brought down by the next moderate storm, is now lessening its angle by 1.5mm at the top every day. This is wiping 23 centimetres or 162 years off its 925-year fall towards the horizontal, and adding at least 350 years to its expected lifespan.
The Tower is now likely to reopen to visitors next June, probably on the festival of its patron saint, when its lean will have been reduced from 45 centimetres to 20cm - or from 5.5 degrees to 5 degrees - at the top. That was the angle of lean before a disastrous intervention by an architect in 1838 caused it to lurch sideways.
Professor Burland said: "I monitor the position at Pisa from my office in London, which is a bit like riding a bicycle by fax. We had a break in August while the drillers were on leave, to see how it behaved when we left it alone. We started again on Monday. This morning's figures show that it is coming up by 1.5mm a day. Last night, it moved slightly less than that."
They are now about halfway through that operation, in which they will remove in all about 60 tonnes of soil. "That is rather less than expected," Professor Burland added. "The tower has behaved rather well. We believe that when we finish, the Leaning Tower will start moving again. However, we believe that that movement will be much slower, and even if it isn't, our calculations show it will not reach the angle it was at when we started for another 350 years."
So, that's all sorted out then. Next job, perhaps, straightening out the Wobbly Bridge
Source:thisislondon
