Time Shift: Is London’s Big Ben Falling Down?
In December 2011 British newspaper headlines warned that London’s Big Ben was about to topple over. Locals instinctively blamed the London Underground’s Jubilee Line Extension, which opened in 1999 and sends trains rumbling under the tower and the Palace of Westminster. What most did not know, however, was that rumors of the tower’s doom, like Mark Twain’s death, were greatly exaggerated; a reporter misread the final report on the extension’s construction and its impact on the tower, which was filed in 2009 and released last year under the U.K.’s Freedom of Information Act. In fact, the builders took careful steps to ensure Big Ben’s stability.
No building is ever completely still: they expand, contract and sway in response to the sun and wind, and they tilt and move when the ground underneath them shifts.
“It’s like a sunflower. It bends with the weather and the time of year,” says John Burland, an emeritus professor at Imperial College London.
Burland has a long, intimate history with the clock tower. He was called in to ensure its stability both when the Westminster station was reconstructed and extended downward to a depth of nearly 40 meters as part of the Jubilee Line Extension project and during the construction of a nearby underground parking lot in the 1970s. Burland also served in the early 1990s on a commission to stabilize the Leaning Tower of Pisa and, he says, now spends a lot of time on other projects that require building underneath sensitive, priceless buildings in built-up areas. The world’s major cities have nowhere to expand but downward.
The technology to manage building movements has become very precise. A key component is accurate monitoring. Inside Big Ben’s tower, a permanent automated plumb line-simply, a weight suspended on a string that serves as a vertical reference with which to compare the tower’s angle of tilt-provides real-time measurements of its movement. A second surveying instrument, an optical plumb, now makes it possible to look up a vertical line from the tower’s base to a target behind the clock.
“We’re detecting movements to an accuracy of 10 millimeters daily.”