IT鈥橲 always leaned slightly, but construction work on a new line for the
London Underground threatened to give the tower of Big Ben a real tilt.
Engineers have had to prop up its foundations to prevent it damaging the rest of
the Houses of Parliament.
To control the tower鈥檚 movement, engineers pumped a cement-based grout into
the soil beneath it. The tower now leans an extra few centimetres, but engineers
say it is stable. The tilt is only visible to the most eagle-eyed observer.
Between 1995 and 1997, contractors building the extension to the Jubilee line
dug a 40-metre-deep hole just 31 metres north of the clock tower. And the new
rail tunnels were bored even closer. John Burland of Imperial College, London,
who recently helped halt the increasing tilt in the Leaning Tower of Pisa and
was an adviser to the Jubilee line project, anticipated the work would affect
the tower. A safety limit of 27.5 millimetres over the existing tilt of 220
millimetres was set as the maximum the buildings could stand without
cracking.
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To keep the tilt within this limit, Burland told the BA about the novel
method of injecting grout under the foundations of the tower. Engineers inserted
steel tubes with rubber-sealed holes under the tower鈥檚 foundations. As the tower
began to tilt, they pumped grout through pipes inside the tubes to the hole of
their choice. There the grout burst the rubber seal and penetrated into the
ground, lifting the foundations above.
More than 300 tonnes of grout were pumped in during construction. The tower鈥檚
additional tilt wandered between 10 and 25 millimetres, but never passed 27.5
millimetres. 鈥淚t certainly never lurched. We had very good control,鈥� says
Burland.
After the construction work was completed in late 1997, a review of movements
in adjacent walls showed that the building is more ductile than previously
thought and the safety limit was raised to 35 millimetres before any action
needs to be taken.
Since 1997, the tower has continued to tilt, Burland told New
快猫短视频. The latest measurements, taken this year, suggest that the
tower鈥檚 lean had just reached 35 millimetres. But following regular reappraisals
of the structure, experts are confident that the tower has stabilised. A
spokesman for London Underground says: 鈥淲e understand the tower has stabilised
and returned to its normal movement cycle.鈥�
