Engineers also had to work out how to stop the River Thames from flooding the tunnels
Thousands of commuters use Westminster Underground station each day without giving it a second thought – But next time you tap in, it’s worth remembering that the station’s construction was perhaps one of London’s most impressive feats of underground engineering.
Building the Jubilee line extension at Westminster in the 1990s was far from a straightforward tunnelling job. Engineers had to create new platforms beneath a busy station, with Big Ben nearby and the River Thames just metres away. They also had to support Portcullis House, the parliamentary office building constructed above the station.
If that wasn’t enough, the existing District and Circle line tracks had to be lowered by about a foot to make room for the new layout. To keep trains running, engineers supported a 130-metre section on a bridge-like structure while digging underneath.
The tracks were lowered bit by bit at night, and below them, workers excavated a 30-metre-deep space for escalators leading to the Jubilee line platforms. The Jubilee line platforms were also stacked one above the other rather than placed side by side to reduce the space needed underground.
Protecting Big Ben was one of the biggest challenges, with both the Elizabeth Tower and the Palace of Westminster at risk of cracking and subsidence as a result of the construction. The danger of two of London’s most iconic landmarks being damaged was taken extremely seriously, and so experts including Professor John Burland of Imperial College London, who also worked on stabilising the Leaning Tower of Pisa, were brought in to offer their knowledge.
The clock tower already leaned about 22cm towards the north-west before construction began, and digging nearby risked disturbing the ground supporting its foundations. Thankfully, engineers found an ideal solution, installing steel pipes and pumping grout into the ground around the foundations to compensate for soil movement and limit further tilting.
Throughout the work, engineers closely monitored the tower using a plumb line – essentially, a weight hanging on a string – with the angle of the string measured to check for any tilting, explained The B1M. An optical plumb was also used, which used a laser beam to create a vertical line, capable of tracking any movement within 10 millimetres.
Construction took place in careful stages, allowing the team to check how the ground and tower responded before continuing.
Meanwhile, the works were also threatened by the Thames flowing mere meters away. Workers reported water leaking through the excavation walls, and one wrong move could have seen the carefully constructed tunnels flooded – but thankfully, that did not happen.
Keeping the tower stable, the works dry and existing trains moving made Westminster an unusually demanding project – but it did not stop the station from successfully opening to the public on December 22, 1999. The station has since won numerous awards for its distinctive futuristic design and was named one of Londoners favourite ‘transport design icons’ in 2015.
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