How a Heathrow Airport tunnel collapsed and almost destroyed part of the London Underground

How a Heathrow Airport tunnel collapsed and almost destroyed part of the London Underground

Heathrow Airport has some of the most advanced technology in the country.

And expect more when the construction of the third runway and all the different changes around the airport begin.

However, there was a time when airport engineers oversaw “the worst civil engineering disaster in the UK in the last quarter century”.

In 1994, while the Heathrow Express was under construction to connect the airport to Paddington, a series of tunnels collapsed in three days, almost eliminating the London Underground Piccadilly line.

And the failure almost meant the end of the extension of the Jubilee line, which diverted trains from Charing Cross to Westminster, London Bridge and Docklands to Stratford, using the same technique to tunnel under London and the Thames.

What went wrong?

In 1994, the Heathrow Express had only been under construction for a year. The main excavation work was to be carried out under West London, as well as the electrification of the track from Paddington.

Two tunnels, each about five miles long, had to be drilled under the airport and surrounding areas to allow trains to go to Heathrow Central and Terminal 4 stations.

But in the early hours of Friday, October 21, the first of these tunnels collapsed.

Buildings were damaged and a crater appeared between the two airport runways
(Image: Vic Crawshaw / Daily Mirror)

Over the next three days, more and more tunnels broke down, causing sinkholes and buildings to collapse around the airport.

The Piccadilly line tunnels serving the airport were also threatened by the ongoing collapses. Old Bailey was later told that the collapse could have “unzipped” the Piccadilly line and crushed the passengers to death.

To avoid this, an emergency concrete plug was installed to bring the line about 20 meters from the disaster.

The airport was also plunged into chaos for several days as the ground below collapsed. A huge crater appeared between the two tracks.

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Despite the dramatic nature of the collapses, no lives were lost. Construction workers in the tunnels had only a few minutes to go before the first tunnel collapsed.

The collapse has always had far-reaching consequences and later, contractors Balfour Beatty and its designer Geoconsult were fined £ 1.7 million, the highest health and safety fine ever recorded. time.

But before they could be fined, the collapsed tunnels had to be repaired, which cost around £ 150 million, almost three times the original cost of the project.

Why did the tunnels collapse?

The tunnels under the airport were constructed using the sprayed concrete tunnel lining method, which quickly sprays liquid concrete on the walls of a freshly dug tunnel to stabilize its walls.

The method itself is still used to this day on Crossrail, the Northern Line extension projects and are universally accepted tunnel drilling techniques.

But construction managers have been heavily criticized by the Health and Safety Executive.

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The court learned that the managers put the results before health and safety considerations, causing the collapses in part.

Balfour Beatty pleaded guilty to failing to ensure the safety of its employees and the public.

Construction was halted for a year, but after assessments it resumed and was completed in 1998. Services started that year and reduced travel time to just 15 minutes.