Remembering the day Heathrow Airport almost disappeared into a hole

Heathrow Airport is Britain’s busiest airport, with over 80 million passengers a year and a plane taking off approximately every 45 seconds during peak hours.

It has four terminals accessible from London by train, bus or underground, with the Heathrow Express rail link connecting the transport hub to London Paddington.

This high-frequency public transport system works like clockwork – but its construction has gone rather less smoothly, putting parts of the airport and even the nearby Piccadilly line at risk.

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In 1994, engineers drilling tunnels for the Heathrow Express under the airport apron experienced three cave-ins in three days, opening huge holes in the tarmac above.

The disaster nearly spelled the end of the Jubilee Line Extension, which was to be built using the same tunneling technique.

So what happened?

Tunnel roof collapse nearly ‘unzipped’ Piccadilly line

Tunnel collapse

Balfour Beatty, a major construction company then and now, submitted the lowest bid for the project and, with the UK economy in recession, they were allowed to proceed.

On Friday October 21, 1994, work on the Heathrow Express had been going on for a year. Engineers had started drilling two tunnels under the airport so trains could reach Terminal 4.

As the canals were drilled, liquid concrete was sprayed on their walls to stabilize them. The shotcrete coating technique is still used, including on the Crossrail project.

But this project was the first time the “New Austrian Tunneling Method” (NATM), as it was then known, had been used in the UK.

And “poor design and planning, lack of quality during construction, lack of technical control and…lack of safety management” at Heathrow would pave the way for disaster.

Only one NATM expert, a consultant from Austrian firm Geoconsult, was recruited by Balfour Beatty instead of the three recommended, to help keep costs down.

He noticed a number of small errors in the work in progress, raising concerns about its quality – but was largely ignored, despite signs of an impending collapse being visible.

Early on October 21, the first of the tunnels collapsed.

Piccadilly Line was in danger

Workers had only minutes to evacuate the excavations after large cracks appeared and concrete began to fall from the ceiling – miraculously no one was injured.

Over the next three days, tunnels continued to collapse around the airport, causing sinkholes to open in the apron and between the two runways.

Buildings collapsed as the ground gave way and a concrete plug was hastily installed underground, preventing the underground instability from spreading further.

The health and safety manager told a month-long trial at The Old Bailey that without this action the collapse could have ‘decompressed’ the Piccadilly line and crushed Tube passengers to death.

The Piccadilly line stops at Heathrow Airport
The Piccadilly line stops at Heathrow Airport

Other projects using the same liquid concrete spraying method – including the Heathrow baggage tunnel and the Jubilee Line extension – were quickly halted pending investigations.

These confirmed that sub-standard construction work had not been checked against deadlines, with managers relying on ‘good fortune’ rather than ‘effective risk management’ to prevent disaster.

Additionally, grouting – whereby grout is injected under the concrete to provide additional support – had damaged the tunnels and adequate repairs had not been carried out.

UK’s ‘worst civil engineering disaster’ for a quarter of a century

Geoconsult and Balfour Beatty have been fined a total of £1.7million over the disaster, the highest health and safety fine ever imposed to date.

The contractor has admitted failing to ensure the safety of its employees and the public, after coming under fire from the HSE for putting results ahead of its responsibilities.

Experts called the collapses “the UK’s worst civil engineering disaster in the last quarter century”, bearing “all the hallmarks of an organizational accident”.

The catastrophe “could have been avoided”, they added, if “the cultural mentality had not focused on apparent economies and the need for production rather than particular risks”.

Repairs to the damage amounted to £150million, triple the expected cost for the boring Heathrow Express project.

Piccadilly Line services to Heathrow have been halted while the route is stabilized. With underground transport disrupted, Heathrow Junction station was hastily built to accommodate passengers.

The station was located in Stockley Park, with a track laid along the route of an old canal. It was used until June 1998 when the Heathrow Express was finally opened.

Balfour Beatty’s reputation was damaged by the disaster, forcing them to adopt new management practices. In 2018 they were ranked as the UK’s largest construction company, with an annual revenue of around £30m.

Balfour Beatty is a major contractor at Heathrow, responsible for rail systems and tunnels - including for the Heathrow Express and the Piccadilly Line
Balfour Beatty is a major contractor at Heathrow, responsible for rail systems and tunnels – including for the Heathrow Express and the Piccadilly Line

UK building legislation has been tightened, while the Institution of Civil Engineers has issued new guidance on the use of NATM in soft soils.

Today, the Heathrow Express delivers five million passengers a year to Heathrow Airport – which has almost disappeared in the holes left by its construction.

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