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Heathrow Express tunnel collapse

From EverybodyWiki Bios & Wiki

Heathrow Express tunnel collapse
Photograph of the ground above following the collapse
DateOctober 20–21, 1994 (1994-10-20 – 1994-10-21)
TypeTunnel collapse
Deaths0
Non-fatal injuries0
InquestHeathrow Report


Background

Map of Heathrow Express layout at Heathrow Airport
Main article: Heathrow Express

In 1991, Heathrow Airport and the British Railways Board were authorized by the government to construct a railway link at Heathrow Airport.[1] Following British Rail's privitisation in 1993, Railtrack plc took over responsibility for the new junction on the Great Western Main Line and the tracks.[2]

The tunnel itself would consist of a two track cut-and-cover tunnel between the railway junction with the main line to the north side of the M4 motorway, transitioning to a twin-bore tunnel until it reached the Central Terminal Area, consisting of Terminals 1, 2 & 3.[2] A single bore tunnel continues on to Terminal 4, crossing under the cargo terminal and south runway.[2][3] Initial drawings early in the project's life planned on using tunnel boring machines for the tunnels carrying the rail lines, with the stations being constructed using tunneling shields and segmented linings.[2]

The use of the New Austrian tunneling method (NATM) was suggested in 1989 by advisors to BAA, with this method being selected for subterranean station construction.[2]

Project

Trial tunnel

The project would be the first NATM tunnel constructed in London clay, which was known for being soft and wet and prone to collapse if not supported.[2][4] A single bore trial tunnel 100 metres (330 ft) was constructed from a shaft in car park 5, located between the cargo terminal and terminal 4.[5] The tunnel was designed and construction supervised by The Dr Sauer Company.[6]

The trial tunnel deviated from what the proejct would actually construct in several ways: a single tunnel instead of three tunnel system, the NATM designer was integrated into the client's design and supervisory team, compensation grouting was not used, the method of supervision of the work was different.[6]

Eventually, this trial tunnel was lined as part of a tunnel for railway line connecting to terminal 4.[6]

Contract structure

On 7 March 1994, Balfour Beatty won the contract to design and construct the tunnels for the project, including railway tunnels, subterranean stations and a concourse tunnel.[4][7] Possession of the CTA site occurred in May of 1994.[8]

As subsidence issues were expected during the project, Mott MacDonald was hired by Balfour Beatty to advise on settlement prediction and controlling measures.[7] Balfour Beatty decided on use of compression grouting as the primary means of addressing settlement issues, and subcontracted Keller Colcrete to install grouting arrays and inject grout.[7]


Balfour Beatty subcontracted design of the tunnel and on-site supervision of the to Geoconsult, who invented the NATM process. Despite Geoconsult's contract including the provision of computer software to assist handing and processing tunnel monitoring data, Balfour Beatty provided their own software. This became a significant problem as data required additional processing before it could be output for review and survey methods used by Balfour Beatty caused data accuracy issues.[8] Geoconsult's engineer recommended changing software and surveying systems to Balfour Beatty in July and September 1994.[8] These suggestions were rejected. Ultimately the software did not meaningfully impact the ability to identify and understand key events occurring in the tunnel in the lead up to the collapse.[8]

Personnel

Geoconsult was contracted by Balfour Beatty to design the NATM works for the project and provide on-site technical supervision and expertise.[7] Conflict arouse early on over the types and amounts of staff to be provided and paid for. Geoconsult suggested that the project have an experienced NATM engineer and two qualified site supervisors.[7] Balfour Beatty disagreed and only approved for the single experienced engineer.[7] This engineer was only on site during the work week, Monday to Friday, despite the project work occurring around the clock.[4]

Balfour Beatty employed three individuals as 'NATM engineers', however these individuals had minimal tunnelling experience and no familiarity with the NATM process in soft ground, like London Clay.[8] In August 1994, another engineer was brought in, focused on addressing quality control and productivity issues in tunnel construction that had been occurring throughout the project. This engineer had NATM experience, but not in soft ground.[8]

Balfour Beatty established a 12-hour shift pattern to enable around-the-clock work, seven days a week.[9] With a central office at the project office just north of the M4 motorway, multiple worksites were established across the project to delegate the work to project managers. At each of these worksites, the managers directed pit bosses, who directed leading mieners who oversaw the miners.[9] In addition three teams of section agents, section engineers and engineers were established to ensure some technical staff were present at any time of day.[9]

Construction

Issues at Camborne House

Despite initial analysis that suggested subsidence of 9 millimetres (0.35 in) above the tunnels, the trial tunnel saw subsidence of 25–35 millimetres (0.98–1.38 in). When the first tunnel passed under the Fuel Depot worksite, passing beneath Camborne House and its extension, which were used as offices for the tunnel project in early July 1994, subsidence was observed

In planning, subsidence in excess of 9 millimetres (0.35 in) hadn't been considered, and no plan had been prepared in advance.[10] The subsidence approaching 25 millimetres (0.98 in) limit specified in the contract. Mitigation measures of compensation grouting had only been expected during construction of the second and third tunnels, and Balfour Beatty was unprepared to implement it during the first tunnel's construction.[11] While it appeared the grouting was correcting the movement of Camborne House, the grout injections had had pushed down on the ground below the grouting, distorting the tunnel lining. Despite this behaviour not being expected, no special monitoring of the area was implemented.[12] A depression developed in the tunnel crown[lower-alpha 1] of approximately 50 millimetres (2.0 in) in the vicinity of Camborne House, well in excess of expectations in planning.

On 16 August, NATM engineers noticed cracking in both walls of the concourse tunnel between chains 61 and 65.[lower-alpha 2] This cracking caused engineers to realize the failure of the invert[13] during the grouting operation, having failed to identify this using available monitoring data.[14] Over the four days, Camborne House resumed sinking, settling 9 millimetres (0.35 in).

Collapse

A catastrophic collapse occurred at approximately 01:15 on 21 October 1994, in the area of the down-line platform tunnel, southeast of the fuel depot shaft[15]

Timeline

Thursday, 20 October 1994
Around 17.00-18.30 Shotcrete began to break away from the lining near the tunnel eye. New movement was detected at repairs completed earlier in the day.
19.00 Start of Central Tunnel Area night shift, consisting of three crews.
Work started
Example
Example
Example
Example
Around 20.30~21.00 A piece of shotcrete fell from the face of the dwon-line platform tunnel 'north'.
Around 22.30 Serious longitudinal cracks appear in both walls of the down-line platform tunnel. Crews examined these cracks and discussed repairs but failed to react to the collapse risk developing in front of them.
The BAA shift engineer returned and photographed the cracks, while miners were instructed to repair the cracks and discontinue excavation work.
The seriousness of the situation was still not recognized by anyone on site and workers remained in the tunnel instead of evacuating as cracking continued to develop.
Around 23.30 Cracking worsened on the left-hand wall of the down-line and was now occuring in the headwall of the side gallery.
The deteriorating state of the tunnel concerned miners, with some leaving the tunnels for the bottom of the Fuel Depot Shaft.
Around 23.45 The 'NATM enginer' noted the crown of the down-line platform tunnel 'north' had dropped 25 millimetres (0.98 in) in 1 hour.
Friday, 21 October 1994
Around 00.15 Section engineer, a leading miner and the pit boss inspected entire concourse tunnel for cracks. They discovered recent repairs badly deformed.
Around 00.30 Pit boss orders some miners to leave concourse tunnel. Remaining workers continued a shotcrete repair work continued at ch. 24
Other miners were concluding shotcrete work in the down-line tunnel 'north'
The site agent, tunnel superintendent and section agent arrived. Geoconsult's site engineer and the 'NATM engineer' arrived shortly after them.
Around 00.30-00.45 Cracking in central wall of the up-line had worsened. Down-line side gallery had extensive damage from horizontal cracking and surface shotcrete was spalling.
The crown had fractured longitudinally at the tunnel face and the failure was progressing along the tunnel
Site agent, section agent and tunnel superintendent entered tunnels and inspected them. Miners had gathered near the base of the fuel depot shaft. The tunnel superintendent ordered a lone worker still applying shotcrete to leave the concourse tunnel.
A miner observed dust rising from the invert of the concourse tunnel, indicating movement was occurring.
Around 00.45 Further cracking occurred within the tunnels. Spalling continued with it spotted around 5 metres (16 ft) into the concourse tunnel.
Site agent and tunnel superintendent, and separately Geoconsult's site engineer realized the situation was irretrievable. The site agent ordered a full evacuation of the tunnel.
Workers were mustered at the base of the fuel depot shaft, and exited the tunnel in two groups via the passenger hoist.
Around 00.50 Upon reaching the surface, staff were instructed to muster in the site canteen, a portable building situated above the course tunnel, and the route to reach from the fuel depot shaft involved crossing above the down-line platform tunnel 'north'.
The second group felt ground movement, as they walked to toward the site canteen and Camborne House.
Around 00.55 Pit boss orders an evacuation of the entire site. The site canteen began to tilt.
Around 01.15 A rapid catastrophic failure occurred in the tunnel system, originating from the down-line platform tunnel north. The initial length of the concourse tunnel also collapsed.

Subsequent Collapses

On Saturday, 22 October, the up-line platform tunnel suffered a collapse, northeast of the fuel depot shaft.[15][16] The following day, 23 October, a third collapse occurred, this one directly under the northern end of Camborne House, inflicting significant damage on both Camborne House and Camborne House Extension. Both buildings would be deemed total losses and later demolished.[15][16][17]

Response

In the aftermath of the collapse, Balfour Beatty staff contacted 999 to summon London Fire Brigade to the scene. London Fire Brigade contacted Heathrow Airport Fire & Rescue Service, who would notify Heathrow Operations Center at 01:21.[18] The initial reports however did not accurately explain what occurred, making it sound like a diesel tank leaking. Heathrow Airport operations only became aware of the actual nature and scope of the incident whe BAA duty manager arrived on the scene a few minutes after report from London Fire Brigade.[18] It was only at this point that London Underground was informed of the situation.[18]

The area around the scene, outside the Fuel Depot Site was not closed and evacuated, such as car parks 1A and 3, pedestrian subways and roads around the site.[18]

London Underground decided to keep Piccadilly line closed around 03:56.[18]

Around noon on 21 October, an inspection team briefly reentered the fuel depot shaft to preform a visual inspection of the aftermath. They discovered the down-line tunnel eye was completely filed with collapsed material. Also observed was that the Up-line tunnel had severe cracking and inspectors could hear movement.

Chester Road, of the site did not show signs of collapse but remained closed to create more working area.[16]

Eventually, the fuel depot shaft was filled with concreate to above the tunnel eyes. A 1.7 metres (5.6 ft) bore hole was dug in car park 3 to enter the down-line platform tunnel to enable inspection. While flooded with around 2 metres (6.6 ft) of diesel fuel contaminated water, it was intact. The up-line platform tunnel was flooded with 4 metres (13 ft) of contaminated water. The intact tunnels and any voids were filled with concrete using bore holes.[19]

8,000 cubic metres (10,000 cu yd) of concrete was used in order to fill and stabilize the collapsed tunnel areas.

Impact

Airport operations suffered significant disruptions due to the closures of roadways near to the collapse area and the closure of the Piccadilly Line.[20][21]

Car park 3 was closed in the event of further subsidence, removing 1,300 vehicle parking spaces from the airport's capacity.[21]

Passengers on the Underground were disembarked at Hatton Cross, with London Underground implementing a bus shuttle to Heathrow from there.[20] By 23 October, the closure was limited to Heathrow Terminal 4 tube station, impacting approximately 6,000 passengers daily.[22] Passengers bound for Terminal 4 continued to disembark at Hatton Cross and use a shuttle bus to the terminal.[23]

Airport operations suffered significant disruptions due to road closures, and the closure of subway links to Terminal 3.[16] Road closures caused a five mile long traffic jam, impacting the A4 and A312 near the airport. Traffic backups on the M4 airport-spur reached the main M4 motorway.

Compounding the issues was the closure of the Underground service to Terminal 4, due to the Underground line passing over the top a Heathrow Express tunnel, and within 18 feet (5.5 m) of Heathrow Express tunnel where the collapses occurred. The Underground closures lasted until

Resuming construction

Following the collapse, a new plan was developed which involved construction of a new, larger shaft, reconstructing the station tunnels using more traditional concrete and cast iron segment linings, installed using Tunnelling shields.[24] Use of NATM was limited to crossovers and points where the soil was undisturbed and the Terminal 4 station, using a revised plan. The supervision and management of the project was also completely redone.[24] Further subsidence and settlement of buildings was minimal in the vicinity of the NATM constructed areas.[24]

The additional work caused by the collapse cost approximately £150 million (equivalent to £Error when using {{Inflation}}: |index=UK (parameter 1) not a recognized index. in 2018) and the Heathrow Express Rail Link opening was delayed until June 1998.[24] The additional cost was around three times the original value of Balfour Beatty's work.[24]

The collapse also resulted in the construction of Heathrow Junction railway station, a temporary surface railway station that operated from January to June 1998, near the tunnel opening, with shuttle busses transporting passengers from the station to the airport.[25]

Investigation

At the time of the collapse, other tunnel projects across London were using the NATM process, which all suspended work as a precaution. The Health and Safety Executive requested the suspensions remain in place until projects could prove they could continue safely.[26] The Health and Safety Commission requested the Health and Safety Executive conduct a two-part investigation, with the first considering if NATM tunnel process was safe in the United Kingdom, and second the specific causes of the Heathrow tunnel collapse.[27][26] Over 100 people were interviewed and substantial amount of documentary evidence was collected. The investigation was complicated by the period of time the problem developed, the significant number of people and organizations involved in the project and the loss of Camborne House, which was a site office for tunneling personnel working at the Central Terminal Area.[28]

On 9 May 1996, HSE published the report into 'part one' of their investigation, Safety of New Austrian Tunnelling Method (NATM) Tunnels.[28] This report considered the history of the NATM process and specific issues raised by geology specific to the United Kingdom, such as London clay.[27] The report ultimately concluded that while NATM tunnels are complex from an engineering perspective, and require careful oversight in the planning, design and construction to ensure a successful tunnel project with minimal adverse risk to workers or the public, that safe construction of NATM tunnels was possible.[29] HSE also determined that no legislation specific to NATM tunnel construction was needed and that a tunnel built with a NATM lining once complete and in service, was as safe as a tunnel constructed by any other means.[29]

Causes

The direct cause of the tunnel collapses was a chain of events involving:

  • substandard construction of in the initial length of the CTA concourse tunnel over a period of some three months;
  • grout jacking that damaged the same length of tunnel plus inadequately executed repairs to it some two months before the collapse;
  • the construction of a parallel tunnel in failing ground;
  • major structural failure in the tunnels, progressive failure in the adjacent ground, and further badly executed repairs during October 1994.
    — Health and Safety Executive, The collapse of NATM tunnels at Heathrow Airport (2000), Page 1-2

Additional factors were issues related to the cast-in-place construction of thin shell linings, failure to recognize risks of compensation grouting and other remedial work to correct settlement of the ground above the tunnel.[30]

The collapses were preventable. Through out the project, from the early stages to the day of the collapse, serious errors were made and indicators of a collapse, present from the early stages of construction, were not recognized.[30] While human errors and oversights contributed to the collapse, the collapse was at its core, an 'organizational accident'. The human errors ultimately traced back to organizational failures that failed to identify hazards, reduce and avoid risk and monitor and control risk that could not be eliminated. The project suffered from poor design and planning, a lack of quality control, engineering oversight and safety management.[30]

Other issues identified included poor contingency and emergency procedures and a lack of means for seeking insight from workers about health and safety issues.[30]

Inquiry report

In July 2000, the Health and Safety Executive released the report into actual collapse, The collapse of NATM tunnels at Heathrow Airport. The release of this report was delayed until the completion of the various legal cases surrounding the collapse, to avoid interference with the outcomes.[31]

In 2024, an individual filed a Freedom of Information request with the Health and Safety Executive for the report, The collapse of NATM tunnels at Heathrow Airport..[32] The requestor noted that the 'part 1' report NATM tunnel method was available online through HSE, but the 'part 2' report into the collapse itself was not available, prompting the request. This request revealed that in 2006, HSE's physical copy of the document was disposed of per HSE's Corporate Retention Policy, and that HSE was unable to locate any copies physical or electronic to fulfill the request.[33] After the requestor made further inquires, stressing the disposal seemed inconsistent with the policy and that the Part 1 report had been published years earlier, and was available through HSE, having had a digital reissue in 2014.[34] HSE then clarified that the hard copy was withdrawn 2006, and should have been digitized at the time, however HSE was unable to locate the digital copy in their internal archive or with The National Archives.[35]

Prosecution

In August 1997 the Health and Safety Executive decided to initiate legal proceedings and informed Balfour Beatty and Geoconsult. Balfour Beatty would plead guilty to violations of sections 2(1) and 3(1) of the Health and Safety at Work Act 1974. The Crown Prosecution Service declined to prosecute for similar issues in Terminal 4.

On 15 February 1999, Balfour Beatty and Geoconsult were found guilty of violating the Health and Safety at Work Act 1974. Balfour Beatty was fined £1.21 million (equivalent to £Error when using {{Inflation}}: |index=UK (parameter 1) not a recognized index. million in 2018), Geoconsult was fined £500,000 (equivalent to £Error when using {{Inflation}}: |index=UK (parameter 1) not a recognized index. in 2018). Both were also ordered to pay prosecution costs of £100,000 (equivalent to £Error when using {{Inflation}}: |index=UK (parameter 1) not a recognized index. in 2018).[36][37] Balfour Beatty's 1.2 million fine was the largest single fine for a Health and Safety Act violation at the time.[37]

In 30 July 1999, Geoconsult's application for leave to appeal against their earlier conviction was rejected, as was change in the financial penalties.[37] In 2000, Geoconsult GES told Uxbridge magistrates' court that they couldn't pay the 500,000 fine and 100,000 in additional costs after missing missing 15 February deadline.[38] As of May 2003, Geoconsult still hadn't paid the 500,000 fines.[39]

See Also

Notes

  1. ↑ Highest point of the tunnel cross-section. Top of the tunnel.
  2. ↑ 2.0 2.1 A point along the axis of a tunnel identified by its distance from a fixed reference point. For the CTA concourse tunnel, the reference point was the centre of the fuel depot shaft.
  3. ↑ 1 chain equals 20 meters or 66 feet.

Bibliography

  • Health & Safety Executive (July 2000). The collapse of NATM tunnels at Heathrow Airport. Health & Safety Executive. ISBN 0-7176-1792-0. Search this book on
  • Vaughan, Adrian (2004). Railway Blunders. Surrey, UK: Ian Allan Publishing. ISBN 0 7110 2836 2. Search this book on
  • Health & Safety Executive (2014). Safety of New Austrian Tunnelling Method (NATM) Tunnels (PDF) (2nd ed.). Health & Safety Executive. ISBN 978 0 7176 1068 6. Retrieved 9 June 2025. Search this book on
  • Connor, J.E. (2000). GWR Disused Stations in Greater London. Colchester: Connor & Butler. ISBN 0-947699-31-7. Search this book on

References

  1. ↑ "Heathrow Express Railway Act 1991". legislation.gov.uk. 9 May 1991. Retrieved 11 January 2024.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 Health and Safety Executive 2000, pp. 4.
  3. ↑ Health and Safety Executive 2000, pp. 5.
  4. ↑ 4.0 4.1 4.2 Vaughan 2004, pp. 139.
  5. ↑ Health and Safety Executive 2000, pp. 1, 12.
  6. ↑ 6.0 6.1 6.2 Health and Safety Executive 2000, pp. 12.
  7. ↑ 7.0 7.1 7.2 7.3 7.4 7.5 Health and Safety Executive 2000, pp. 15.
  8. ↑ 8.0 8.1 8.2 8.3 8.4 8.5 Health and Safety Executive 2000, pp. 16.
  9. ↑ 9.0 9.1 9.2 Health and Safety Executive 2000, pp. 24.
  10. ↑ Health and Safety Executive 2000, pp. 31-32.
  11. ↑ Health and Safety Executive 2000, pp. 31.
  12. ↑ Health and Safety Executive 2000, pp. 32.
  13. ↑ Lowest apart of the tunnel cross-section. The bottom of the tunnel.
  14. ↑ Health and Safety Executive 2000, pp. 34.
  15. ↑ 15.0 15.1 15.2 Health and Safety Executive 2000, pp. 48.
  16. ↑ 16.0 16.1 16.2 16.3 Health and Safety Executive 2000, pp. 51.
  17. ↑ Pilkington, Edward (24 October 1994). "Second slide strikes tunnel". The Guardian. The Guardian. Retrieved 31 August 2025.
  18. ↑ 18.0 18.1 18.2 18.3 18.4 Health and Safety Executive 2000, pp. 49.
  19. ↑ Health and Safety Executive 2000, pp. 52.
  20. ↑ 20.0 20.1 Laville, Sandra (21 October 1994). "Heathrow chaos as landslip shuts roads and Tube". Retrieved 31 August 2025.
  21. ↑ 21.0 21.1 Morton, Ian; Tweedie, Neil. "250,000 hit as airport tunnel troubles go on". Evening Standard. London. p. 7. Retrieved 31 August 2025.
  22. ↑ Murray, Dick (28 October 1994). "Heathrow Tube station shut for 3 more weeks". Evening Standard. p. 7. Retrieved 31 August 2025.
  23. ↑ Coates, John (23 October 1994). "Heathrow tunnel fall 'triggered by Tube'". Sunday Telegraph. The Telegraph. p. 4. Retrieved 31 August 2025.
  24. ↑ 24.0 24.1 24.2 24.3 24.4 Health and Safety Executive 2000, pp. 53.
  25. ↑ Connor 2000, pp. 68.
  26. ↑ 26.0 26.1 Health and Safety Executive 2000, pp. 54.
  27. ↑ 27.0 27.1 Health and Safety Executive 2014, pp. 5.
  28. ↑ 28.0 28.1 Health and Safety Executive 2000, pp. 55.
  29. ↑ 29.0 29.1 Health and Safety Executive 2014, pp. 15.
  30. ↑ 30.0 30.1 30.2 30.3 Health and Safety Executive 2000, pp. 2.
  31. ↑ Murray, Dick (5 July 2000). "Heathrow tunnelling chiefs 'put lives at risk'". Evening Standard. London. p. 22. Retrieved 30 August 2025.
  32. ↑ "HSE Investigation into Heathrow Express Link". whatdotheyknow.com. 19 January 2024. Retrieved 30 August 2025.
  33. ↑ Burslem, Amalia (5 February 2024). "HSE Investigation into Heathrow Express Link". Health and Safety Executive. Retrieved 30 August 2025.
  34. ↑ "HSE Investigation into Heathrow Express Link". whatdotheyknow.com. 6 February 2024. Retrieved 30 August 2025.
  35. ↑ Burslem, Amalia (8 February 2024). "HSE Investigation into Heathrow Express Link". whatdotheyknow.com. Retrieved 30 August 2025.
  36. ↑ Vaughan 2004, pp. 142.
  37. ↑ 37.0 37.1 37.2 Health and Safety Executive 2000, pp. 56.
  38. ↑ Hall, Macer (27 February 2000). "Airport tunnel fall firm cannot pay fine". The Sunday Telegraph. 2020. London. p. 4. Retrieved 30 August 2025.
  39. ↑ Evans-Pritchard, Ambrose (9 May 2003). "Britons will be tracked down to EU fines". The Daily Telegraph (46004). The Telegraph. p. 2. Retrieved 30 August 2025.


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