Infographic

Story of the collapse

Analysis

Incident details

The project centred on a stretch of twin-track railway running from Gerrards Cross station towards London in a 12m-deep cutting between two road bridges: Packhorse Road to the north-west and Marsham Lane to the south-east.

This section was being tunnelised, which involved installing precast concrete arch sections for about 324m along the lines. Material such as crushed stone was added to the surrounding space to fill the cutting and bring the ground level up to meet that of the surrounding area. The eventual span would be 20m and the structure would be around 8.5m high to allow for future overhead line electrification.

A retail development was to be built on this newly formed land above the tunnel. The retailer, Tesco, was the project’s client. Network Rail and Buckinghamshire County Council owned the railway and the roads respectively. An agreement between Network Rail and Tesco made the latter responsible for construction safety.

To minimise disruption to rail services, arches were placed during overnight possessions and backfilling was done in the daytime.

The arches were designed to flex upwards as fill was placed to the sides and then settle back down when fill was applied on top. (credit: Timo Newton-Syms/Wikimedia Commons)

The tunnel’s design concept

Each arch comprised half-arch panels that interlocked at the crown. Each of the 343 concrete sections measured 1.82m wide by 350mm thick and weighed between 21.5t and 22t. Supporting these sections at their base, some 1,100 concrete piles with a 450mm diameter, topped by a pile cap, were cast in situ. The backfill around each section would contribute to the resistance of the whole structure, constraining lateral arch deflections under vertical loads.

A specific backfilling sequence would need to be followed in accordance with how the arches had been designed. This would also comply with the then Highways Agency’s standard for earthworks, Specification for Highway Works – Series 0600 (SHW). 

The method involved applying and compacting class-6N crushed limestone fill across the full width of the cutting by backfilling either side of an arch first and then loading the crown. When fill was placed to the sides, the crown of the arch lifted. When fill was placed on the crown, it settled back down. 

The arches were designed for this kind of movement to achieve the correct geometry upon completion. The use of a deflection monitoring system was specified, so that the arches’ actual movements could be checked against those calculated in the design.

Two senior engineers from Network Rail and one from the council signed this off using Network Rail’s process document, known as Form A, to confirm that the designer’s concept had been checked and “had no unusual design features”. Their undersigning of Form A indicated that they were satisfied that the design would conform with familiar standards and so presented “no unusual risk”.  

On the basis of this document, the national body responsible for rail safety, HM Railway Inspectorate, issued a “no-objection letter” concerning the design. The design-build contractor with by far the lowest quote, which had raised no queries about the proposed design and construction method during tender meetings with Tesco and the original designer, went on to win the contract. 

A significant change of plan

The successful contractor then proposed using a less dense backfill material called incinerator bottom ash aggregate (IBAA) in addition to the 6N. Both the arch supplier and the original designer checked the likely structural effects of this change, but neither submitted any new calculations. Although it had assumed responsibility for design and construction, the main contractor didn’t submit any such calculations either.

Adding a new type of backfill material should have required Form A to be resubmitted, complete with new sign-offs, but that didn’t happen. 

Proposing the use of two fill types rather than the one first specified also necessitated a new application method. This required the original 6N material to infill the arch sides up to a certain depth and be applied around the structure as an annulus, creating a “trapezoidal sleeve”. IBAA would then be applied up to ground level. 

The independent reviewer on the project did not check the design for these intermediate construction stages arising from the use of two fill types.

The new backfilling sequence no longer complied with the SHW. This deviation from standard was later addressed when the arch supplier added the original method statement, which had complied, as an addendum to the new one.

Network Rail raised concerns about the change of method statement, but it was eventually satisfied that the new backfilling process would remain compliant with the Specification for Highway Works

The 6N fill would need to be applied around the sides of the arch to a certain height, followed by a 1m- to 2m-wide section flanking the arch, with the remaining width made up by the addition of IBAA to the sides of the embankment. After 6N had been applied to a depth of between 1m and 2m over the crown of the arch, IBAA could then be added across the full width of the cutting. 

The method required that there could never be more than a 500mm difference between the heights of fill being applied either side of the arch. 

Network Rail raised concerns about the change of method statement, but it was eventually satisfied that the new backfilling process would remain compliant with the SHW. 

The two application sequences remained in the method statement, which caused some confusion onsite about the correct backfilling process to follow.

Two key deviations from standard procedure

Backfilling practices differing from those in the method statement are known to have happened on two occasions. The day before the collapse, fill was applied on top of the crown – to provide a temporary roadway for a concrete pump to be placed onsite – before enough material was in place at the sides. And, on the day of the collapse, a section of 6N side fill was removed to enable a leak repair and then immediately replaced. 

Arch deflections were not monitored from the start of the construction phase. When monitoring eventually began, it found significant variations from the movements anticipated in the design. Despite this, the discrepancies were ignored and no action was taken. Compared with the heights stated in the design, crown levels were found to be about 150mm lower before backfilling and 250mm lower afterwards. 

At 7.34pm on 30 June, 29 arch sections, each weighing 22t, and thousands of tonnes of backfill collapsed on to the live railway. No trains happened to be on that stretch of line at that moment and the site was already closed for the day, owing to a lighting fault in the tunnel, so there were no casualties. 

The line was shut for seven weeks while repairs took place. The project restarted in 2008, using a new design and construction method.

The chief causes of the failure and the lessons learnt can be grouped into the following categories and are discussed in detail in the below chapters:

  • Cause 1: Inadequate communication of the complex design and construction method using the Form A sign-off
  • Cause 2: Selection of the least engaged contractor with the lowest quote by far
  • Cause 3: Inadequate review of the change from using one type of fill material to two
  • Cause 4: Lack of appreciation of the warning signs of structural failure
  • Cause 5: Temporary application of fill on the crown, contrary to the method statement, without review

It took seven weeks to clear the site of debris and restore rail services (credit: Johnny Green/Alamy/PA Images)

Lesson 1

Complexity poorly communicated

The inadequate communication of the complex design and construction method using the Form A sign-off contributed to the failure in the following ways:

  • There was a lack of understanding of the complexity of the design and the tunnel arches’ dependence on the sequence of backfilling.
  • There was a lack of understanding of the project’s complex nature overall. It involved a wide range of stakeholders, disciplines and working practices in both highly regulated sectors and unregulated ones.
  • The fact that numerous qualified specialists working for different stakeholders signed Form A indicated that the design had no unusual features. People stopped thinking that it might contain some complexity or that any misjudgement might have occurred. This widespread complacency enabled errors and inconsistencies based on Form A to be carried forward.

Lessons on complexity

TRAINING 1. Ensure that people can identify and understand soil-arch interaction.

TRAINING 2. Ensure that people can identify and appreciate optimisation in designs and their construction methods where co-dependency creates complexity.

TRAINING 3. Ensure that people can spot and tackle knowledge gaps and overlaps that create or heighten risks on a complex project.

TRAINING 4. Ensure that people can both identify a range of stakeholder motivations and working practices and understand how these can affect the design, the construction method and their alignment with the project’s aims.

TRAINING 5. Ensure that people can identify weaknesses in a decision or process and know how to report this to senior management.

PROCUREMENT. The responsible person must assess the design’s complexity and suitability with respect to the procurement strategy and the skills of the delivery team. Collaborative Reporting for Safer Structures (CROSS) recommends that the alignment of the procurement strategy and contracts be checked against the complexity of the project and the competence of those involved.

PRACTICE/PROCESS 1. Understand that changes to any design (especially a proprietary system) that is sensitive to its method of construction will not be straightforward.

PRACTICE/PROCESS 2. Be aware that approvals issued by means of certain process documents may simply be continuations of previous documents’ approvals rather than an extra level of scrutiny.

CULTURE 1. Create an environment that both fosters a common language with respect to the project’s aims and addresses knowledge gaps and overlaps that create or heighten safety risks.

CULTURE 2. Be aware that, even though several qualified people have signed something off, errors could still have crept through.

CULTURE 3. Factors such as the pressure to deliver on time and the fear of admitting failure can lead people to ignore the warning signs. Construction projects need to create an environment of open reporting where people aren’t inhibited from sharing safety concerns. CROSS exists for this purpose. Concerns should ideally be dealt with on a project as soon as they arise.

Lesson 2

Lowest-quoting contractor

The selection of the least engaged main contractor with the lowest quote by far contributed to the failure.

The contractor may have been ruthlessly motivated to win the contract or it may simply have priced the job incorrectly. In either case, it may have failed – intentionally or unintentionally – to identify risks posed by the arch-backfill construction system.

Lessons on contractor choice

TRAINING. Ensure that contractors understand that under-pricing and/or failing to identify the project’s risks could create grave dangers to people’s safety and impose severe reputational costs.

PROCUREMENT. Carefully check and/or reject any quote that is alarmingly low, especially if the bidder offers no discussion showing their appreciation of the project’s complexities and risks.

CULTURE. Contractors need to feel able to admit oversights and submit amended quotes with more realistic prices.

Lesson 3

Change of fill design

The inadequate review of the change from using one type of fill material to two contributed to the failure in the following ways:

  • There was a widespread assumption, given that Form A had been signed off, that the change could be accommodated under the terms of the contract.
  • There was also a widespread assumption that the change was straightforward and that it still complied with the SHW. Since the arch supplier and client’s designer told the contractor that the arch and foundation could accommodate the change, the contractor went ahead. There is no mention in any publicly available report of whether their calculations considered loadings imposed during the construction process as well as the final loadings.
  • The use of an addendum in the revised method statement created confusion. The new method involved applying the original 6N fill material as a “trapezoidal sleeve” around each arch and then applying the new IBAA fill on top. According to the Health and Safety Executive’s investigation report, the designer had originally intended that “all backfill materials should be placed across the full width of the excavation. The two methods of backfilling were therefore inconsistent.” A Network Rail engineer rightly queried this deviation from the original plan, but they were eventually satisfied and the conflict was never resolved.

  • The chance to resubmit Form A, and thereby highlight the significance of the change, was missed on three occasions:

    1. when there was a change of fill type;
    2. when the new backfilling method was proposed; and 
    3. when the conflict between the two method statements was identified.

The tunnel under construction days before the collapse (credit: Timo Newton-Syms/Wikimedia Commons)

Lessons on the change of fill design

TRAINING 1. Ensure that people can identify a significant specification change.

TRAINING 2. Ensure that people appreciate that a multidisciplinary change review is a matter of safety management.

TRAINING 3. Ensure that people understand that changes of fill materials on an arch backfill system will require the designer to review both the design and the construction procedure.

TRAINING 4. Ensure that people appreciate that any proposed deviations from standard earthworks specifications need to be addressed and, possibly, rejected.

TRAINING 5. Ensure that people conducting independent checks of changes can identify and analyse intermediate construction loadings.

TRAINING 6. Ensure that people can identify the nature of the conflict and raise their concerns to ensure that it’s resolved rather than pushed down the line.

TRAINING 7. Ensure that people understand that an addendum won’t solve the problem if it creates confusion.

TRAINING 8. Ensure that people can identify a mechanism for re-evaluating standard sign-off documents such as Form A.

TRAINING 9. Ensure that people appreciate the importance of resubmitting standard documents and can trigger reviews relating to safety.

PROCESS/PRACTICE 1. A change must be re-analysed and design calculations resubmitted by the contractor to ensure that its potential effects are well understood and communicated to the project team so that it can give its feedback before sign-off.

PROCESS/PRACTICE 2. Standard documents must be resubmitted when there is a change. This will make the project team aware of the potential impact and give it the chance to report back.

PROCESS/PRACTICE 3. Certain technical issues require a detailed multidisciplinary review.

CULTURE 1. Proposed changes should be rejected if they would create complexity that cannot be safely accommodated without adding significantly more skills and resources to the project and/or changing the delivery strategy.

CULTURE 2. Even when a design has been signed off as having no unusual features, it doesn’t mean that changes to it can be accommodated and accepted without first being reviewed along with that sign-off.

CULTURE 3. Accept that a safety-critical review of a change may delay a project.

CULTURE 4. Factors such as the pressure to deliver on time and the fear of admitting failure can lead people to ignore the warning signs. Construction projects need to create an environment of open reporting where people aren’t inhibited from sharing safety concerns. CROSS exists for this purpose. Concerns should ideally be dealt with on a project as soon as they arise.

Lesson 4

Failure to appreciate warning signs

The lack of appreciation of the warning signs of structural failure contributed to the collapse in the following ways:

  • No one resolved the issue of the two conflicting method statements. 
  • Worsening arch deflection readings were ignored.
  • Arch deflection monitoring was specified but not deployed effectively, so no one knew whether deflections were within the expected range or whether they had reached levels dangerous enough to warrant safety measures.
  • There would have been more awareness of the arch’s sensitivity to loading if its movements under backfilling had been checked against the predicted values.
  • The site was not closed and the train operating company was not alerted when excessive deflections were reported.
  • Responses to the leak and the lighting failure in the tunnel were both inadequate.

Lessons about warning signs

TRAINING 1. Ensure that people can identify contradictions – e.g. conflicting information in a method statement – or anything that doesn’t make sense and report these to higher management.

TRAINING 2. Ensure that people know how to use deflection monitoring systems onsite, so that safety measures can be taken if the readings exceed set trigger values.

TRAINING 3. Ensure that everyone can identify warning signs of a structural failure – e.g. leaks and lighting faults – and report these to trigger safety measures.

PROCUREMENT. Include deflection monitoring in the contract, so that it’s budgeted for and responsible project partners are aware about design sensitivities and the expected accuracy of the construction. If readings exceed acceptable levels, agreed safety measures can be taken swiftly. 

PROCESS/PRACTICE 1. A qualified independent person checking design intent onsite should be appointed to address contradictions and errors.

PROCESS/PRACTICE 2. Include deflection monitoring and check progress against predicted levels. This is a transparent and time-efficient way to address problems collectively when things don’t go to plan and provide assurance when they do. Monitoring would have alerted the contractor to the arch’s sensitivity to backfilling. 

CULTURE 1. Use deflection monitoring data. Monitoring systems can check actual readings against predicted values, especially for sensitive or complex designs. This is a transparent and time-efficient way to address problems and/or provide assurance.

CULTURE 2. Contractors that make errors may be putting lives at risk, so they should work with the project team to resolve them as quickly as possible.

CULTURE 3. Leaks and lighting faults must be taken seriously and acted on, as they could be signs of an imminent structural failure. 

CULTURE 4. Factors such as the pressure to deliver on time and the fear of admitting failure can lead people to ignore the warning signs. Construction projects need to create an environment of open reporting where people aren’t inhibited from sharing safety concerns. CROSS exists for this purpose. Concerns should ideally be dealt with on a project as soon as they arise.

Lesson 5

Deviation from the method statement

The temporary application of fill on the crown, contrary to the method statement, without review contributed to the failure.

IBAA was applied to the crown to provide access for the concrete pump without sufficient side fill. This happened without review, as did the temporary removal of a section of 6N fill to investigate a leak shortly before the tunnel collapsed.

Lessons about temporary fill

TRAINING. Ensure that people adhere to the method statement and understand that any proposed deviation from it must first be reviewed.

CULTURE 1. Accept that a safety-critical review of a change may delay a project.

CULTURE 2. Factors such as the pressure to deliver on time and the fear of admitting failure can lead people to ignore the warning signs. Construction projects need to create an environment of open reporting where people aren’t inhibited from sharing safety concerns. CROSS exists for this purpose. Concerns should ideally be dealt with on a project as soon as they arise.

Timeline

How events unfolded