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.

The main appointments for the design and construction were:

• The project designer, who was also appointed as the planning supervisor under the Construction (Design and Management) Regulations 1994.
• A design-build contractor, who was also appointed as principal contractor.
• A design checker for the permanent works design.

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

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 and then, once that process was complete, backfilling above the crown. To keep the loading balanced, backfilling needed to proceed on both sides of the arch so that the difference in levels did not exceed 500mm.

When fill was placed to the sides, the crown of the arch was predicted to rise. When fill was placed on the crown, it would then settle back down to its design position. 

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 by the designer, 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 approved the design in principle using Network Rail’s approval in principle 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 the relevant Network Rail standards without derogation, meaning that it 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 the lowest quote by far (£3m), 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. 

Following the approval in principle, the normal process would be for the detailed design to be completed and then to be subject to an independent design check. The detailed design and independent checking for a structure of this nature would normally include:

• The assumed construction sequence, so that the designer and independent checker can assure themselves of adequate structural performance under the intended sequence. This would include constraints on the sequence, such as the backfilling specification, sequence and levels.
• Monitoring proposals to ensure that the structure performs as designed.

The normal process would be for the detailed design and the independent check to be confirmed through the so-called Form B design and check certificates. But the report of the Health and Safety Executive’s investigation into the incident does not specify whether this occurred.

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.

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. 

Adding a new type of backfill material and changing the sequence should have required a Form A resubmission, complete with new sign-offs, but that didn’t happen. 

The independent reviewer did not check the design for these intermediate construction stages arising from the use of two fill types. Such a fundamental change to the construction sequence and backfill material should have required a new set of design calculations from the designer and a new independent design check, both confirmed by a new Form B. Again, the investigation report made no mention of this.

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.

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. 

It remained a requirement 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 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 contractor with the lowest quote by some margin, which had “raised no queries” during tender meetings
• 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

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 dependence between the sequence of backfilling and the structural integrity and performance of the precast arches.
• 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.
• Numerous qualified specialists working for different stakeholders signed Form A to indicate that the design had no unusual features. This introduced a significant risk that the team did not appreciate that it was designing and building a complex and sensitive structure. This complacency enabled errors and inconsistencies based on Form A to be carried forward.

TRAINING 1. Ensure that the team can identify and understand the dependencies between design and construction sequence. In this case it was also important for the team to understand the soil-structure interaction.

TRAINING 2. Ensure that people can identify the risks when designs and their construction methods are changed to shorten the schedule or reduce costs. This is particularly important where there is complexity in the design, the construction sequence or their interaction.

TRAINING 3. Ensure that people can spot knowledge gaps that create or increase risks on a complex project – and that they can highlight these at the right level in their organisations.

TRAINING 4. 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. The procurement strategy and contracts have to be selected with full recognition of the complexity of the project to secure a suitably skilled delivery team, with a commercial environment that enables it to make the right decisions.

PRACTICE/PROCESS 1. Ensure that the design documentation includes construction sequence information and the construction criteria/assumptions on which the design depends. It is important that design and construction teams are coordinated and share any subsequent developments regarding sequencing.

PRACTICE/PROCESS 2. Understand that changes to any design that’s sensitive to its method of construction won’t be straightforward and will need to follow a robust checking process.

PRACTICE/PROCESS 3. 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 a culture that aligns values and objectives in terms of safety and fosters the use of a common language to address gaps and overlaps that could create or heighten safety risks. This is especially important where there are several stakeholders with differing priorities,

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.

The selection of the main contractor with the lowest quote by some margin, which raised no queries during tender meetings, probably contributed to the failure.

The contractor may have been motivated to win the contract through low pricing or it may simply have priced the job incorrectly. In either case, it may have failed – intentionally or otherwise – to identify risks posed by the arch-backfill construction system. It did not raise any queries during the second-stage tender meeting, whereas the other three on the shortlist made comments about the designer’s two-hour presentation on the construction method and its constraints.

TRAINING 1. Clients need to understand that procurement processes that are heavily weighted towards price can lead to higher risks in delivery, especially for complex projects.

TRAINING 2. Contractors need to understand that under-pricing and/or failing to identify risks could endanger people’s safety and impose severe reputational costs. Low pricing can often mean that too few resources are devoted to managing a project safely, with the team either lacking access to the required expertise or coming under pressure to manage a large number of parallel activities. Should it emerge that commercial factors are jeopardising the safe delivery of a project, particularly a complex one, clients and contractors must hold a constructive discussion to resolve the problem.

PROCUREMENT. Carefully check and/or reject any quote that is worryingly 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 acknowledge mistakes or oversights and submit amended quotes with more realistic prices. Any failure to address these could lead to much greater impacts on the client and the contractor, including accidents causing substantial losses.

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 update the design and check certificates (Form B) – 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.

TRAINING 1. Ensure that the team is competent enough to identify a significant change of design, specification or construction sequence.

TRAINING 2. Ensure that people appreciate that a multidisciplinary change review for any potentially significant alteration is a vital part of engineering safety management.

TRAINING 3. Ensure that people understand that alterations to the construction method, such as a change of fill material or backfill sequence on a buried structure, will require the designer to review both the design and the construction procedure.

TRAINING 4. Ensure that people undertaking designs and independent design checks (including original designs and changes) appreciate that they are required to understand, define and analyse construction sequences and intermediate loadings.

TRAINING 5. Ensure that the design and construction documents are updated clearly after any change, so that the design, construction sequence and critical information are communicated unambiguously.

TRAINING 6. Ensure that people understand that, if design or construction information is unclear, work must stop until clarity is obtained from the responsible person or organisation.

TRAINING 7. Ensure that people understand the triggers and process for re-evaluating and updating design assurance documents such as Form B and appreciate the importance of this.

PROCESS/PRACTICE 1. Changes that could invalidate the design basis and/or design assumptions must be resubmitted by the contractor to ensure that their potential effects are well understood, approved and communicated to the project team.

PROCESS/PRACTICE 2. Design and construction 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, reinforced by the senior leadership team. CROSS exists to share learning across the industry, but concerns need to be dealt with on a project as soon as they arise.

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.

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 structural performance monitoring (defined in the instrumentation and monitoring plan) in the contract, so that it’s budgeted for and responsible project partners are aware of design sensitivities and the actions expected if readings exceed trigger values.

PROCESS/PRACTICE 1. Design documents and specifications for buried structures, retaining walls, tunnels and other large/complex structures must include instrumentation and monitoring plans to check that structures are performing as expected. These should include the type, location and frequency of monitoring, along with trigger values to determine if a structure is meeting expectations. The instrumentation and monitoring plan should include the actions to be taken if trigger values are breached. This is a transparent and 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. 

PROCESS/PRACTICE 2. A qualified responsible person should be appointed to ensure that monitoring data is recorded and assessed, and that actions are undertaken as required by the instrumentation and monitoring plan. These would include predetermined actions to alert external bodies – e.g. utility companies – if their assets/operations are at risk.

PROCESS/PRACTICE 3. A process should be established to ensure that any design queries/errors can be addressed by a qualified responsible person from the design organisation.

PROCESS/PRACTICE 4. For complex structures, there should be a process established to ensure that the design intent is maintained and respected throughout the construction phase.

CULTURE 1. Ensure that all members of the site team understand the safety-critical importance of reviewing structural performance monitoring data and acting upon it. Monitoring systems are important to check actual readings against predicted/trigger values, especially for a sensitive or complex design, and to ensure that action is taken if readings indicate any level of structural performance that wasn't predicted in the design. This is a well-established way to manage structural performance problems and provide assurance to stakeholders.

CULTURE 2. Designers and construction organisations that make errors may be putting lives at risk, so they should work with the project team to highlight and resolve these as quickly as possible.

CULTURE 3. Unusual events such as leaks and lighting faults must be investigated and acted upon, 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. This has to be reinforced by senior leaders and all management teams on civil engineering and infrastructure projects. CROSS exists to share learning across the industry, but concerns must be dealt with on a project as soon as they arise.

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.

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 multi-disciplinary review of a change is essential, even if it delays a project. Senior leaders and management teams need to foster a culture where it’s acceptable to stop work and check if things change.

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 raising safety concerns.