Series of errors that led to 2005 Tesco tunnel collapse finally revealed

A newly unearthed Health and Safety Executive (HSE) report into the 2005 Gerrards Cross tunnel collapse has revealed the sequence of events that led to the structure's failure during construction.

Gerrards Cross tunnel was mooted decades ago as a potential way of creating new land on which to build offices, technically making it a covered railway, rather than a tunnel. In the 1990s, Tesco was identified as a partner to sponsor the scheme, with the intention of building a new supermarket on top. The council refused the plan in 1998, but this was overturned by then secretary of state for the environment, transport and the regions John Prescott.

Jackson Civil Engineering was appointed as main contractor for the works in a contract worth approximately £25M. Piling works started in June 2003 and main works in July 2004, with the railway remaining live the whole time. The structure was to be a 324m long reinforced concrete tunnel made out of 343 sections that interlock at the crown of the arch then back filled to the sides and over the top.

The tunnel in April 2005; backfill can be seen at the far end

On the evening of 30 June 2005, part of the tunnel collapsed, with the crown caving in on itself. This saw 29 arch sections, each weighing 22t, and thousands of tonnes of backfill fall onto the track and block it. Nobody was on the affected section of the line at the time, but two trains were approaching it and one had to apply its emergency brakes. There was potential for multiple fatalities, which was narrowly avoided. The train line was closed for seven weeks, causing massive disruption.

HSE undertook an investigation of the incident but its report was never published and requests for it to be released were repeatedly refused.

Railway engineer and writer Gareth Dennis discussed the incident on an April 2021 edition of his Rail Natter podcast and talked about how the report had never been released. A listener to this episode decided to take the problem into their own hands, asking for the report from HSE through a Freedom of Information request. This was denied, but not on lawful grounds. As described by Dennis in the latest episode of Rail Natter, the "intrepid" fan then spent the next two years going through a long and arduous process of involving the Information Commissioner's Office and sending dozens of follow-up emails.

Finally, a 2010 draft was released to the intrepid fan, who submitted it to Dennis. He has now dissected it on the latest episode of his podcast, noting that some of the concluding pages have been redacted. "It's very annoying and very inappropriate," he said. "It's like they’re embarrassed over the mistakes they’ve made."

Castigating HSE further for withholding the report, Dennis pointed out "this was a major civil engineering failure and it was a novel civil engineering structure. Up until this report had been published, we had no idea what had gone wrong." The report is now available to view on the Railways Archive.

The report describes how four contractors tendered for the construction: Skanska, Costain, Jackson and Taylor Woodrow. Jackson raised no concerns about the design at this stage and offered no alternative proposals. By submitting a design and build tender, it accepted responsibility for managing the design risks in the project. It was awarded the contract in April 2003, coming in around £3M lower than the next offer.

White Young Green (WYG) - now part of Tertra Tech - designed the tunnel, incorporating a specialist reinforced concrete arch design developed by Reco and using its Tech Span system. This is a three pin "arch" design consisting of segmental pre-cast steel reinforced concrete sections that interlock together.

The tunnel was initially designed to house two tracks, but, according to HSE's report, it was later decided to futureproof the tunnel by making it wide enough to one day accommodate four rails, potentially for freight. However, the report states "due to the floor level and height of the proposed store needing to remain, the designer kept the crown of the arch at the same level... the effect of maintaining the crown level while increasing the span was create a much flatter arch. This had implications on the design."

HSE's report explains that when the erection of the sections is complete, the backfilling commences with compaction to the required specification. Asymmetrical loading of the backfill can influence the instability in all arch structures and the report says that the Tech Span system is particularly "sensitive" to asymmetric loading. The Tech Span manual emphasises that backfilling must be carefully controlled so that the differential between both sides is no greater than 500mm. HSE quotes from the manual which saying that if this guidance is not followed, the stresses in the structure will be increased and the overall safety factor of the design decreased.

There was also an addendum to the backfilling arrangements to ensure that they complied with the Specifications for Highways Works – Series 600.

Jackson proposed an alternative backfilling arrangement. It envisaged using a 6N material (a limestone quarried, crushed stone material) around the tunnel and an alternative bulk fill elsewhere. The fill placement procedure involved placing the trapezoidal "sleeve" of 6N material around the arch of the tunnel, along its length, to be followed later by other filling material. The method statement was not sent to Reco or WYG for inspection, although discussions did take place.

Jackson was responsible for managing the delivery of its own backfill procedure and sub-contracted Clark Construction for the job, but did not foresee any issues. "We are not aware of any calculations undertaken prior to the collapse," HSE's report states.

It continues: "It is not clear how the fill should progress in the statement method. The use of the word ‘sequentially’ is ambiguous." It says that the method saw backfilling around the arches, but not across the full width of the tunnel, as the specification required.

The report goes on to explain that WYG requested deflection monitoring of the arch units but that engineers on site were not aware of this initially. When Jackson did start monitoring the units for deflections, the results were ignored and some "very large deflections" (over 150mm) did not result in any actions by the site management. "Thus, had the works been completed without collapse, large areas of the structure would have been in a condition not foreseen by its designers and hence well outside its design parameters," the report states. "Nobody on site was aware of this situation at the time it occurred."

HSE says that the site engineer who was in charge of the backfill never saw the method statement prior to the tunnel collapse as he could not find it. He instead interpreted the drawings and was briefed by a colleague who was departing for Australia.

Despite this, the backfilling did take place in a way that followed the method statement, except it was not in accordance with the Highways Specification Series 600 Earthworks. When, in the weeks leading up to the collapse, Jackson engineers inspected the intrados levels at the east and west portals, it was found that the west portal was 125mm lower than its design position. The results were passed on but no actions were taken. Similarly, the crown level at the east end was found to be 147mm below those shown in WYG's drawings, but again this was glossed over. Two days before the collapse, the levels were measured again and found to be even lower, but nobody is sure what was done with this information, according to HSE.

The day before the collapse, engineers were informed that a temporary access road was to be constructed up to and over the tunnel so that a concrete pump could be brought on top of the arch. It was discovered that the sides of the tunnel were not up to level with the fill, but priority was given to making the access road as backfilling could be done later, it was presumed. There is a disagreement in accounts on what happened in this case, with another engineer stating that they ordered the backfilling to be completed before the access road went over it. In the end, the top was covered in backfill material for the access road platform, but the sides of were not backfilled – contrary to the method statement prepared for the works.

Earlier on the day of the collapse, a 1.2m section of the 6N material had been dug out of the tunnel to fix a leak. This was around the point of collapse.

The collapse occurred at 7.30pm on the evening of 30 June. Fortunately, workers in the tunnel had left earlier in the day and no trains were in the tunnel at the time. On the north side 14 sections collapsed and on the south side 15 sections collapsed. "The fill profile over the adjacent damaged units that did not collapse (though were later removed in a controlled manner) shows approx. 2.5m fill over the crown with 2m to 2.5m at the haunches," HSE's report states. "The access road sloping down on the north side from east to west."

HSE's report then quotes from a 2005 report into the incident that was signed off by Jackson and WYG: "Network Rail's view on the cause of the collapse was inadequate fill material around the arch haunches compared to the fill directly over the crown. In [its] opinion this was a classic mode of failure, with the arch deflecting under the weight of the backfill over the crown with insufficient fill to the sides to resist the forces. Consequently, plastic hinges formed at the haunches and this allowed the crown pin to snap through or dislocate and resulted in collapse."

It also includes this conclusion from an earlier draft of HSE's report into the incident: "The engineering team review has concluded that the cause of the collapse was due to the placing of too much type 1A fill over the area of the crown of the tunnel prior to placing and compacting sufficient material in the area of the tunnel haunches adjacent to the north and south site access roads towards the east end of the site. This overloading induced excessive moments within the arch, leading to formation of plastic hinges at approximately quarter points in the precast arch units, thereby inducing collapse.

"There is no evidence of any other contributory factors."

Costain was appointed by Tesco in 2006 to construct the tunnel and the supermarket atop it. A more cautious approach was taken to the tunnel using precast continuous arch segments. It completed in 2010.

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