Tunneling failures offer lessons about how to effectively manage risk, particularly by encouraging a culture of transparency when things go wrong. There are risks with tunnels that do not exist anywhere else. Tunnels have got the ground around them, so the materials that you’re working with are out of sight. Frequently what you’ve constructed is also out of sight,” says independent consultant Bill Grose, who over the past decade has focused on project leadership risk management. Grose says tunneling failures are surprisingly common, much more so than public perceptions indicate. Recent and historic incidents seem to support his claim. In the UK, the appearance of a sinkhole above the Chiltern Tunnels route of High Speed 2 in May prompted project promoter HS2 Ltd to launch an investigation – ongoing at the time of writing – with theories emerging that the hole is linked to a pre-existing fissure that had partially collapsed. In the same month, the buried Health & Safety Executive (HSE) report into the Gerard’s Cross railway tunnel collapse in Buckinghamshire in June 2005 surfaced. This incident involved the partial collapse of precast concrete tunnel elements under the weight of backfill, with the tunnel’s crown caving in on itself (NCE, last month).The report points to design and construction errors. These include the decision to widen the tunnel to span four tracks instead of two while maintaining the crown at the same level as the original design. The implications of tunneling disasters are sizeable for contractors. The Heathrow Express tunnel collapse in London in 1994 is considered one of the worst civil engineering disasters on record in the UK. Balfour Beatty was ultimately fined £1.2M and had to pay £100,000 worth of prosecution costs for its part in the collapse. The disaster occurred after engineers excavating tunnels for the station under the airport’s central terminal area experienced three collapses in as many days, opening giant holes in the ground above. The HSE report on the Heathrow Express collapse identified design and management errors, poor workmanship and quality control as root causes. The project was using the New Austrian Tunneling Method, which was relatively untried in soft ground. Using this method, excavation was carried out incrementally with a sprayed concrete lining applied as work progressed. The collapse occurred because substandard construction work had gone unchecked, while grout jacking to stabilize a building above had also damaged the tunnel. Overall, the report found the incident exhibited “all the hallmarks of an organizational accident”.
Grose investigated the Heathrow Express tunnel collapse and says one factor was lack of joined up communication and action when problems became apparent. “They were measuring convergence, which entails measuring the widest distance across the tunnel. It shouldn’t change very much but on that project, it changed a lot. While this was being recorded by one party, it wasn’t really being detected as a problem by others,” he explains. Another significant incident also investigated by Grose was the Nicoll Highway collapse in Singapore in 2004. There a 110m section of cut and cover tunnel excavation for the City’s Circle Line collapsed. The official report into the disaster found that design and construction errors led to the failure of the strut and beam retaining wall system. Engineers were found to have used an inappropriate soil simulation model that overestimated the soil strength at the site and underestimated forces on the retaining walls. An inclinometer was being used to monitor the movement of the retaining walls on each side of the excavation, Grose says, “and those walls were moving a lot, much more than they should have been. Again, somebody was monitoring it, but nobody was spotting it.” What was striking about both the Heathrow Express and Nicoll Highway failures was that there were key omissions or construction mistakes and early warning signs that either went unheeded or were not followed up, Grose says. Mott MacDonald project principal, transportation tunnels account lead Rosa Diez agrees that common tunnel failure themes are communication blockages and human error. She gives the example of monitoring the movement of a tunnel structure during construction. “The truth is that monitoring tends to become quite a monotonous task,” she says. This is when tunneling conditions can become dangerous because “you tend to overlook patterns”. She adds: “Interestingly, in many projects where you do back analysis, the trends were clearly there. But of course, that is always clearer with hindsight.” Diez highlights the fact that many factors can contribute to a tunnel collapse. In the early stages of her career as a civil engineer, she was involved in the recovery work after the Heathrow Express failure and one of her responsibilities was to log the tunnel linings after the collapse. While she declines to comment directly on the Heathrow Express incident, she notes that lack of continuity of workmanship can contribute to tunnel failures. “I’m not talking about Heathrow here, but theoretically, if something like a lining is not thick enough or applied incorrectly or with the wrong methodology or sequencing, that can mean joints are incorrectly formed. Even the failure to clean surfaces properly between applications can cause problems.” She adds that inadequate testing and analysis can also contribute to tunnel collapses. Diez says: “It could be that you have misinterpreted the ground – for instance, that you are designing the tunnel in which you have a ground prone to swelling; you have thought of the possibility of swelling and you have done testing. The testing might not show swelling, but it turns out that the testing was limited.”
With tunnel collapses often stemming from a convergence of factors, Grose cites the “Swiss Cheese Model”, a project management technique which views a project as a series of layers like slices of Swiss cheese. Those layers might represent stages in a tunnel’s construction, including ground investigation, design, supervision and the employment of suitably qualified people. Importantly, the holes in each slice should not align, so that if something slips through one slice, it stops at the next. “It’s only when multiple holes line up all the way through that something goes wrong,” says Grose. Meanwhile Diez adds that technology can play a key role in reducing tunnel failure risks. She says digital technologies like Sequani’s 3D modelling software Leapfrog, building information modelling (BIM) software and data capture have a growing role. “With tunneling you gather a lot of information and the big data revolution means that you can analyze and back analyze,” Diez explains. “There’s a lot of information which you can then potentially use in real time to optimize your design or your construction methods.” Procurement Ultimately the way in which participants take responsibility for projects also has an impact on outcomes. LBA Construction chair Martin Knights says new procurement methods and changes to frameworks via which projects are delivered will go a long way to shape behavior and remove the “blame culture” from schemes when they go wrong. Knights gives the example of Project 13, a partnership initiative developed by the ICE, the World Economic Forum and the Civil Engineering Contractors Association. This has been designed to support collaboration and transparency. Project 13 is based on the concept that a project is an enterprise and entails a series of transactional agreements with the supply chain. The enterprise comprises a client, advisor, integrator and suppliers, who are all incentivized to deliver outcomes through collaboration. Knights points to the recent application of Project 13 principles by National Grid for the London Power Tunnels project (NCE, August 2022). He says this project demonstrated Project 13’s benefits in engaging all participants to collaborate early in a scheme. It is seen as success story because work progressed without incident through effective management and monitoring of settlement. “A key player in that project has been the cable manufacturer, which played a part defining the logistics of the project,” Knights says. “The thinking is that the main civils contractor needs to be on board, liaising directly with the cable contractor before work starts.” Grose adds that re-framing responsibility is a key element of avoiding tunnel failure and makes an important distinction. He adds: “Taking responsibility is not to do with blame. Responsibility is a proactive thing.” He says efforts to move away from situations that can result in cover ups when errors are made or steps are missed are reflected in the third edition of the Code of Practice for Risk Management in Tunnel Works published in March. The publication includes revisions that address behavior and culture as well as a new section on digital modelling. It is published by the International Tunneling & Underground Space Association and the International Association of Engineering Insurers. Expanding on the areas of behavior and culture where engineering organizations could improve, Grose says many should also reassess their emergency detection and response policies. He adds: “How will they respond if it looks as if something is going wrong? Who has responsibility to stop the job to take emergency action? Sometimes it’s not all that clear, even in major projects, who has their finger on the stop button.”