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Phoenix from the flames

Experience, automation and innovation helped Eurotunnel build on the experience from the Channel Tunnel fire of 1996 when carrying out repairs after last September’s blaze. NCE reports from the scene as the operator prepared to reopen the damaged section of running tunnel.

On the afternoon of 11 September 2008 a fire broke out on a train carrying freight lorries through the twin bore Channel Tunnel.

The blaze lasted for more than 16 hours, causing damage to a 650m stretch of Interval 6 in the northern running tunnel, 11km from the French portal. The concrete lining was severely damaged and electrical and ventilation systems destroyed. It was a major incident. Yet it was not a first. The first fire in the tunnel was in November 1996. It too caused serious damage and operator Eurotunnel lost six months of full service plus millions of pounds in operating revenue as a result.

This time, the operator was determined it would not incur the same service disruption or costs as it sought to repair the damage and restore operating capacity. And after using a combination of experience, improved technology and more effective project management, Eurotunnel was this week due to reopen the fire damaged section, restoring the tunnel to full operating capacity, just five months after the blaze.

“When we told people when we planned to open, they said we couldn’t do it,” says Eurotunnel chief operating officer Jean-Pierre Trotignon. “But now it looks like we might even finish early, ready to get on with testing.” Full re-opening of the tunnel was scheduled for Tuesday as NCE went to press.

A three and a half month construction timetable was demanding, says Trotignon, but each day the damaged section is closed costs Eurotunnel €1M (£880,000) in fares, so there was plenty of motivation to work quickly. And that is why teams have been working in shifts around the clock to complete the work.

After the fire had been put out, repair work had to be held back until 18 October. This was so that engineers could check the tunnel and the judicial investigation into the cause of the blaze could get underway.

Contracts were let a week before work began, on 10 October. The project team is all-French, led by project supervisor Setec and group leader Freyssinet. Freyssinet took responsibility for civil engineering works, Eurovia Travaux Ferroviaires for track and overhead cateneries and Vinci Energies for other equipment. Vinci subcontractor Axima was made responsible for the cooling system.

“The first thing we wanted to do was secure the tunnel for further works,” says Eurotunnel logistics and infrastructure maintenance director Jean-Luc Pochet. Here, the geology threw the tunnel repair team its first technical challenge.

The Channel Tunnel runs for 50.5km from Folkestone to Sangatte on the French coast. It runs an average of 40m below the seabed. The two running tunnels run for most of their length through a 15m to 30m thick band of blue chalk marl, which is relatively stable and easy to tunnel through. However, before tunnelling began in the late 1980s, geophysical investigation boreholes revealed strata riddled with faults close to the French side.

Click here for Channel Tunnel geology and profile

September’s fire struck the tunnel where it sits among these faults, seriously damaging the lining. This raised questions about whether the fire had weakened the lining enough to destabilise the tunnel. “We weren’t worried [that the tunnel would collapse],” says Pochet. “But the fracturing near to this section where the fire was meant we did have a question in our mind. Using rock bolts was our response to that question.”

As soon as site workers were granted access they began installing 50mm diameter hollow rock bolts where the concrete had suffered its worst spalling in the 7.6m diameter tunnel. Pochet describes the affected section as a “very small area” in a 20m stretch of tunnel, where a 400mm thickness of concrete had been burned away.

The stabilisation job called for just over 1,000 bolts – eight in any single 1.5m long concrete lining section. Seven lining sections make up each tunnel lining ring. Once the 3m-long bolts were drilled to depth through the tunnel walls, site workers pumped grout through their hollow cores to secured the annulus between the anchor and tunnel wall. Ground strength tests conducted after the bolts were in place reassured engineers that the tunnel bore was strong enough for work to continue safely.

This meant that attention could then turn to the fire damaged concrete tunnel lining. This had to be completely stripped away and replaced. The team opted to use hydro-demolition – water sprayed under high pressure – to do this. The decision to use mobile hydrodemolition machines led to the team’s decision to build a temporary roadway inside the tunnel to make it easier for plant to travel up and down the repair site. The roadway was 620m long and comprised 2,500 timber sleepers laid on 3,200t of ballast. “It gave the freedom you get from working on a road rather than along a railway,” says Eurotunnel head of communications, UK John Keefe.

In addition, a 550m long, two-level scaffolding working platform was erected along both sides of the tunnel to maximise the number of site workers able to operate in any area at any given time. The hydrodemolition rigs run on tyres, so it was possible to manoeuvre them easily into place along the roadway against the tunnel wall. For the benefit of safety, this operation was carried out remotely from 50m away.

Using hydrodemolition made the repair effort more efficient than after the 1996 fire when hand held pneumatic hammers were used. “This time around it was about industrialising the process using automatic and robotic machinery. It was a much quicker way of working,” says Keefe. “And being inside the works this time was nothing like last time. “In 1996, the tunnel had been open just over two years. We only had two years’ experience of running what was the most modern, high-tech tunnel in the world. We hadn’t even carried out a full cycle of maintenance in that time. Now it’s been running for so much longer and we’ve changed the catenary and track often – we’ve got so much more knowledge.”

The four hydrodemolition rigs on site pumped 200l/min of water at 1,000bar pressure to strip away the concrete. The high pressure water caused less damage to the reinforcement than hammers, so less reinforcing steel had to be replaced. In reality, the replacement of rebar was minimal, says Pochet.

When it came to replacing the concrete, using like-for-like precast concrete sections was ruled out in favour of shotcrete. “Reinstalling panels would have been an extremely long process and unnecessary,” says Pochet. From early December last year, site workers spent a month spraying 4,000t of the shotcrete in layers between 100mm and 150mm thick the full length of the 650m stretch of fire damaged tunnel. The shotcrete has achieved strengths of 55mPa – perfectly adequate for strengthening the tunnel.

Machines once again enabled a speedier application compared with repair work following the first fire, when all concrete was placed by hand. By 15 December, some of the shotcreting work was nearly complete and work to fix the supports for 1.8km of cable trays made of fire, smoke and toxinproof plastic for the electrical cables followed on. In addition about 150 steel brackets were fitted at 10m intervals to support cooling pipes and 120 catenary support brackets were bolted to the tunnel roof at 27m intervals.

The whole process was closely organised, as the scaffolding was immediately struck in the sections where concreting and fixings were complete. Since the New Year the roadway has been removed and work has continued to re-equip the repaired tunnel section.

A 40-strong team has unwound over 2km of 21kV mains power supply cabling from three 8t reels. In addition, 750m lengths of 3.3kV cables were installed to connect electrical pumping equipment to the power supply. Installation of 400V cables for lighting and earthing wires completed the electrical work.

At the end of January, works trains transported 10 mammoth, 96m-long replacement cooling pipes into the tunnel and the replacement of fire mains pipes was nearly complete. Track laying took place in two days in late January, clearing the way for non-passenger trains to carry out a sequence of tests to ensure that the damaged section can be reopened.

Since the fire, Eurostar trains have been diverted from the northern running tunnel to the southern tunnel via crossover caverns, to take them around the fire damaged area. When the repaired section reopens Eurotunnel services will be once again running through the whole of the northern tunnel to France in 35 minutes, shaving 10 minutes off the time it has been taking since works began. Eurostar is expected to restore its full service shortly afterwards on 23 February to allow time to amend its timetables.

Although construction work is set to come in at just under Eurotunnel’s initial budget of £53M, and despite its claim that it is “really well insured”, the tunnel operator has confirmed that losses resulting from the fire will extend beyond the £175M mark. Eurotunnel expects to provide detailed figures on the loss in its next set of financial results, which are due out on 4 March. It is also hoping to hear the outcome of the judicial investigation into the fire which is being carried out by France’s Bureau d’Enquêtes sur les Accidents de Transport Terrestre.

Eurotunnel has also brought in new safety procedures. More changes can be expected following the release of the official investigation report (News last week).

  • Repair Timeline

  • 11 September
    Fire breaks out in the tunnel, damaging concrete lining

  • 12 September
    Judicial investigation begins. Engineers carry out safety checks

  • 8 October
    Start of preparatory works including rock anchoring

  • Mid October
    Temporary road way and scaffolding supported access platform installed

  • December to January
    Damaged concrete removed and replaced

  • 15 December to 15 January
    Supports for overhead cateneries, cables and cooling pipes fitted

  • 20 December to 15 January
    Scaffolding struck and temporary roadway removed.

  • 11 January to 22 January
    Electrical works

  • Mid-January
    Cooling pipes and fire mains installed.

  • 29 to 30 January
    Track replaced

  • Early February
    Test trains run

  • 9/10 February
    Tunnel section reopens to freight and car services

  • 23 February
    Full Cross Channel Eurostar resumes

Third Time Around
Unfortunately for operator Eurotunnel, September’s fire was not the first to hit the Channel Tunnel.

Two and a half years after its opening on 18 November 1996 a truck fire on one of the train’s open HGV wagons caused severe damage to a 500m section of concrete lining in the south tunnel 17km from France. Nobody was seriously hurt, but the blaze left the chalk behind the tunnel lining exposed.

Repairs took six months to complete and resulting losses amounted to €290M (£255M). A less severe incident hit the tunnel on 21 August 2006 when a lorry aboard the penultimate wagon of a shuttle train caught fire. The train was brought to a stop 20.5km from the UK. The fire burned for two hours before being extinguished. There was damage to the tunnel crown to a depth of 30mm along 10m of the tunnel. In this case, normal service through the whole tunnel – including the damaged section – resumed within 24 hours.

At 2.57pm on 11 September 2008 a third fire broke out on a freight shuttle carrying 27 vehicles. Fire fighters began tackling the blaze in the north tunnel, 11km from the French portal, 75 minutes later and extinguished it by 7am on 12 September.

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