Left:
The Chiltern Tunnel, which passes through the chalk geology of the Chiltern Hills, is the longest on the HS2 route
When TBM ‘Cecilia’ broke through on HS2’s 16.1km- long Chiltern Tunnel on 21 March, smiles on the faces of those involved were irrepressible. Arriving at the north portal, Cecilia joined TBM
‘Florence’, which had broken through three weeks earlier on 27 February, marking the end of boring on HS2’s longest tunnel. Not only is the twin-bore tunnel on C1 – HS2’s Central
Phase 1 section – the longest on the high-speed railway, it is more than three times longer than a typical drive for a TBM. “We are very proud of what has been achieved –
32km in 33 months. When we broke through, there were many happy faces,” says Align JV’s underground construction director, Didier Jacques. HS2’s head of delivery, Mark Clapp, says that pride
was well deserved. “It was a real case of the team delivering,” he says,
adding that everyone, from the surveyors, to the TBM crew, to the team managing the spoil works, was proud of the achievement. “It hasn’t all been plain sailing, we’ve had our
challenges but what has been notable is at each of those potential stumbling points, we’ve closed ranks, identified the best for the programme, and the team have found the right solution to deliver the works without major incident. What more can you ask of a construction project?” Align – a joint venture of Bouygues Travaux Publics,
Sir Robert McAlpine, and VolkerFitzpatrick – operates as an integrated project team (IPT), a collaborative approach that was established between all parties at the outset and it has been working well. “Align worked really hard to understand the
challenges and also to optimise the delivery vehicle,” says Clapp. An early example of this was Align’s three-
year collaboration with Herrenknecht on the TBM specification. “It would not be unfair to say these two TBMs are
Didier’s babies,” says Clapp. “He is an absolute mine of information on TBMs and every aspect of these TBMs has been thought about.”
The result was two variable density 10.26m-diameter TBMs – the first to be used in the UK and the largest TBMs to be used on a rail project – designed for the chalk and flint geology of the Chiltern Hills. The partnership between Align and Herrenknecht
produced TBMs with an impressive amount of innovation – enabled by the space in the tunnel and motivated largely by the length of the tunnel and the desire to improve safety. “We wanted staff to have the safest TBM to work on,”
says Jacques. In a world-first for boring technology, the TBMs
feature the Krokodyl and Dobydo robots reducing the need for operatives to work in confined spaces, at height and near plant and machinery, when handling the tunnel segments. The Krokodyl system removes the timbers that separate the segments when they are delivered on to the TBMs and drops them into a dedicated storage area, while the Dobydo robot installs the dowels on the segments, required before installation. Stepped access everywhere, rather than ladders, was
another safety feature on the TBMs. The technological innovations made on the Chiltern
Tunnel project are a change from established practices and, after some initial scepticism, the team embraced the changes. “The first time they saw the robot they were sceptical,
but they saw it made life easier and safer. They took ownership,” says Jacques. The logistics of building such a long tunnel also
shaped operations – not least the slurry management. Slurry processing is usually carried out in tunnels
of 3km-4km lengths, says Jacques, but in this case Align was applying it to a tunnel four times as long, and with the added challenge of a 110m difference in height between the South Portal, where the TBMs were launched, and the North Portal. In the slurry box on the TBM, recycled water is added
to the excavated material and the resulting slurry is pumped to the slurry treatment plant – the largest in the world – on the South Portal construction site. There, a total of 24 filter presses processed the resulting
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