Florence and Cecilia are set to bore through the Chiltern hills as part of the first phase of HS2. Named after nursing and astronomy innovators Florence Nightingale and Cecilia Payne-Gaposchkin, the German-built machines will start work next spring on the longest transport tunnel in the UK.
Like their namesakes, the Herrenknecht TBMs are designed to be innovative under pressure. Once they commence boring next spring, the 2,000 tonne German giants will work for the next three years, spending the majority of the time under the ground.
Efficient and reliable
Owing to the magnitude of the task the TBMs are designed to be as reliable and efficient as possible and will utilise variable density technology to cope with the demands of different ground conditions.
Utilising both slurry and pressure balance technology, the TBMs will be expected to cope with challenges such as an aquifer situated under the route of the tunnels.
Align, comprising Sir Robert McAlpine, Bouygues Travaux Publics and VolkerFitzpatrick, has re-engineered the machines to achieve maximum efficiency. Where grout would normally be pumped to the TBM, these machines will carry their own grout plant. Robotic arms will automatically join sections of concrete lining and remove the planks they were transported on.
The TBM design is intended to remove the need for people in a hazardous zone and the machines will work in a continuous boring mode installing segments as they excavate.
Preparation for tunnelling
Before the TBMs are ready for launch, they'll undergo a six month building process on-site. A similar effort has been required by the engineering team who have been working since 2019 to prepare the ground for tunnelling.
Align says the tunnel is on the critical path of HS2 Phase One and put in the order for Florence and Cecilia even as the oakervee review was underway. The team is currently removing some 850,000m3 of chalk soil in order to build the TBMs and build a raft of other construction facilities at the site.
It's a big challenge as chalk has up to a third water content, making it all but unworkable after heavy rain. Work stalled over the winter but the fine summer allowed over 500,000m3 to be removed. Everything has been thought through, with the creation of a 17-metre high headwall for the TBMs to break through when they commence tunnelling.
Align South Portal construction director Mick O'Hare explains that, as a TBM is launched, its entire cutting head must be buried straight away so Florence and Cecilia will require vertical earth to cut into. The work to get the headwall ready for the TBMs took seven months.
Digging the approach cutting and engineering the reinforced concrete slab from which the two giants will launch was another major undertaking. The design of the technically challenging launch pad took 18 months of collaboration between Align and its design joint venture Jacobs and Ingerop-Rendel.
Work to create the launch slab is well underway, ahead of the arrival of Florence and Cecilia in the coming months.