Work on High Speed 2 is ramping up ahead of tunnelling on the first phase of the route.
When tunnel boring machines (TBMs) Florence and Cecilia are launched next spring, the 2,000t German-built giants will work relentlessly and largely unseen for three years to create the UK’s longest transport tunnel.
Named after local nursing and astronomy legends Florence Nightingale and Cecilia Payne-Gaposchkin, the Herrenknecht TBMs will create 16km of twin-bore passageway under Buckinghamshire’s Chiltern Hills for High Speed 2 (HS2).
Challenging conditions
And like their namesakes, the bespoke machines will display elements of innovation working in challenging conditions.
Due to the length of the tunnel and the amount of time they will be underground, a huge amount of design effort has gone into making the TBMs as efficient and reliable as possible.
For a start, it will be the first time variable density TBMs have been used in the UK. This technology allows the machines to adapt to different ground conditions.
“There are two main types of conventional closed-faced tunnelling machine – slurry shield and earth pressure balance,” explains James Reilly, TBM engineer at contracting consortium Align, which is responsible for the Chiltern Tunnel project as part of a £1.6bn package of works on phase one of HS2.
“Given the constraints we have – such as an aquifer underneath the tunnel and not being able to use certain chemicals – we are to use a variable density TBM, which uses both slurry and pressure balance technology.”
Align comprises Bouygues Travaux Publics, Sir Robert McAlpine and VolkerFitzpatrick. It has thought beyond normal constraints to maximise the efficiency of the TBMs.
“We have changed the design,” says Reilly. “For example, grout used to fill the gaps between segments of the tunnel wall would usually be pumped to the TBM or brought on vehicles to the machine, but given the length of the tunnel this was not practical – so we will have a grout plant on the TBM itself. This means we will not rely as much on logistics on the outside.”
The boring machines also feature robotic arms that will automatically screw in dowels to join concrete lining segments together as well as removing timber planks that separate these segments while they are transported.
“It removes a person from a hazardous area,” says Reilly. “We’ve done a lot of development with the supplier to put the correct equipment on the TBM.”
The TBMs will operate in a “continuous boring” mode that hasn’t been used on site before.
“Usually you would excavate, stop, install a ring, continue excavating,” says Reilly. “We are doing a form of continuous boring where segments can be installed as the TBM excavates. Align has been working very closely with its supplier to make sure it is working; we’ve had to change the interface with the cabin, the hardware, the programming and logic to make the system work.”
Florence and Cecilia will arrive on site in parts this autumn and the team will undertake a six month building phase before launching them next spring. But before the machines’ epic journey can begin, a similar effort has already been required from the engineering team preparing the ground along the tunnel alignment.
The consortium took over the South Portal site – which has its own access slip road between junctions 17 and 18 of the M25 – in September 2019 to prepare for tunnelling as well as construction of the 3km long Colne Valley Viaduct.
Levelling this sprawling, hilly, chalky site and ensuring everything is ready for Florence and Cecilia has required an unblinking focus, especially in the face of a government-ordered Oakervee review of the entire HS2 project, closely followed by a global pandemic.
“The tunnel is on the critical path of phase one of HS2,” says Align construction director David Whiteford. “The decision was taken last year to order the TBMs.
“The tunnel is on the critical path of phase one of HS2,” says Align construction director David Whiteford. “The decision was taken last year to order the TBMs.
“We have targets to meet so we cracked on with the plans as the Oakervee Review happened. We could not afford to pause as we are working to a tight plan that includes procuring buildings and processes.
The team is moving in the region of 850,000m3 of chalky earth to create the right conditions to build a raft of construction facilities and to build and launch the TBMs.
Align South Portal construction director **** O’Hare says the high water content of chalk means it can become unworkable after heavy rain.
“A big challenge was that chalk has an extremely high water content – up to 33% – so any more water makes it unworkable. We were therefore unable to move a lot of the material through the winter. We started at the beginning of April and we’ve done about 500,000m3 over the summer.”
As the groundworks are delivered, the construction aspects of the project can begin. One major structure that the consortium has already created is a 17m high headwall for the boring machines to break through when they are launched. “When you launch a TBM forward, you have to bury the whole of the cutting head straight away, so you need vertical ground to press into,” says O’Hare.
Almost 700 reinforced polymer soil nails, each up to 20m long, were driven into the wall to stabilise it. A specialised rotary drill took up to two hours to create each bore hole before each nail could be installed.
“Building and nailing the headwall took seven months, starting in December 2019 and completing this June,” adds O’Hare.
All the precasting for this section of HS2 will be done on facilities that Align is purpose building on site.
“We’re creating three batching plants to produce all the concrete,” says Whiteford. “There will be tunnel segment and viaduct deck precast facilities.
“We will also be processing 3M.m3 of arisings from the tunnel and placing them into the recently quarried landscape. Water has to be added to the chalk at the face of the tunnel to slurrify it, then it can be pumped down the tunnel and we immediately squeeze the water out. The slurry treatment plant is a process in its own right. We have a facility to reuse as much of the water as possible.”
Digging the tunnel approach cutting and creating the reinforced concrete launch slab for the TBMs is another major project.
“It is a technically challenging reinforced concrete structure,” he says. “It’s only an 18m section and has 207t of steel due to the weight and thrust that will go through it when we launch.”
Designing the slab took 18 months of close collaboration between the Align team and its design joint venture Jacobs and Ingerop-Rendel. It is described by Whiteford as among the largest co-ordinations of permanent and temporary works he’s ever worked on.
As this issue of NCE went to press, work to build the launch slab was well underway ahead of the arrival of the TBM components.