UK: The 1·6 km long twin-bore tunnels that will take High Speed 2 beneath Long Wood in Warwickshire are moving from the civils phase to the fit-out stage.
According to project promoter HS2 Ltd, the tunnel is the first of the bored structures on the route to be see civil works completed; no less than 44 route-km of twin-bore tunnels are being built between London, Birmingham and a junction with the West Coast Main Line near Lichfield.
In March 2023, boring machine Dorothy completed the second and final drive, and since then the main works contractor Balfour Beatty-Vinci, responsible for building the West Midlands section of HS2, has worked with subcontractors to pour the floor slab and walkways and excavate emergency cross passages between the two tunnel bores.
This process first involved pouring the floor slab or invert, using a specialised concrete paver. Once the invert was complete, subcontractors working on behalf of BBV began pouring the concrete for the two emergency and service walkways on either side of what will be the track.
The walkways were constructed using a process known as slip forming, which is more commonly used to construct concrete highway barriers. The specially designed machine has formwork on one side, into which concrete was poured. There is an arm on the other side, attached to which is a set of wheels. The wheels pushed against the opposite side of the tunnel to resist the lateral forces of the wet concrete as the machine moved forward.
Before work began in the tunnel itself, the slip forming method was trialled on a 30 m long ‘half-pipe’ section constructed in a nearby worksite using leftover concrete tunnel ring segments. This was to ensure that the concrete mix used would be wet enough so that it could be poured, yet dry enough to keep its form, as the machine continued to move forward before the concrete had cured fully.
Tunnel equipment access
The walkway closest to the tunnel cross passages has been designed such that it can be used when there is a train failure or other emergency. It will allow passengers to disembark, and cross safely from one running tunnel to the other. A second walkway will be used for servicing of equipment and other asset management tasks; in particular, it will help staff maintain the jet fans deployed at both ends of the tunnel bores.
After the completion of the walkways, tunnelling teams started to excavate the three cross passages which will connect the tunnel bores. This involved first excavating the passages using small mining machines. Sprayed concrete was applied after each metre of the passage was excavated to stabilise the earth during the excavation process.
Once fully excavated a waterproof membrane was installed and a secondary layer of concrete was poured using a formwork system which produced the final shape of the cross passage.
Concrete doorways also had to be built, which will house steel fire protection doors. The doors are being supplied by Bolton-based Booth Industries, which won the contract to supply fire-resistant doors for the HS2 tunnels in 2020. Although construction of the cross passages is now complete, installation of the doors will not take place until 2027, when the railway track and systems contractors take over from BBV.
Ready for slab track
The installation of slab track through the tunnels will also include the pouring of a second floor slab. This will raise the level of the floor by approximately 200 mm. The second pour is necessary to bring the floor up to a specific level and tolerance required by the track and systems contractor.
Last November, HS2 Ltd announced the award of the contract for Track Systems, Lots 1, 2 and 3 to a joint venture of Ferrovial Construction and BAM Nuttall. This contract includes the design and installation of the track infrastructure for the section between Old Oak Common in northwest London and Curzon Street in the centre of Birmingham. Switches and crossings and precast slab track will be delivered by Voestalpine and Porr UK. Costain Group will provide lineside mechanical and electrical systems, which include equipment within the tunnels, shafts and cross-passages.
Although the invert, walkways and cross passages are now complete, there are still some outstanding civil works left to do, such as the construction of the porous portals at either end of the tunnel. The walls for these structures are being built using in situ poured concrete, while the porous roof sections will be built using precast panels which will be covered with a final layer of poured concrete. Material will then be backfilled against the outer walls, with the portals being blended into the landscape. The work on both of the portals is expected to be fully complete by mid-2026.
