By rosebell
Following arrival of the final TBM at Green Park Way in Greenford, west London, digging of HS2’s Northolt Tunnel was completed on 26 June.
As the fourth of four TBMs used to bore the 13.5km twin-bore tunnel, TBM Anne before emerging into a vent shaft, installed the final concrete ring underground.
Transporting HS2’s high-speed trains between West Ruislip at the outer edge of the capital and the new Old Oak Common super-hub station in west London, the Northolt Tunnel is one of five deep twin-bore tunnels on the 225km route and the second longest behind the Chiltern Tunnel, which is 16.1km long.
Responsibility of excavating the Northolt Tunnel, which is 35m beneath ground at its deepest point, was up to HS2’s London tunnels contractor Skanska Costain Strabag JV (SCS JV). While the 8km western section from West Ruislip was built by two TBMs, two of the machunes built the 5.5km eastern section from Victoria Road in North Acton.
TBM Anne, named after Lady Anne Byron, has now finished the second bore of the 5.5km eastern section, completing the tunnel excavation.
Fig1
The drive of all four Herrenknecht machines finished at Green Park Way where they are to be lifted out from the underground shaft, whereas the date of finishing the journey pf third TBM, Emily, was earlier this month, and it is due to be lifted out of the ground later this summer. The first two machines to arrive, from West Ruislip, were lifted out earlier this year.
In a complex engineering operation, high water pressure in the ground at the site at Green Park Way has meant that the TBMs are finishing their drives in steel reception cans installed in two underground shafts. The reception can method allows the TBM to maintain pressure while sealant can be applied from the tunnel lining, preventing water ingress. It is then depressurised before it is opened and the TBM is lifted out.
Being commissioned in April 2024,TBM Anne has spent 14 months mining the tunnel. In order to completing the work, three teams of 48 workers have operated the machine around the clock and this 1,700-tonne, 9.11m-diameter TBM’s daily advance at peak is almost 38m.
The excavated London clay was taken to HS2’s London Logistics Hub where it was taken away by rail for beneficial reuse across the UK.
The total amount of dug London clay by the four TBMs is 4.16 million tonnes and they lined the tunnel with 14,300 concrete tunnel rings, made up of 94,233 concrete segments. Being transported to site by rail, the tunnel segments used by TBM Anne were made in Hartlepool by Strabag.
Considering that among the eastern and western section inside width of the tunnel varies, 8.10m in the east and 8.80m in the west, the tunnels for HS2 are wider for the sections of the route where the trains run faster given aerodynamic needs.
According to SCS JV managing director Richard Adam: “TBM Anne’s arrival was a major milestone for both HS2 and SCS.This is the fourth and final TBM that completes the 8.4-mile twin-bore Northolt Tunnel. TBM Anne’s journey has been one of efficiency and precision, delivered safely and on schedule. As we move forward, work continues at pace on the cross-passages, tunnel fitout and surface structures and we are preparing to launch our final two TBMs to Euston early next year.”
Additionally, SCS JV will continue to build the tunnel invert where the high- speed tracks will be laid and complete the five headhouses.
