[VIDEO] 8.4 mile HS2 tunnel breaks through beneath London

Northolt Tunnel, the second-longest on the HS2 route, has been completed using four tunnel-boring machines (TBMs)

The Northolt Tunnel will take HS2’s new high-speed trains between West Ruislip and the new Old Oak Common super-hub station in west London.

The tunnel, which is 35 metres below ground, has been bored by HS2’s contractor Skanska Costain STRABAG JV (SCS JV). Two of the TBMs built the 5-mile western section from West Ruislip, and two built the 3.4-mile eastern section from Victoria Road in North Acton.

The final TBM, named Anne, installed the last concrete ring underground after 14 months of work.

Excavated 4,160,000 tonnes of clay

TBM Anne advanced at roughly 38 metres per day, inserting 7 tonne-concrete segments and cutting through earth and clay. The excavated clay is then taken away to be reused by the UK’s rail network.

In total, the four machines have excavated 4,160,000 tonnes of London clay and lined the tunnel with 14,300 concrete tunnel rings, made up of 94,233 concrete segments.

Due to high water pressure in the ground, the tunnel had to remain sealed during the dig. This was achieved using a reception can method, allowing TBM Anne to keep pressure and apply sealant from the tunnel lining to stop any water from leaking through. The tunnel can then be safely depressurised when the machine has finished its dig.

Next steps

Now that the tunnel excavation is complete, the team will soon move to the next stage of construction, including the flat tunnel base where high-speed tracks will be laid.

Alan Morris, construction delivery director, HS2 Ltd, said: “Completing the excavation of this 8.4 mile-long tunnel on HS2 is a real achievement and one the team should be immensely proud of.

“We’re building HS2 for the future, to increase capacity on our rail network and improve journeys for millions of rail users.

“The construction of HS2 is already bringing benefits, with £20bn economic benefit already being delivered at either end of the line.”

HS2 Northolt Tunnel is one of five deep twin-bore tunnels. The longest tunnel is the Chiltern, registering a whole 10 miles in length.

A ‘fundamental reset’ for HS2

HS2 is currently going under a fundamental reset, but recently, transport secretary Heidi Alexander spoke in the House of Commons earlier this month, saying that there is “no reasonable way” that HS2 will be delivered on the then-scheduled due to a “litany of failures.”

Issues with the project include:

• The infamous £100m bat tunnel
• Signing contracts when advised not to
• Drawing up expensive plans to redesign London’s Euston Station- which were then scrapped

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