Four massive tunneling machines operating beneath west London have successfully finished the 8.4-mile (13.5 km) Northolt tunnel excavation, stretching from West Ruislip to a new station at Old Oak Common, as part of the HS2 initiative.

According to HS2, these Tunnel Boring Machines (TBMs) have removed over four million tons of London Clay and laid approximately 100,000 concrete segments to construct the tunnel walls.

Engineers have now initiated the process of bringing the last of the four machines to the surface.

On Sunday, the first segment of TBM “Anne,” which had been underground for 16 months, was recovered from a shaft at Green Park Way in Greenford, located in west London.

Named after Lady Anne Byron, an advocate for educational reform and philanthropist who founded the Ealing Grove School, England’s first co-operative school in 1834, Anne weighs 1,700 tons and measures about 490 feet (150 meters) in length, which includes a cutter head with a diameter of 30 feet (9.11 meters).

Excavation for HS2’s second-longest tunnel commenced in October 2022.

The TBMs operated by a team of 17 workers, managed to remove clay and other materials at a rate of approximately 50 feet (15 meters) per day, functioning around the clock in shifts.

The machines, which were launched from the West Ruislip and Old Oak Common locations, converged in the center to complete the Northolt tunnel in June.

HS2 reported that the enormous amounts of clay extracted were transferred to “designated sustainable disposal sites and rail freight hubs” to reduce reliance on road transport, enabling the material to be repurposed or reintegrated into the local environment.

The excavated soil is moved on conveyor belts from the TBMs to rail terminals and then transported by train to logistics centers, including the Willesden Euro Terminal, for further distribution across the UK.

Efforts are still underway to construct the remaining 4.5 miles (7.2 km) of tunnels linking Old Oak Common to Euston station in central London.

In October 2023, the future of the high-speed rail project connecting Birmingham to London faced uncertainty when then-prime minister Rishi Sunak indicated that extending the project from Old Oak Common to Euston would depend on private investment.

Nevertheless, in her October 2024 Budget announcement, Chancellor Rachel Reeves secured the necessary funding to commence tunneling work towards Euston.

When the government first announced the high-speed line connecting the capital with regional cities in 2009, it was anticipated that the London to Birmingham segment would be completed by 2020 at a cost of £7 billion.

This phase of HS2 is now projected to be operational between 2029 and 2033.

According to the Department for Transport, the remaining cost of the project was estimated at £45 billion to £54 billion in 2019 prices, while HS2 management projected it to range from £49 billion to £57 billion.