HS2’s Chiltern Tunnel Structure Complete: UK Rail Milestone Reached

HS2’s Chiltern Tunnel structure is complete after 33 months of boring. This milestone shifts focus to systems installation, a critical step towards operational readiness for high-speed rail.

January 23, 2026 10:39 am

🛑 Key Takeaways:

Structural work on HS2’s 16 km Chiltern Tunnel, the project’s longest, is now complete.
The 33-month boring operation excavated 3 million tonnes of chalk using two 2,000-tonne tunnel boring machines (TBMs).
The project milestone shifts focus from heavy civil engineering to the next phase of mechanical, electrical, and track systems installation, a critical step toward operational readiness for Phase One.

BUCKINGHAMSHIRE, UK – The Align JV consortium has completed all structural civil engineering work on the 16 km Chiltern Tunnel, the longest twin-bore tunnel on the HS2 high-speed rail line. The milestone concludes a 33-month continuous boring operation that began in May 2021. The project now transitions to the internal fit-out phase, though specific timelines and costs for this next stage have not been released.

Category	Specification / Detail
Project Segment	HS2 Chiltern Tunnel
Length	16 km (10 miles) twin-bore
Contractor	Align JV (Bouygues Travaux Publics, Sir Robert McAlpine, Volker Fitzpatrick)
Max Design Speed	320 km/h (200 mph)
Excavation Volume	3 million tonnes of chalk
Boring Duration	33 months (TBMs launched May 2021, emerged early 2024)

Operational & Technical Details

Two 2,000-tonne TBMs were launched from a site near the M25 motorway to construct the separate northbound and southbound tunnels. The machines advanced at an average rate of 16 meters per day. During the drive, they installed a total of 112,000 concrete tunnel lining segments, each weighing 8 tonnes. These segments were produced at a purpose-built factory located near the tunnel’s southern portal to ensure a constant supply. With the TBMs having emerged near Great Missenden, work has continued on 40 cross-passages and internal walkways. The next phase involves installing all mechanical, electrical, and plumbing (MEP) systems, followed by the slab track and overhead catenary systems. Once operational, trains will transit the tunnel in approximately three minutes.

Market Impact Analysis

The completion of the Chiltern Tunnel’s structure is a significant physical and symbolic milestone for the HS2 project, which has faced intense scrutiny over its budget and schedule. This achievement provides tangible evidence of progress on Phase One’s most complex single civil engineering work. It de-risks the project’s construction profile, shifting the primary challenge from large-scale excavation to systems integration. For the Align JV partners, the delivery strengthens their credentials for securing future large-scale UK and international infrastructure contracts. The milestone also occurs as other elements of the HS2 project, such as spoil management strategies at Euston station, continue to be revised, underscoring the ongoing logistical complexities of the wider program.

FAQ: Quick Facts

What is the status of the Chiltern Tunnel?

The main civil engineering and structural work is complete. The project is now moving into the next phase, which includes the installation of mechanical, electrical, plumbing, and railway systems.

When will the Chiltern Tunnel be operational?

A specific date for the start of train services has not been announced. The completion of the civil structure is a major step, but the subsequent systems installation phase must be completed first.