TBM Caroline - HS2 Northolt Tunnel West
West London, UK (Old Oak Common to Ruislip) · 2022-2025 (Started October 2022, completed May 2025) · £87.7bn - £102.7bn (HS2 project total, 2025 prices)
The judgement call
Account-gated at launchTBM Caroline was the second of four TBMs to complete a drive for the Northolt Tunnel. Named after Caroline Herschel, a pioneering astronomer. Completed its 8km journey under London.
Key engineering challenges
Tunnelling under densely populated urban areas. - Managing ground movements and settlement. - Excavating 34 cross passages.
Project facts
- Client / owner
- HS2 Ltd.
- Lead contractor
- SCS JV (Skanska Costain STRABAG Joint Venture)
- Lead designers
- —
- Project type
- new build
- Scale
- Tunnel length: 13.5 km (8.4 miles) twin-bore tunnel; Drive length: 8km (5 miles); TBM diameter: 9.48 meters
- Disciplines
- civil; structural; mechanical; electrical; systems; tunnelling; geotechnical
- Standards & frameworks
- UK railway standards, CDM Regulations (implied).
Sources: GEplus.co.uk (https://www.geplus.co.uk/news/hs2-first-tbm-ends-northolt-tunnel-bore-in-reception-can-23-12-2024/) - Tunnelling Journal (https://tunnellingjournal.com/hs2-lifts-out-second-northolt-tbm/) - Railway-News.com (https://railway-news.com/in-pictures-hs2-tbm-caroline-emerges-at-green-park-way/) - HS2.org.uk (https://www.hs2.org.uk/work-items/update-atlas-road-logistics-tunnel-works/)