Chiltern Tunnel (C1) HS2
Delivering performance at scale
The Chiltern Tunnel (C1) on HS2 is one of the UK’s most complex infrastructure projects, combining high-volume on-site precast manufacturing with technically demanding in-situ construction.
Through early-stage collaboration and advanced admixture technologies, Chryso enabled efficient production, innovative construction methods and consistent concrete performance across underground works beneath the Chiltern Hills.
The Project
The Chiltern Tunnel is a 16.04km twin-bore tunnel on HS2’s Central 1 route, integrating large-scale on-site precast segment manufacture with complex in-situ works beneath the Chiltern Hills Area of Outstanding Natural Beauty.
The project combined large-scale precast manufacturing with complex in-situ construction, including 38 cross passages and five deep ventilation and service shafts. More than 112,000 precast tunnel lining segments, installed in around 16,000 rings, were produced on site using a fully automated manufacturing process operating continuously for almost three years.
From the earliest stages, Chryso worked in close collaboration with Tarmac, Align JV (Bouygues Travaux Publics, Sir Robert McAlpine and VolkerFitzpatrick) and HS2 Ltd, providing bespoke admixture solutions that enabled innovation, productivity and consistent concrete performance across every element of the works.
The Challenge
The Chiltern Tunnel placed exceptional demands on concrete performance:
- Early demoulding within 6–8 hours to support high-output precast production
- Full compliance with HS2 durability and fire resistance requirements
An innovative lidless mould system used in segment manufacture introduced further complexity. The concrete needed precise rheology control, remaining workable during placement, then rapidly stiffening to hold its shape immediately after compaction.
Beyond precast production, the project required robust and reliable mixes for:
- Deep diaphragm walls up to 78m with large single pours
- Slipformed shafts, demanding predictable setting and strength development
- Secondary linings for cross passages, delivering long open times and high post-cracking performance
Products used
- Superplasticizer: Chryso®Optima 1325
- New generation water reducing plasticiser: Chryso®Omega 141
- Accelerators: Chryso®Xel Time 68
The Solution
Through early contractor involvement, Chryso supported extensive laboratory testing, full-scale trials and performance validation ahead of on-site production.
For tunnel lining segments, Chryso developed a bespoke accelerating superplasticiser system based on Chryso Optima and Chryso Omega technologies. Tailored specifically for the lidless mould process, the solution delivered:
- Controlled workability during batching and placement
- Rapid early-age stiffening for immediate shape retention
- Excellent surface finish and repeatable early strength performance
The system was enhanced with Chryso Xel early strength technologies, enabling fast demoulding and handling without compromising long-term durability or structural performance.
Fully integrated into an automated production environment, Chryso admixtures supported output of up to 200 segments per day, using approximately 700m³ of concrete, while reducing waste through re-use and digital tracking of every segment within the project BIM model.
Across in-situ construction, Chryso delivered tailored admixture solutions for each application:
- Extended consistency retention and controlled heat evolution for deep diaphragm walls
- Stable rheology and predictable setting for slipform operations, even in high summer temperatures
Chryso admixture technologies played a key role in enabling new construction methods at scale, improving productivity, reducing material use and delivering the consistent, durable concrete performance demanded by HS2.
The Chiltern Tunnel demonstrates how Chryso helps transform ambitious engineering concepts into buildable, efficient and resilient infrastructure solutions.
Gallery
Project participants
Client: HS2 Ltd
Contractor: Align JV (Bougues Travaux Publics, VolkerFitzpatrick, Sir Robert McAlpine)
Concrete producer: Tarmac Ltd
Other references
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