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.

112K

Tunnel Segments

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.

Stills of the South Portal Chiltern Tunnels exterior and interior

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

Chryso®Optima 1325

Chryso®Optima 1325 is a high‑range water‑reducing superplasticiser delivering extended workability retention for fluid concretes.

Chryso®Omega 141

Chryso®Omega 141 is a new generation water reducing plasticiser, which allows optimisation of cement content in order to obtain a specific slump class. Chryso®Omega 141 has great flexibility: with a large dosage range it can be used in a great range of concretes, including low water cement ratio mixes, or to allow a jump in slump class without requiring additional cement. Chryso®Omega 141 can be used to produce high workability concretes without additional increase in mix water content.

Chryso®Xel Time 68

Chryso® Xel Time 68 is a chloride-free set accelerating admixture designed to speed up the setting and hardening of concrete. It acts as a catalyst during the early stages of cement hydration, particularly in low-temperature conditions where reaction rates are reduced. By accelerating the initial hydration process, the product reduces setting time and significantly increases early-age strength development. This makes it particularly effective in cold weather concreting, where it helps minimise delays and reduces the risk of frost-related damage to fresh concrete. Chryso® Xel Time 68 is suitable for use in a wide range of applications, including ready-mix, precast, and prestressed concrete. It is compatible with all types of cement, including mixes containing fly ash or ground granulated blastfurnace slag (GGBS), allowing flexibility across different mix designs.

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.

View of the tunnel segments from the gantry crane at the South Portal, October 2022

Partial walkway completed at the South Portal, November 2022

Last tunnel segment produced from precast factory. December 2023

Project participants

Client: HS2 Ltd

Contractor: Align JV (Bougues Travaux Publics, VolkerFitzpatrick, Sir Robert McAlpine)

Concrete producer: Tarmac Ltd