HS2 M6 Viaduct Construction: Multi-Stage Sliding Technique Minimizes Motorway Disruption

HS2’s construction of twin viaducts over the M6 near Birmingham Airport has seen engineers employing a multi-stage sliding technique to minimise traffic disruption.

The project demonstrates how modern infrastructure engineering addresses the challenge of building over active motorways while maintaining operational efficiency.

The M6 South viaducts present a complex engineering challenge that requires carrying high-speed rail traffic over one of Britain’s busiest motorways.

Traditional construction methods would require extended road closures, creating substantial economic and logistical impacts. Engineers from BBV—a joint venture between Balfour Beatty and VINCI—developed an innovative approach that addresses these constraints through off-site assembly and precision sliding techniques.

The twin 320-meter-long viaducts will carry a total of four tracks, with the East Deck accommodating southbound trains toward London and the West Deck handling northbound services. This configuration requires precise engineering to ensure structural integrity while managing the substantial loads imposed by high-speed rail operations.

The construction methodology centres on a multi-stage sliding process that pushes completed viaduct sections across the motorway infrastructure. This technique represents a significant departure from conventional bridge construction, where structures are typically built in place.

The sliding operation employs several critical engineering components:

• Strand jack winch systems that generate controlled forward motion at 6-8 meters per hour
• Non-stick friction pads using materials similar to household frying pan coatings to reduce resistance
• Precision guidance systems to maintain structural alignment during movement
• Load distribution mechanisms to manage increasing deck weights throughout the process

The first section of the East Deck, measuring 119 meters and weighing 1,300 tonnes, will initiate the sliding sequence. As subsequent sections are added, the total weight will increase to approximately 3,000 tonnes—equivalent to 214 double-decker buses. This progressive weight increase requires careful engineering calculations to ensure the sliding mechanism can handle the evolving load conditions.

Engineers face an additional challenge as the railway operates on a slight gradient, with the finish point positioned 4.2 meters higher than the starting position. The uphill sliding approach provides enhanced control during the construction process, allowing engineers to manage the substantial masses involved while maintaining precise positioning accuracy.

https://x.com/HS2ltd/status/1933485236983124117

The viaduct design incorporates advanced materials and structural engineering principles to ensure long-term durability and reduced maintenance requirements.

Both viaduct spans utilize hollow double-box structures manufactured from weathering steel. This material develops a protective oxidization layer that provides:

• Natural corrosion resistance, eliminating regular repainting requirements
• A protective rust-like surface finish
• Reduced lifecycle maintenance costs compared to conventional steel structures
• Enhanced structural longevity in exposed weather conditions

The structural support system comprises four pairs of concrete piers, with the tallest reaching a height of 9.9 meters. These foundations must accommodate the substantial loads from high-speed rail operations while providing stable support during the sliding construction process.

Noise mitigation measures include a 4.5-meter high parapet on the side facing Chelmsley Wood, demonstrating the integration of environmental considerations into the structural design.

The construction process follows a carefully orchestrated timeline designed to minimize operational disruption:

Phase 1: East Deck Construction (June 2025)

The initial sliding operation is scheduled to occur during the weekend of June 7-8, 2025, with the first 119-meter section being moved over the motorway slip road. This represents the first practical application of the multi-stage technique for this project.

Phase 2: Motorway Crossing (2025)

Subsequent weekend closures will advance the East Deck across the main motorway carriageway and the M6-M42 Link Road. Each phase adds complexity as the structure’s weight increases and the sliding distance extends.

Phase 3: West Deck Assembly (2026)

The second viaduct will undergo assembly and sliding operations in 2026, necessitating four separate sliding operations due to considerations regarding pier spacing. This phased approach allows engineers to apply lessons learned from the East Deck construction.

The multi-stage sliding technique significantly reduces traffic disruption compared to conventional construction methods. The approach limits road closures to Single weekend closures for slip road operations and two weekend closures for combined motorway and slip road crossings.

This strategy addresses the economic implications of major motorway closures while maintaining construction efficiency. The approach builds on the experience gained from previous bridge sliding operations, including those on the nearby M42/M6 link viaducts.

The M6 South viaduct project shows how modern engineering addresses complex infrastructure challenges through innovative construction methodologies. The multi-stage sliding technique represents a significant advancement in bridge construction, particularly for projects requiring construction over active transportation infrastructure.

If the sliding operations proceed as planned, the technique could establish new benchmarks for similar infrastructure projects. The approach demonstrates how careful engineering planning can strike a balance between construction efficiency and operational continuity, addressing the growing challenge of maintaining transportation networks while expanding infrastructure capacity.

The integration of weathering steel construction, precision sliding mechanisms, and phased implementation strategies creates a comprehensive approach to complex bridge construction. As infrastructure projects increasingly face constraints from existing transportation networks, such innovative methodologies become essential for effective project delivery.

The successful completion of the M6 South viaducts will contribute to HS2’s broader objective of creating additional rail capacity while maintaining existing transportation networks. The project’s engineering solutions address fundamental challenges in modern infrastructure development, where construction must occur within increasingly constrained operational environments.