Excavation of Longest HS2 Tunnel Breaks Through With Machine Digging

The Cross Country Route of the High Speed 2 rail project in the United Kingdom recently achieved a major milestone with the completion of the excavation of the longest tunnel for the route.

The excavation work was carried out by Florence, a 1,000-tonne tunnel-boring machine manufactured by Herrenknecht in Germany. Florence broke through to the north portal, north of Chalfont St Peter in Buckinghamshire, after digging through a challenging mix of chalk and flint beneath the protected landscape area.

A Challenging Process with Innovative Technology

The excavation of HS2’s longest tunnel was an immense challenge that required innovative technology and engineering expertise. As reported, the tunnel boring machines (TBMs) named Florence and Cecilia were custom-designed for the Chiltern’s geology.

Equipped with state-of-the-art technology, the TBMs were able to excavate the tunnel efficiently at an average of 16 meters per day while lining it with concrete segments.

Excavation of the Chiltern Tunnel

Two tunnel boring machines (TBMs), named Florence and Cecilia, were used to excavate the chalk under the Chiltern Hills. Launched in May 2021, Florence broke through at the northern end of the tunnel in 2024, marking a major milestone for the HS2 project.

The TBMs excavated over 3 million cubic meters of chalk, which was transported back through the tunnel and processed to be used for landscaping and habitat restoration. Traveling at 16 meters per day, each machine lined the tunnel with 56,000 precast concrete segments.

Benefits and Support

Once completed, the full HS2 high-speed rail network will reduce journey times between London and Birmingham by half, free up capacity on the existing West Coast Main Line for more local services, and drive economic growth in cities served by HS2 stations.

Research indicates the West Midlands will receive a £10 billion economic boost over the next decade from the arrival of high-speed rail. The HS2 project has faced both support and opposition.

Environmental Considerations

The 3 million cubic meters of chalk and other material removed during tunneling are being used to create 127 hectares of new grassland and wildlife habitat.

The habitat will include chalk grassland, an endangered ecosystem found mainly in southern England and the Isle of Wight. Water quality and flow monitoring found no significant impacts during construction.

The Operation

The TBMs were operated by a crew of around 17 people working in shifts to ensure 24-hour operation. Over 100 additional workers on the surface provided logistical and operational support. In total, 450 people worked on tunnel construction and support over three years.

A dedicated team produced 112,000 precision-engineered tunnel wall segments, completing production just before Christmas. Another team processed the spoil, or excavated material, from the tunnels.

Innovative Technologies and Advancements

The TBMs incorporate innovative technologies introduced in the UK to improve performance and safety. This includes ‘semi-continuous boring,’ which allows the TBMs to install the tunnel lining segments without stopping.

The TBMs were designed collaboratively between the main works contractor, Align, and the TBM manufacturer Herrenknecht to meet the unique challenges of excavating twin 32-mile tunnels. Over 450 people worked for three years operating the TBMs and in support roles to complete the tunnel excavation.

Environmental Considerations During Tunneling

To preserve local wildlife and habitats, Align reused the 3 million cubic meters of chalk removed to create 127 hectares of chalk grassland, an endangered habitat, at the South Portal site. The chalk was pumped as a slurry back through the tunnel, processed to remove flint, and dewatered before being used in landscaping.

The new grassland will provide space for orchids, invertebrates, and other plants and animals native to the Chilterns.

Preservation of The Water Quality

Groundwater, surface water, and water quality were closely monitored before and during construction to avoid pollution. The tunnel passes under two rivers, the M25 motorway, and local rail lines, at its deepest point of 80 meters.

Monitoring has shown no significant impacts on water resources thus far. Careful monitoring, habitat restoration, and technological innovations to improve efficiency can achieve a balance between development and conservation.

What Completing the Tunnel Means for the HS2 Timeline

With the breakthrough of ‘Florence,’ HS2’s first tunneling machine, the excavation of the longest tunnel in the HS2 rail project is now complete. This milestone demonstrates significant progress in building the high-speed railway that will connect London and Birmingham.

The completion of the tunnel excavation keeps the project on schedule to begin high-speed rail service between London and Birmingham by 2026.

Potential Benefits of the High-Speed Rail Link

The HS2 rail project will provide significant benefits once completed. It will drastically reduce travel times between London and Birmingham. According to HS2 Ltd, the high-speed rail link will cut journey times from London to Birmingham by half, to around 49 minutes.

This will make it faster to travel between Britain’s two largest cities by rail rather than air. The time savings will also benefit business travelers and commuters.

Freed Up Capacity on The Main Line

HS2 will free up capacity on the existing West Coast Main Line for more local and freight services. By diverting high-speed intercity trains to the new HS2 line, space will open up on the conventional network for additional regional and suburban trains to run, especially during peak periods.

This could reduce overcrowding and provide more options for travelers. The released capacity may also allow for more freight trains, taking some heavy goods vehicles off the roads.

Hope For Economic Growth

HS2 is expected to drive economic growth around station sites and boost local job opportunities. Research by KPMG indicates that HS2 could generate over £10 billion in growth for the West Midlands economy over the next decade.

The high-speed rail project will support hundreds of jobs during construction and lead to more employment and business opportunities in the area once operational.

Light At The End of The Tunnel

The excavation of the 32-mile Florence tunnel for the High-Speed Two (HS2) rail project marks a major milestone in what will be Europe’s largest infrastructure project this decade. With the breakthrough of the tunneling machine, the excavation phase is now complete.

The Florence tunnel will play a critical role in the HS2 network, helping connect London to the Midlands and the North with faster and higher capacity train lines.

A Great Milestone For HS2

With the completion of this tunnel through the Chilterns, the ambitious HS2 project has overcome one of its biggest engineering challenges. While debate continues around the environmental impact and economic benefits of HS2, construction is powering ahead.

The scale of engineering required for tunnels like this one demonstrates the complexity and ambition of building Europe’s largest infrastructure project.