The UK has a long tunnelling history. Brunel not only invented the shield method but also built the world's first underwater tunnel beneath the Thames, which is still in use today. London Underground's vast tunnel network also reflects the UK's tunnelling construction level.

At the 2024 World Tunnel Congress, UK peers shared the development trends of the UK tunnelling industry in recent years—'smarter, larger, more economical, greener'—through innovative technologies and methods applied in a series of major projects such as the Thames Tunnel, HS2 high-speed rail project, and Crossrail.

Smarter Tunnel Design

In recent years, the amount of information designers obtain from geological surveys has greatly increased. Data analysis has become an indispensable skill for geotechnical and tunnel engineers, playing a crucial role in optimizing processes, improving efficiency, and supporting tunnel design decisions.

For large projects like Crossrail, BIM was used to create a vast information resource network that not only meets construction needs but also provides information support for subway operations. All Crossrail designs were completed in 3D, using a central software model to manage all BIM 2D/3D models. CAD drawings were reviewed and integrated into the central software model. The model was operated by Crossrail, shared with its construction contractors, and eventually handed over to the railway's operations and maintenance party.

Larger Tunnel Diameters

From the historic London Underground to modern new infrastructure projects, the trend of increasing tunnel diameters is evident. Advances in shield technology have facilitated this process, making it possible to build ever-larger tunnels. However, increasing tunnel diameter also brings various challenges, including construction and logistics aspects, shallow cover launching, and soft ground.

In particular, building large-diameter tunnels usually requires large-diameter shields, and the installation and launching of shields require huge shafts. To meet this demand, a new solution called the 'caterpillar' shaft has attracted much attention and has been successfully applied.

Space optimization: The shaft provides ample space to accommodate the size and requirements of large boring machines. The design of such shafts focuses on structural integrity and efficiency, ensuring safe shield launching.

Reduced support: It reduces the need for additional structural support, thereby simplifying the construction process and reducing related costs.

Minimal ground settlement impact: It helps minimize the impact of ground settlement, thereby reducing potential impacts on the surrounding environment.

More Economical Temporary Works 

To reduce construction costs, the UK tunnelling industry has made technological innovations in lining.

When constructing cross passages in segmentally lined tunnels, crescent-shaped steel supports or cast iron tunnel segments are generally relied upon to control deformation within the tunnel structure. However, in the HS2 project, a type of 'special segment' was adopted. Such special segments abandon reliance on supports and instead inject high structural bearing capacity into the segments around the opening area. This structural reinforcement can effectively transfer shear forces to adjacent rings and is a key factor in controlling deformation.

Traditional sprayed concrete lining uses a double-layer structure, with a waterproof membrane sandwiched between the primary and secondary linings. In the UK, a more novel method has been increasingly applied, which involves spraying a waterproof layer onto the structure, which then hardens to achieve a hardness similar to concrete. This technology can significantly reduce the thickness of the secondary lining, optimizing the efficiency and sustainability of tunnel construction.

Greener Tunnel Construction

The HS2 project under construction is at the forefront of implementing a low-carbon emission strategy: low-carbon concrete is widely used in construction to reduce carbon emissions; fiber-reinforced tunnels are used in soft ground sections, shallow cover areas, and near cross passage openings; hydrogen fuel rotary drilling rigs have been successfully used to construct four 30m-deep piles, reducing CO2 emissions.

The process of trying new technologies is always difficult. The UK has invested enormous human and financial resources to achieve efficient, smart, and green construction, and has adopted a series of innovative technologies in numerous projects under construction. But there is no doubt that the UK tunnelling industry has accumulated valuable experience for the entire underground engineering industry to achieve energy conservation and emission reduction, which will serve as a reference for peers.