Tunnel Segments FAQ

Accordion Tunnel Segments are precast concrete pieces that form the tunnel lining, the tunnel’s structural wall.

Historical Note: While modern linings are predominantly precast concrete, materials such as cast iron (traditionally used for the London Underground) and steel liners have been used in the past. Concrete linings are favoured today for their smooth finish, durability, and inherent fire resistance.Content

They are manufactured under controlled factory conditions, ensuring high quality and precision.

The segments are assembled into rings as the tunnel boring machine (TBM) advances, providing immediate, robust, and watertight support to the excavated ground.

While they typically form circular rings, they can be cast to suit any tunnel shape.

A concrete mould (or mould) is the vessel used to shape wet concrete. For precast concrete, such as tunnel segments, these moulds are high-precision, heavy-duty steel forms designed to withstand the hydrostatic pressure of wet concrete and the rigours of mass production.

Concrete formwork is a broader term for the temporary or permanent frame that holds wet concrete in place until it sets (cures).

Permanent Formwork: This formwork stays in place and becomes an integral part of the final structure, often contributing to insulation or aesthetics. An example is Insulated Concrete Forms (ICFs), which use foam material (such as expanded polystyrene—EPS) into which concrete is poured, providing excellent thermal insulation.

Temporary Formwork: This reusable frame (often made from timber, steel, plywood, or plastic) is removed after the concrete is strong enough to support itself.

A concrete shaft is a vertical or steeply inclined excavation that is reinforced with a concrete lining. In tunnelling, shafts are essential for:

Support: Providing structural support, often serving as a starting or retrieval point for the TBM, and can be used in conjunction with tunnel segments to provide additional strength and durability for the entire infrastructure project.

Access: Providing entry and exit points for workers, materials, and machinery (like the Tunnel Boring Machine, or TBM).

Ventilation: Allowing fresh air into the underground network.

Tunnels are constructed using various methods, with the choice depending on factors such as ground conditions (soft clay vs. hard rock), water presence, tunnel length, diameter, depth, and the final use.

The three basic types of tunnel construction are:

1. Bored Tunnels: Constructed in situ (in place) without removing the ground above. This is typically done using a Tunnel Boring Machine (TBM), which excavates the ground and simultaneously installs the precast concrete tunnel segments to form the circular or horseshoe-shaped lining.
2. Cut-and-Cover Tunnels: Constructed in a shallow trench that is excavated, built, and then covered over. This is common for shallow subway lines or underpasses.
3. Immersed Tube Tunnels: Used to cross a body of water. Precast concrete or steel tunnel sections are fabricated on land, towed to the site, sunk into a dredged trench on the waterbed, and then joined together.

Manual shield tunnelling was an early method that used a protective shield to safeguard labourers while they excavated the tunnel face.

The system was crucial for soft, unstable ground (such as the clay beneath London) and was famously used in the construction of the original London Underground.Tunnels are constructed using various methods, with the choice depending on factors such as ground conditions (soft clay vs. hard rock), water presence, tunnel length, diameter, depth, and the final use.

The shield was inched forward incrementally, and workers would manually dig the material out ahead of it.

As the shield advanced, workers would progressively install prefabricated sections of the tunnel wall (often cast-iron or brick lining) to replace the space vacated by the shield.

Herrenknecht AG is a major German manufacturer of Tunnel Boring Machines (TBMs) of all sizes. They are globally recognised as a world market leader for heavy TBMs and tunnelling equipment, headquartered in Allmannsweier, Germany

The shift to factory-produced precast segments offers significant advantages over older methods:

Durability and Watertightness: The segments are designed for high strength and often feature integrated gaskets to ensure the tunnel structure is robust and waterproof for a service life often specified for 100 years or more.

Speed and Efficiency: Segments are mass-produced off-site under controlled conditions, drastically reducing on-site installation time and accelerating the entire construction timeline.

Quality and Precision: Factory conditions allow for superior quality control and millimetre-level precision, ensuring a perfect fit between segments.

Safety: The continuous ring installation by the TBM provides immediate ground support, significantly enhancing worker safety and ground stability.

Modern technology is continually advancing the field:

Digital Integration (BIM): Using Building Information Modelling (BIM) to plan, design, and simulate the entire tunnel construction process digitally, ensuring greater accuracy and coordination between all project phases.

Fibre-Reinforced Concrete: Replacing traditional steel reinforcement with steel or polymer fibres to improve durability, crack resistance, and speed up production.

Real-Time Monitoring (IoT): Embedding sensors within the segments and TBM to monitor stress, temperature, ground pressure, and alignment in real-time, allowing for immediate adjustments and enhanced safety.

Sustainable Materials: Developing concrete mixes that incorporate recycled aggregates or supplementary cementitious materials (SCMs) like fly ash or slag to reduce the carbon footprint of production.