Segment tunnels are tunnel tubes assembled from precast concrete elements and produced by mechanized excavation with tunnel boring machines. The individual segments, often referred to as tubbings, form ring-shaped sections of the inner tunnel lining. This construction approach is precise, fast, and proven in urban settings, beneath water bodies, and in complex geological formations. In the planning, construction, maintenance, and deconstruction of a segment tunnel, selective demolition and adaptation works play a key role—for example when creating openings, niches, or cross-passages. Depending on the task and boundary conditions, tools such as concrete pulverizers or hydraulic wedge splitters from Darda GmbH are considered in the application areas of concrete demolition and deconstruction, gutting works and cutting, as well as rock demolition and tunnel construction.
Definition: What is meant by a segment tunnel
A segment tunnel is a tunnel with a ring-shaped inner lining made of precast, reinforced concrete segments. These segments are placed, bolted, and sealed immediately after each advance under the protection of a tunnel boring machine (TBM) using a segment erector. The resulting tunnel lining provides watertightness, structural stability, and load transfer to the surrounding ground and groundwater. In practice, terms such as tubbing tunnel, segmented inner lining, or segmental lining are common.
Construction method and components of a segment tunnel
The typology of the segment tunnel is based on precisely fitting precast concrete elements that are assembled into closed rings. Each component fulfills a defined function to ensure that assembly, sealing, and load transfer are safe and reproducible.
Tubbings and ring assembly
A ring consists of several identical segments and a wedge or closing piece. Shape, number, and thickness depend on geometry, load assumptions, and the shield used. The joint faces are machined, often with tongue-and-groove profiles and inserted seals. Dimensional accuracy and surface quality are critical for ring geometry and subsequent watertightness.
Sealing and joints
Elastic sealing profiles are inserted in the segment joints to create a watertight joint under axial and radial pressure. Shear forces and moments are transmitted via shear dowels, locating pins, or interlocking teeth. The annular space between the outside of the tubbing and the ground is filled after assembly with annulus grouting (cementitious or hybrid) to minimize settlements and establish load transfer.
Connectors and reinforcement
Segments are joined by bolted connections, anchors, or coupling plates to form a force-locked assembly. Reinforcement is designed for crack width control, shear and flexural tension capacity, and durability. Local embedded components (e.g., inserts, lifting anchors) serve for assembly and later retrofitting.
Construction process: excavation, assembly, and annulus grouting
In mechanized tunneling, the tunnel boring machine supports the tunnel cross-section with the shield skin and a support medium (EPB, slurry shield). After each advance stroke, one ring is assembled:
- Segment supply via logistics train or conveyor belts
- Positioning with the erector and temporary fixing
- Joining, bolting, and aligning the ring
- Annulus grouting in several phases until fully filled
- Quality checks of tolerances, watertightness, and bolt pre-tension
Precise guidance, monitoring of excavation forces, and rigorous documentation of assembly ensure dimensional accuracy and durability. When correcting individual segments or removing damaged parts, selective, low-vibration work is required—typically a field for concrete pulverizers or hydraulic wedge splitters from Darda GmbH.
Fields of application and boundary conditions
Segment tunnels are used for metro lines, long-distance gas and district heating corridors, cable tunnels, wastewater and drinking water pipelines, road tunnels, and special structures. The construction method is suitable for small cover depths and high groundwater pressures and enables high advance rates with controlled settlement.
Geotechnics and hydrogeology
Soil type, pore water pressure, and in-situ stresses influence shield selection, tubbing dimensions, and the sealing concept. In swelling, abrasive, or heterogeneous strata, tailored seal profiles, higher concrete strengths, and low-wear connectors are common. Annulus grouting is matched to pumping distance, setting time, and final strength.
Materials, durability, and fire protection
Tubbings are typically made of high-strength, dense concrete with low water absorption and appropriate reinforcement. Durability concepts consider exposure to sulfates, chlorides, and soft waters. For fire protection, fiber-reinforced concretes are used to mitigate spalling; additional linings may be required depending on use. Joint and connection details should be planned to allow inspection and replacement of individual components.
Planning, design, and quality assurance
Design is carried out considering construction stages (excavation, assembly, transport) and final stages (earth and water pressure, traffic loads, temperature). Essential aspects include:
- Segment geometry, joint detailing, and ring rotation
- Verification of watertightness, crack widths, and reserve capacity
- A concept for manufacturing tolerances and factory tests
- An installation manual and documented bolt preload torques
- A maintenance concept with clear inspection intervals
Quality assurance ranges from formwork checks and concrete production to assembly control in the tunnel. For future adaptations, openings, cable penetrations, and niches should already be planned.
Maintenance, modifications, and deconstruction in the segment tunnel
Over the life cycle, inspections, rehabilitations, and conversions are common: installation of cable trays, ventilation niches, emergency exits, or service crossings. These activities require controlled interventions in the tubbing lining.
Selective concrete removal
For low-vibration, controlled removal, concrete pulverizers are particularly suitable for breaking out segment areas with reinforcement. They allow precise force application with low dust and noise emissions—a benefit in confined tubes and during ongoing operation.
Splitting instead of blasting
Hydraulic wedge splitters and rock wedge splitters are used where boreholes can be drilled and a defined crack path without vibrations is required. In the vicinity of sensitive installations or adjacent infrastructure, this method is often a suitable alternative to percussive techniques.
Cutting steel components
For reinforcement, segment connectors, embedded parts, or secondary steel, steel shears, combination shears, or multi cutters are suitable. Hydraulically driven tools work with low sparking and compact dimensions, supporting occupational safety in confined spaces.
Interfaces to products and application areas of Darda GmbH
The application areas concrete demolition and special demolition, gutting works and cutting, and rock breakout and tunnel construction are directly related to segment tunnel practice. Typical tasks include:
- Creating openings for cross-passages and cable and pipeline penetrations
- Reworking defects, spalls, and damaged joint zones
- Deconstruction of temporary construction states, repair of sealing elements
- Dismantling embedded parts, brackets, and auxiliary structures
- Targeted widening of niches where access is limited
In all these scenarios, depending on component thickness, reinforcement, and environmental conditions, concrete pulverizers, hydraulic wedge splitters, steel shears, and the required hydraulic power packs from Darda GmbH are considered. Critical factors are careful work preparation, dust and noise reduction measures, and a structurally verified approach.
Best practice: Procedure for creating an opening in the segment ring
- Survey and records: determine measurements, structure documentation, and the position of reinforcement and connectors.
- Structural assessment: define the ring’s load-bearing behavior and temporary supports; consider load redistribution.
- Work organization: plan ventilation, lighting, shoring, and water and dust management.
- Marking and preparation: mark the contour, drill boreholes, and consider seal locations.
- Selective removal: release concrete in a controlled manner with concrete pulverizers or split it in a targeted way using hydraulic wedge splitters; protect joints and seal profiles.
- Steel separation: cut reinforcement and connectors with steel shears or combination shears; minimize sparking.
- Edge finishing: trim the opening edge, treat cut faces, and apply corrosion protection to exposed steel.
- Installation: fit frames, penetrations, or sealing systems; check for watertightness and dimensional fit.
Alternative lining systems compared
Compared with in-situ concrete inner linings or shotcrete lining, segment tunnels offer high prefabrication quality, short assembly times, and early watertightness. Cast-in-place concrete can offer advantages for varying cross-sections and massive embedded parts, while shotcrete excels in complex geometries and for temporary support. The choice of lining system depends on geology, use, groundwater conditions, schedule targets, and logistics.
Occupational safety and environmental protection
In the tunnel, protection against dust, noise, water ingress, and limited escape options is paramount. For demolition and adaptation works, low-vibration, hydraulic methods have proven effective. Careful planning of ventilation, material flow, emergency communication, and disposal, as well as the coordinated use of suitable Darda GmbH tools, support safe operations and the protection of the existing tubbing lining.