Tunnel closure

The tunnel closure marks a pivotal step in tunnel construction: it closes the excavation so that it becomes structurally effective and enables the safe transition from the heading to the final lining. In practice, this moment is less a single blow than a sequence of precise work steps – from reprofiling and joint preparation to ring closure. Especially around sensitive structures, in densely built-up areas, or in operating facilities, low-vibration methods play a key role. Here, tools such as concrete pulverizers and hydraulic rock and concrete splitters are used, powered by high-performance Hydraulic Power Units and complemented by cutting technology for reinforcing steel and steel beams. Accurate tolerances, clean edges, and controlled load transfer are decisive for watertight, durable results.

Definition: What is meant by tunnel closure?

Tunnel closure is understood as the structurally effective completion of a tunnel cross-section or a segment ring. This can be achieved by installing the closing piece in the segment ring (key segment) in mechanized tunneling, by the arch closure in shotcrete construction (creating a closed ring action, usually with the invert), by the breakthrough as the connection of two headings, or by the structural connection to existing structures, cross passages, and caverns. The common denominator is always the transition from an open, temporarily secured excavation situation to a closed, load-bearing structure with defined load redistribution, stiffness, and tightness across all joints.

Significance in the construction sequence and practical execution

The tunnel closure affects safety, deformation, watertightness, and construction time. For load-bearing capacity, precise geometry is crucial – excessive over- or under-profiles, open joints, or inadequately prepared contact surfaces jeopardize the ring action. In execution, a structured approach has proven effective and reduces rework and risks:

1. Surveying and target/actual comparison of the profile, including tolerance definition
2. Reprofiling of rock and concrete contours
3. Cleaning and preparation of joints and contact surfaces
4. Installation of reinforcement, sealing elements, or segments
5. Production of the ring closure (setting the key segment, casting the invert)
6. Grouting, sealing, and quality control with documented parameters
7. Documentation and release for subsequent steps (inner lining, equipment)

For reprofiling and material-specific removal in confined conditions, concrete pulverizers and hydraulic wedge splitters are frequently used. These work tools reduce vibrations and dust generation compared to percussive tools, which supports the protection of sensitive neighboring structures and occupational safety in the tunnel. Selective methods also improve the surface quality of contact areas, which enhances bond and sealing performance.

Term in the construction process: variants of tunnel closure

In mechanized tunneling with segments, tunnel closure is created ring by ring. Installing the key segment completes the ring and activates its load-bearing action. In shotcrete construction (NATM), the arch closure is formed via the invert, which completes the temporary shotcrete supported by anchors and steel arches into a closed cross-section. In a breakthrough, the last meter of excavation closes the void between two drives; the subsequent lining and sealing create the permanent closure. The same principles apply on a smaller scale in cross passages, niches, and caverns, where geometry, joint tightness, and controlled load transfer remain decisive.

Ring action and load redistribution

With the tunnel closure, a closed ring action is established. Deformations are limited, and loads are redistributed between crown, bench, and invert. Prerequisites are accurate contact surfaces, sufficient bond, and a tight joint. In practice, this means: protrusions are removed in a controlled manner, joints are prepared sufficiently rough and clean, and the bond is secured by grouting or contact concrete. Irregularities in the contour are specifically reworked before the closure. Sealing profiles, gaskets, and grout connections must be continuous and free from inclusions to ensure watertightness under operating loads.

Tools and methods for reprofiling

Concrete and reinforced concrete: concrete pulverizer in use

Concrete pulverizers are suitable for the selective removal of shotcrete, cast-in-place concrete, and protruding edges in the arch or at the invert. They allow opening joints, exposing reinforcement, and removing excess concrete without excessive vibrations. In areas with high reinforcement density, hydraulic demolition shears and Multi Cutters support the cutting of reinforcing steel, connection plates, and embedded components. Clean cuts and defined fracture lines facilitate subsequent sealing and improve the quality of the bonding surfaces.

Rock and unreinforced concrete: hydraulic wedge splitter

Hydraulic wedge splitters work with rock splitting cylinders that are inserted into drilled holes. Using hydraulic pressure, rock or concrete is split in a controlled manner – with low vibration and dust generation. This approach is particularly suitable for remaining meters at breakthrough, for niches with a sensitive environment, or for reprofiling near sensitive structures. Hydraulic power packs provide the required energy in a compact setup. The method minimizes collateral damage and maintains the integrity of adjacent support elements.

Steel components: steel shear and cutting torch

For the removal of temporary steel arches, lattice girders, splices, or pipelines, steel shears and cutting torch are used. This includes shortening bracing members, cutting off anchors, and adapting embedded parts. Multi cutters assist wherever different materials must be separated in one pass. Spark control, fire watches, and protection of nearby sealing systems are part of the standard setup.

Work steps in detail: from raw profile to ring closure

1. Surveying and target profile

Precise setting-out with laser or scanning methods provides the basis. Markings for over- and under-profiles define where material must be selectively removed or added. Digital comparison to the design model supports traceable acceptance of the final geometry.

2. Selective material removal

Excess concrete, protruding rock edges, or segment overhangs are removed in a targeted manner. Concrete pulverizers enable layer-by-layer work at sensitive locations. Hydraulic wedge splitters open rock seams without shock waves, which is particularly advantageous with low overburden or in zones sensitive to structures. Edge protection and dust suppression maintain visibility and surface quality.

3. Joint and contact surface preparation

Contact surfaces are cleaned and prepared to create a durable bond. Where required, joints are widened or edges are chamfered so that injection or contact mortar has sufficient space and bonding surface. Defined roughness, dry and dust-free conditions, and continuous sealing elements are verified before closure.

4. Installation and closure

Depending on the method, reinforcement, sealing elements, or segments are installed and aligned. The closing element (key segment or invert) is set or concreted. This is followed by grouting and sealing. Injection pressures, volumes, and leak-tightness checks are documented for quality assurance.

5. Inspection and documentation

The completed tunnel closure is surveyed, visually inspected, and documented. Records on joints, grout volumes, and surface quality support quality assurance and traceability. If required, short-term monitoring of deformation and leakage complements the acceptance.

Typical decision criteria for method selection

• Boundary conditions: overburden, neighboring structures, permissible vibrations, groundwater
• Material: rock, shotcrete, cast-in-place concrete, segment construction, degree of reinforcement
• Accessibility: working space, visibility, ventilation, escape routes
• Occupational safety: dust, noise, sparks, hydraulic safety
• Construction logistics: power supply via hydraulic power pack, hose routing, disposal
• Execution risks: overbreak, uncontrolled cracking, impact on existing sealing systems
• Quality targets: tolerances, watertightness class, durability and maintenance requirements

Quality assurance, tightness, and durability

The durability of the tunnel closure depends on well-prepared contact surfaces, complete grouting, functional sealing systems, and documented execution. Clean, defined edges achieved by careful removal with concrete pulverizers and controlled splitting improve bonding. After closure, settlements and deformations should be monitored to fine-tune early, for example by supplementary injections. Typical checks include:

• Verification of geometry and continuity of the ring action
• Inspection of sealing elements and joint tightness under load
• Documentation of injection parameters and achieved fill

Work safety and environmental protection

Work on the tunnel closure often takes place with limited ventilation and in confined conditions. Organized hose management for hydraulic systems, effective dust suppression, adequate lighting, and safe positioning of work equipment are essential. Low spark generation and low vibration levels are advantages of hydraulic wedge splitters and concrete pulverizers in areas with sensitive infrastructure. Standards and regulations must be observed at all times; the implementation is project-specific and the responsibility of the executing parties. Additional measures such as energy isolation, hose burst protection, and hot-work permitting increase process safety.

Special situations in tunnel closure

Breakthrough

At breakthrough, the importance of low-vibration methods increases. The remaining partition can be opened in a controlled manner with hydraulic wedge splitters. Protruding edges are then trimmed flush with concrete pulverizers before sealing and outfitting follow. Careful sequencing minimizes uncontrolled stress release and supports tight connections.

Portal zones and existing structures

Near portals or in existing tunnels, strict emission limits often apply. Selective removal methods as well as precise cutting and splitting protect neighboring structures. Steel shears, hydraulic demolition shears, and multi cutters support the removal of temporary support frames, anchor heads, and embedded items. Noise and dust management plans, together with monitoring, ensure compliance.

Cross passages and niches

Freedom of movement is limited in small cross-sections. Handheld, hydraulically powered tools offer advantages here: low dead weight, high performance, and controlled material separation without large-area damage. Compact power packs and short hose runs improve handling and reduce setup times.

Equipment planning and construction logistics

The selection of equipment depends on material, geometry, and access. Decisive factors include sufficient power reserves of the hydraulic power pack, hose lengths, transport routes, power supply, and the disposal concept. Where reinforcing steel and structural steel are present, the package of concrete pulverizers and cutting technology is a sensible addition. In rock, controlled splitting in combination with drilling predominates. Availability of spare tools and quick-change systems reduces downtime during critical closure windows.

Sustainability and resource efficiency

Precise methods reduce overbreak and rework. This lowers material consumption for shotcrete and the inner lining and facilitates the segregated separation of concrete and steel. Lower vibrations and noise protect the environment and the workforce and improve working conditions on long drives. Efficient hydraulic systems and selected reuse of temporary steel elements contribute to resource conservation.

Example sequences by construction method

Shotcrete method (NATM)

1. Profile scanning and marking of protrusions
2. Selective removal with concrete pulverizers, exposing reinforcement
3. Controlled splitting of rock noses with hydraulic wedge splitters
4. Joint preparation, cleaning, roughening
5. Reinforcement, sealing, invert concrete – arch closure
6. Grouting, surface inspection, documentation

TBM with segments

1. Ring assembly and alignment in the advance cycle
2. Insertion of the key segment, torque and joint inspection
3. Backfilling grouting, leakage test
4. Follow-up works: shortening bolts, separating embedded parts with steel shear/multi cutters
5. Local reprofiling of connection areas with concrete pulverizers

Practical terms related to tunnel closure in context

Headings are often subdivided into crown, bench, and invert. With the invert, the ring closes; the crown relaxes, and deformations decrease. In mechanized tunneling, the key segment secures the segment ring, while at breakthrough the last partition is opened in a controlled manner. Reprofiling, joint preparation, and grouting tie these steps into a load-bearing, tight system – supported by selective removal and cutting techniques up to controlled splitting of rock and concrete. Consistent terminology and documented interfaces between excavation, temporary support, and closure help avoid gaps in responsibility and ensure durable performance.