Tunnel shotcrete

Tunnel shotcrete is a central construction material and support system in underground construction. It stabilizes the excavation, forms the load-bearing shotcrete shell, and serves as the basis for further outfitting. From tunnel face support via the top heading to the bench, shotcrete enables a flexible, layered lining—often in combination with lattice girders, fiber reinforcement, or conventional reinforcement. In rehabilitation and selective deconstruction, shotcrete layers are removed in a controlled manner, thinned, or fully taken off. Depending on the task, this involves the use of concrete pulverizers as well as hydraulic splitters in combination with suitable compact hydraulic power units to work material-appropriately, with low vibration and high precision.

Definition: What is meant by tunnel shotcrete

Tunnel shotcrete is a concrete or mortar mix applied on site under pressure to the substrate, which achieves sufficient early strength in a short time through accelerators, water addition, and consolidation by the spray jet. It serves as temporary or permanent support and lining for tunnel structures, caverns, drifts/adits, and shafts. Depending on the process (wet- or dry-mix), composition (e.g., with steel or synthetic fibers), and layer thickness, tunnel shotcrete covers tasks from immediate excavation support to final lining.

Structure, composition, and properties

The composition is guided by subsoil requirements, installation method, and the demanded durability. Typical are a low water–cement ratio, fine aggregates, suitable plasticizers and stabilizers, as well as low-alkali or alkali-free accelerators. Depending on the loading, steel fibers or polypropylene fibers are added to improve tensile capacity, crack distribution, and fire performance (spalling reduction).

• Binders and admixtures: Cements with tailored hydration, accelerators to increase early strength, plasticizers for pumpability and a consistent spray pattern.
• Aggregates: Well-graded particle size distribution for nozzle efficiency, low rebound, and a dense matrix.
• Fibers: Steel fibers increase toughness and flexural tensile capacity; PP fibers reduce explosive spalling under fire exposure.
• Fresh and hardened concrete properties: Rapid setting, sufficient bond strength to the substrate, low porosity for improved durability.

Fiber reinforcement and reinforcement strategies

Fiber-reinforced shotcrete is often combined with locally supplemented conventional reinforcement or with lattice girders. The design considers flexural tensile capacity, rotation capacity, and redistribution of internal forces. During later selective removal, fiber-bearing layers can be controlled by scoring, gripping, and separating with concrete pulverizers; localized stress relief can be prepared with hydraulic splitters to reduce stresses.

Processes and application in tunnels

Installation is performed using the wet- or dry-mix process. The choice and parameterization influence rebound, dust generation, adhesion, and strength development.

Wet-mix process

In the wet-mix process, a premixed concrete is pumped and only accelerated with compressed air at the nozzle. Advantages include consistent quality, lower dust, and controllable rebound values. It is suitable for higher daily outputs, thicker layers, and areas with increased surface quality requirements.

Dry-mix process

In the dry-mix process, a dry mix is conveyed to the nozzle; water is added only there. The process offers high flexibility for smaller material quantities, is more sensitive to water dosing, and can show increased dust generation. In tight tunnel face areas and for repairs in existing structures, it is common due to quick readiness for use.

Accelerators and early strength

Accelerators control setting and strength gain. Low-alkali or alkali-free products reduce potential impacts on durability. Dosing depends on temperature, moisture, substrate, and the desired layer thickness. Excessively rapid stiffening can impair adhesion; overly slow setting delays advance.

Substrate preparation, application, and curing

A load-bearing, clean, and roughened surface is crucial. Loose material, breakout edges, and spray shadows must be removed; water-bearing areas are captured. Curing prevents drying out and promotes hydration. In installation phases with many starts and stops, concrete pulverizers facilitate clean trimming of edges and openings.

1. Inspect the substrate, remove loose components, pre-wet if necessary.
2. Adjust nozzle technology (water quantity, air pressure, nozzle stand-off distance, impact angle).
3. Adapt layer thickness, plan overlaps and joints, and consistently remove rebound.
4. Ensure proper curing (keep moist, protect from drafts and temperature spikes).

Interfaces with deconstruction: selective removal, openings, and adjustments

Tunnel projects often involve changed cross-sections, niches, drilling bays, cable ducts, or retrofits. When removing shotcrete, controlled methods are required to limit vibrations, protect adjacent structures, and avoid impacting ongoing operations. Here, hydraulic splitters prove effective for targeted crack initiation, and concrete pulverizers for form-fitting gripping and peeling of the layer. Hydraulic power packs provide the necessary energy supply for compact, hand-held tools in confined workspaces.

Gentle detachment of shotcrete

Before removal, cuts or relief boreholes are made to control stress redistribution. Subsequently, hydraulic splitters generate controlled splitting forces along these lines. In this way, defined areas can be released from the shotcrete shell—with low vibration and reduced dust generation compared to purely percussive methods.

Separating embedded items and reinforcement

Once reinforcement layers or embedded parts are exposed, concrete pulverizers remove residual concrete material-appropriately. For reinforcing steel and sections, steel shears or multi cutters are available; for complex embedded items, a combination of gripping, cutting, and splitting is used. Hydraulic power packs enable a mobile, modular way of working within the tunnel cross-section.

Application areas in tunnel construction and specialized deconstruction

Tunnel shotcrete covers a wide spectrum—from advance to rehabilitation. The following application areas have a close interplay with mechanical separation and splitting methods:

• Rock excavation and tunnel construction: Immediate shotcrete support after excavation; adjustments with concrete pulverizers and hydraulic splitters during cross-section corrections.
• Concrete demolition and special demolition: Selective removal of excessive shotcrete with concrete crushers for selective removal, removal of damaged zones, preparation for waterproofing systems.
• Strip-out and cutting: Exposing embedded items, cable troughs, ventilation shafts; separating reinforcement and steel profiles with shear tools.
• Natural stone extraction (adjacent applications): Experience from rock splitting transfers to the gentle removal of shotcreted surfaces.
• Special duty: In special situations, such as confined shafts or rehabilitation under operation, compact hydraulic solutions allow controlled work.

Quality assurance, testing, and key parameters

The quality of tunnel shotcrete is ensured through process control and testing. Essential aspects:

• Fresh concrete: Consistency, temperature, accelerator dosing, water–cement ratio.
• Hardened concrete: Compressive strength (early and final), bond tensile strength to the substrate, flexural tensile capacity with fiber concrete.
• Layer thickness and homogeneity: Measurement via pins, core samples, or scanning; rebound management.
• Durability: Porosity, chloride resistance, freeze–thaw de-icing salt resistance in line with exposure classes.

Occupational safety, health, and environment

Safety takes priority. Dust and mist exposure are minimized by suitable methods, extraction, and ventilation. Personal protective equipment, coordinated escape routes, and communication rules are mandatory. During deconstruction, hydraulic splitters and concrete pulverizers help reduce vibrations and noise. Handling chemical accelerators requires proper storage and dosing. Water and material cycles should be closed wherever possible to limit environmental impacts.

Typical damage and rehabilitation of tunnel shotcrete

Common damage patterns include debonding, voids, cracks due to restraint, efflorescence, and local spalling. The rehabilitation approach follows the principle “inspect – expose – repair”:

1. Diagnosis by visual inspection, sounding, and, if necessary, pull-off and core testing.
2. Selective removal of damaged zones: concrete pulverizers for controlled opening and lifting, hydraulic splitters for stress relief and delimitation.
3. Substrate preparation, cleaning, and, if necessary, corrosion protection of the reinforcement.
4. New application of shotcrete, aligned with layer thickness, accelerator, and fiber content.

Planning, logistics, and power supply for tools

In tunnels, short routes, low installation heights, and reliable power supply are crucial. Hydraulic power packs feed concrete pulverizers, hydraulic splitters, and shear solutions. Line routing, couplings, and pressure stages are matched to the tool requirements. An orderly material flow for aggregates, accelerators, and water prevents cycle interruptions. For deconstruction, clear interface planning between the spraying crew and the deconstruction team is recommended, including isolation and cleaning cycles.

Standards, guidelines, and project requirements

Execution is based on relevant standards and regulations for shotcrete construction as well as project- and country-specific requirements. These include requirements for materials, testing concepts, exposure classes, minimum strengths, pull-off values, and durability verification. For deconstruction, occupational and environmental regulations must also be observed. Concrete application is project-specific; legal requirements may vary by region and must be coordinated case by case with site management and the competent authorities.

Practical guidance for execution and deconstruction

• Spray pattern: Keep nozzle stand-off distance and impact angle constant, build up thin and uniform layers, and consistently remove rebound.
• Cut line planning: Mark planned openings in advance, set split lines, then cleanly expose with concrete pulverizers.
• Low-vibration methods: In sensitive areas, prefer hydraulic splitters and cutting methods.
• Quality assurance: Continuously document results (layer thickness, material quantities, test results) and adjust parameters promptly.