The tunnel wall forms the visible and usable interior surface of a tunnel. It is more than a surface: as a load-bearing and protective structural element, it provides load transfer, waterproofing, fire protection and ensures durability. During construction, operation, repair and selective deconstruction, precise, controlled interventions on the tunnel wall are paramount. Depending on the task, non-blasting concrete demolition methods and tools such as concrete demolition shears or a hydraulic splitter together with a hydraulic power pack are typically used in the application areas of concrete demolition and special demolition, gutting works and concrete cutting as well as rock demolition and tunnel construction.
Definition: What is meant by tunnel wall
A tunnel wall is the inner lining of a tunnel structure. It can be executed as a temporary primary lining (usually shotcrete) or as a permanent inner lining: as a cast-in-place concrete vault, as a half-shell with vault supports, or as a segmented lining behind a tunnel boring machine. The tunnel wall transfers rock and earth pressure, indirectly takes traffic loads via the structure gauge clearance, protects against water ingress, and fulfills requirements for fire protection, noise and vibration mitigation, and for a uniform airflow. Depending on the construction method, the wall build-up consists of structural concrete, waterproofing (membrane or watertight concrete design), drainage, waterstops and, where applicable, additional fire protection layers. In soft rock, waterproofing is more frequently in the foreground; in rock, load-bearing function, shotcrete support and rock stabilization system dominate.
Structure, materials and construction methods of the tunnel wall
The build-up of the tunnel wall follows the geotechnical situation, the excavation method and the intended service life. Key methods are shotcrete primary linings, cast-in-place concrete inner linings and segmented segmental linings. A two-shell build-up with a waterproofing membrane between primary and inner lining is often selected. With high groundwater or aggressive media, reliable joint sealing, crack-width-controlled reinforcement and drainage routing are of central importance.
Materials and typical layers
In practice, various material combinations have proven themselves that are matched to the function of the tunnel wall and impose different requirements on processing, construction logistics and later interventions.
Shotcrete primary lining
- Function: Immediate support, protection against rockfall, load redistribution in the rock mass.
- Material: Shotcrete (often with steel fibers), lattice girder beam, anchors, rock bolts.
- Particularities: Irregular geometry; further concrete finishing or inner linings are necessary for later smoothing.
Cast-in-place concrete inner lining
- Function: Durable, smooth interior surface with defined load-bearing and watertightness functions.
- Build-up: Formwork traveler concreting, pre-installed waterproofing membrane with spacers, waterstops, reinforcement.
- Follow-up work: Recesses, niches, cable ducts and drainage are produced or later introduced in a controlled manner.
Segmental lining
- Function: Segmented interior lining in shield and TBM tunneling.
- Build-up: Concrete segments with sealing profiles and bolts; backfill grout where applicable.
- Particularities: The joint and sealing concept is a central element of serviceability.
Actions and design criteria
The tunnel wall must be designed for combined actions. Essential actions are rock and earth pressure, hydrostatic pressure, thermal actions, shrinkage and creep, vibrations and, in the event of fire, rapidly rising temperatures. From this follow requirements for crack-width control, watertightness, reinforcement layout and joint sealing. In practice, design is carried out for the limit states of load-bearing capacity and serviceability; in operation, crack and deformation behavior is just as important as durability. For subsequent breakthroughs or cross-passages, the redistribution of internal forces in the tunnel wall must be explicitly considered.
Construction, quality assurance and documentation
Construction follows the selected excavation and lining method. For cast-in-place concrete inner linings, formwork travelers, concreting stages, joint and membrane systems determine the process. For segments, segment production, logistics, assembly and backfilling are in the foreground. Decisive are controlled concrete curing, documented joint sealing, and the verifiability of watertightness. Accompanying measurements (convergence, settlements, water levels) provide the basis for the component assessment.
Occupational safety and environmental protection
- Dust and noise reduction through dust extraction, spray mist and noise reduction measures during equipment operation.
- Minimization of vibrations—especially near sensitive structures, existing facilities and operating tunnels.
- Media and fire protection: appropriate spark prevention, readiness of firefighting media, orderly hose and cable routing.
- Legal requirements are project-specific; the recognized rules of technology and applicable regulations are authoritative.
Deconstruction, openings and repair on the tunnel wall
Interventions on the tunnel wall are frequent over a tunnel’s life cycle: niches and emergency exits, cross-passages, cable and equipment rooms, repair of damaged areas or strengthening measures. In confined conditions, controllable methods are required that work precisely, generate low vibration levels and protect the surroundings. Depending on wall thickness, reinforcement ratio, construction method (cast-in-place concrete or segments) and inflows, concrete demolition shears, hydraulic splitters, combination shears, multi cutters, steel shears, and the associated hydraulic power packs from Darda GmbH are used in the application areas of concrete demolition and special demolition, gutting works and concrete cutting, rock excavation and tunnel construction, or also in special assignments.
Procedure for selective concrete demolition
- Record the as-built: drawings, cast-in-place concrete/segments, wall thicknesses, reinforcement layout, waterproofing, utility lines and cable ducts.
- Locate and expose: rebar and utility locating, marking of cutting edges, protective measures for waterproofing.
- Pre-cut and delimit: saw cutting, core drilling or milling to define intended break lines.
- Removal: use concrete demolition shears for controlled breaking and biting; for massive areas, supplement with hydraulic splitters to release components with low internal stress.
- Cut reinforcement: cut bars, sections and lattice girder beam with steel shears, multi cutters or combination shears.
- Removal and logistics: piece-by-piece removal, fall protection, organized haulage logistics in the tunnel.
- Finishing: edge treatment, reprofiling, corrosion protection of exposed reinforcement, produce watertight connections.
Targeted splitting instead of blasting
Hydraulic splitters enable the opening of thick concrete shells and the controlled lowering of partial areas with very low vibration levels. In tunnels this is particularly advantageous when blasting is not permitted or operating systems in the immediate vicinity must be protected. The splitting action is introduced via core drilling; load transfer and the sequence of splitting operations must be defined in the work planning.
Concrete demolition shear in tunnels
Concrete demolition shears are suitable for precise removal of shotcrete, biting off protruding concrete edges and exposing reinforcement without unnecessarily jeopardizing intact waterproofing. When creating niches or removing damaged zones, the work can be carried out step by step and with low vibration levels.
Separating steel content and embedded items
In the tunnel interior fit-out, steel appears in the form of reinforcement, lattice girder beam, embedded parts and rail profiles. Steel shears, combination shears and multi cutters are suitable for their deconstruction. In special assignments—such as dismantling underground tanks in service rooms—tank-cutting tools are an option, provided the fire protection and occupational safety framework permits.
Power supply and deployment organization
Hydraulic power packs supply the tools used with the required energy. In tunnels, space requirements, exhaust management, ventilation and safe hose routing must be considered. Work planning additionally covers material logistics, water management (collection, discharge), environmental protection measures and escape routes.
Typical damage patterns on tunnel walls
- Cracks due to temperature, shrinkage or restraint; critical to watertightness in water-loaded structures.
- Spalling and honeycomb structure in concrete, often at edges, impact zones or with poor compaction.
- Water ingress, sinter and efflorescence formations (calcium carbonate) at joints or defects.
- Reinforcement corrosion due to concrete carbonation or chloride contamination, recognizable by rust staining and spalling.
- Fire damage (spalling, changes in the matrix), detachment of fire protection layers.
- Alkali–silica reaction with network cracking and matrix damage.
Repair methods and strengthening
The choice of method depends on the cause of damage, construction method and operating conditions. As a rule: damaged areas must be cut back to sound substrate, durably rehabilitated, and executed so that watertightness and load-bearing capacity are restored.
Overview of methods
- Crack injection with suitable injection resin to restore bond and watertightness.
- Reprofiling and concrete replacement with matched mortars; surface preparation often with concrete demolition shears followed by abrasive blasting.
- Strengthening by additional shotcrete layers, supplementary anchors or fiber-reinforced layers; renew joint sealing.
- Segment repair (segments): joint sealing, bolt replacement, local concrete patching, injection into backfill voids.
- Openings and cross-passages: combined approach of pre-cutting, hydraulic splitters and reinforcement cutting, followed by permanent lining and sealing.
Operation, inspection and monitoring
Regular walk-throughs and monitoring programs (convergence, moisture, leaks) ensure the proper condition of the tunnel wall. Drainage systems must be maintained; even minor leaks are repaired early to avoid consequential damage. Documentation of interventions—including small deconstruction measures with concrete demolition shears or splitting tools—is part of systematic asset management.
Special situations in rock and soft rock
In rock construction, the shotcrete primary lining together with anchors provides the load-bearing function; the inner lining ensures uniformity, watertightness and protection. In soft rock, waterproofing and drainage dominate. For subsequent works on the tunnel wall—for example under overbuilds or when creating cross-passages—controlled splitting can reduce the impact on the rock mass. Techniques originating from natural stone extraction (targeted splitting) have proven to be a gentle option and are adapted to the subsurface conditions in rock excavation and tunnel construction.
Planning interventions on the tunnel wall
A structured approach increases safety and quality. The following points have proven effective:
- Damage and structural analysis, definition of assets to be protected (waterproofing, load-bearing function, operation).
- Define work and escape routes, ventilation concept, water management and emission protection.
- Selection of methods and tools: concrete demolition shears for selective removal, hydraulic splitters for low-vibration separations, shears for steel components, hydraulic power packs sized appropriately.
- Trial area and stepwise approach to verify assumptions.
- Monitoring, documentation and acceptance in accordance with the recognized rules of technology.