A sprayed wall is an applied shotcrete lining that secures and lines terrain, excavation pits, or cavities on a short-term to permanent basis. It is created by applying shotcrete to rock, soil, or existing structural elements and combines load-bearing action, surface protection, and the leveling of irregularities. For deconstruction, adjustments, and openings in sprayed walls, practical day-to-day work often uses concrete pulverizers as well as hydraulic rock and concrete splitters, especially in sensitive environments such as tunnel heading, existing structures, and confined inner-city settings.
Definition: What is meant by a sprayed wall
A sprayed wall is a structural safeguard in the form of a thin-walled shotcrete shell. It is applied to the prepared substrate in layers using the wet-mix or dry-mix spraying method. Typical tasks include primary support in tunnel construction, temporary bracing of excavation pits and slopes, protection against weathering and spalling, and the leveling of uneven rock and concrete surfaces. Depending on the project, the sprayed wall serves as a temporary support until the final lining or as a permanent load-bearing lining. Reinforcement (steel mesh, lattice girder beams), steel or plastic fibres, and anchors complement the load-bearing action. The required bond tensile strength and layer thickness result from geology, loading, and the planned service life.
Structure, materials, and layer thicknesses
A sprayed wall consists of shotcrete with coordinated aggregate, cement, water, and, if necessary, admixtures (e.g., accelerators) and additions (e.g., steel fibres). The layer is applied to a load-bearing, cleaned, roughened substrate. Typical nominal thicknesses—depending on the stabilization objective—range from about 50 to 300 mm, and more for multilayer application. The layer is often coupled with lattice girder beams, mesh reinforcement, and anchors to safely transfer bending moments, shear forces, and membrane stresses into the ground. So-called rebound (rebounding material) must be minimized and properly removed. Substrate preparation includes removing loose components, knocking off overhangs, and exposing joints. In practice, hydraulic tools can be used for compact material removal; for adjustments, touch-ups, or local thickenings, precise, low-vibration methods such as concrete pulverizers or—if underlying rock must be separated—hydraulic wedge splitters are considered.
Manufacturing methods: wet-mix and dry-mix spraying
In the dry-mix process, dry components are conveyed to the nozzle and mixed with water only at the nozzle. The wet-mix process conveys a premixed suspension. Selection criteria include component geometry, layer thickness, accessibility, desired surface quality, and environmental requirements. Important parameters are spray nozzle stand-off distance, angle of incidence, reinforcement position, temperature, and moisture. Smooth, even nozzle guidance reduces voids and ensures high density. Controlled layer build-up is crucial, particularly overhead and on steep faces. The accelerator is dosed to steer setting behavior, early strength, and rebound without impairing adhesion and durability.
Fields of application: tunnel construction, excavation pits, slope protection, and existing buildings
Sprayed walls stabilize the tunnel face and crown in rock breakout and tunnel construction, secure excavation pits in inner-city areas, protect slopes from erosion, and serve in concrete demolition and special demolition as temporary protection or separation layers. In existing structures, they help level irregular substrates or take up interim loads. For subsequent adjustments—such as service routes, shafts, or drainage—openings are often produced selectively. For this, concrete pulverizers with finely controllable force are suitable to remove layers without large-scale vibration. If the rock behind a sprayed wall needs to be opened with minimal induced stress, hydraulic wedge splitters are a proven tool, especially where vibration or noise limits must be observed (special demolition).
Deconstruction, openings, and finishing on sprayed walls
During construction or later refurbishment, recesses, niches, and adjustments are often required. The aim is a controlled, material-conserving intervention that maintains bond and load-bearing action in adjacent areas. In practice, the following steps have proven effective:
- Survey: detect thickness, reinforcement position, anchor locations, utilities, and embedded items.
- Edge definition: mark cuts and separation joints, define vibration and dust control.
- Selective removal: concrete pulverizers for controlled release of the shotcrete shell; combine with core drilling in more massive areas.
- Low-stress intervention: where the substrate requires it, use hydraulic wedge splitters to open rock or thick layers.
- Severing reinforcement and embedded items: depending on the material, optionally use hydraulic demolition shear or Steel Shears for mesh, lattice girder beams, and sections.
- Reprofiling and surface preparation for re-spraying (reprofiling, edge chamfers, cleaning).
Hydraulically operated tools are powered by hydraulic power units (power packs). Their benefits lie in controllability, optionally quieter operation, and reduced vibration—an advantage in gutting works and concrete cutting as well as for work in sensitive neighborhoods or during ongoing operations.
Load-bearing behavior, load transfer, and design principles
Sprayed walls act as thin shells bonded to the substrate. In combination with anchors and lattice girder beams, they take support, bending, and membrane forces. In tunnel construction, the sprayed wall serves as primary support; load transfer occurs via hoop stresses and systematic interlock with the tunnel face. For slope stabilization, the shell couples surface forces to anchor points. Design follows recognized engineering practice, geotechnical reports, and project-specific requirements. Specifications for minimum thicknesses, reinforcement ratios, fibre contents, and pull-off bond values must be determined for the specific project. Legal and normative requirements must always be reviewed in general and project-specifically; binding case-specific statements are not possible here.
Quality assurance and monitoring
For a durable, high-performance sprayed wall, planning, execution, and control must be closely interlinked. Key measures include:
- Substrate verification (bearing capacity, roughness, moisture), protection against contamination and icing.
- Trial panels to optimize mix design, nozzle handling, and accelerator dosing.
- Control of layer thicknesses, voids, and rebound; proper disposal of rebound.
- Pull-off bond and density tests, visual inspection for cracks, honeycombing, and spalling.
- Documentation of work parameters, temperatures, material batches, and curing.
Occupational safety, emissions control, and sustainability
Spraying generates dust, noise, and rebound; deconstruction adds vibration and potential particle flight. Protective measures include respiratory and eye protection, shielding, dust extraction, and wetting of the work zone, as well as careful logistics for material separation. Low-vibration methods—such as opening with concrete pulverizers or low-stress splitting with hydraulic wedge splitters—reduce emissions and protect adjacent structures. Rebound, drill cuttings, and overspray must be collected separately; recycling of concrete and steel fractions improves the footprint. Water from cleaning must be collected and—where required—neutralized.
Interfaces with drilling and anchor works
Sprayed walls often work in tandem with temporary or permanent anchors. Drilling should be low in dust and vibration; embedment depths, drilling angles, and anchor plates are project-specific. When creating openings, anchor rows must be located in advance so as not to impair load paths. For exposing anchor bodies and anchor plates, controlled methods with high edge quality are advantageous; concrete pulverizers permit fine corrections in edge zones without unnecessarily disturbing the bond of adjacent areas.
Typical damage, causes, and repair
Common observations include debonding (voids), cracking due to early shrinkage or restraint, spalling from inadequate substrate preparation, as well as efflorescence and damp patches. Causes often lie in unsuitable mix design, incorrect nozzle distance, high rebound rates, or insufficient curing. Repair involves the targeted removal of damaged areas, restoration of the bonding surface, and re-spraying with a suitable mix. For selective removal, precise, controllable tools are effective; depending on boundary conditions, concrete pulverizers can expose the damaged area cleanly. If the substrate must also be opened, hydraulic wedge splitters provide a controlled option.
Planning practice: sequence in the project workflow
- Investigation and support concept (define geology, groundwater, loads, service life).
- Substrate preparation (cleaning, removing loose material, edge chamfers, if necessary preliminary work with hydraulic tools).
- Installation of anchors, lattice girder beams, and mesh; define joints and intended crack locations.
- Spraying in layers with documented quality control and curing.
- Installations, openings, and penetrations as planned; in case of changes, selective removal with concrete pulverizers, and, if required, low-stress opening with hydraulic wedge splitters.
- Final inspection, documentation, protective and surface treatment as required.
Practice-oriented tips for deconstruction and adjustments
For clean opening edges, a combination of pilot boreholes, defined separation cuts, and subsequent removal is recommended. Reinforcement should be exposed before severing; suitable cutting tools (e.g., steel shear in hydraulic operation) help avoid sparks. In confined areas, compact handheld devices with hydraulic power units are advantageous. The sequence of work steps—relieve first, then cut, then remove—reduces the risk of uncontrolled cracking. Finally, prepare the bonding surface for re-spraying: remove loose edges, roughen the surface, clean, and pre-wet if necessary.