Sprayed concrete lining (commonly abbreviated to SCL) is one of the most widely used and technically sophisticated applications of sprayed concrete in modern civil engineering. From Crossrail to HS2, from metro extensions to highway tunnels, SCL has become the preferred tunnelling method for a huge proportion of underground infrastructure in the UK and across the world.
But what exactly is sprayed concrete lining, how does it work, and when is it the right choice? This guide answers those questions in practical terms.
What Is Sprayed Concrete Lining?
Sprayed concrete lining is an engineered tunnel support system formed by the sequential application of reinforced sprayed concrete to the excavated profile of a tunnel, shaft, or underground cavern. Rather than inserting pre-cast concrete segments, SCL builds the structural lining in situ applying concrete directly to the exposed ground face immediately after excavation.
The result is a continuous, monolithic lining that conforms perfectly to the excavated profile, accommodates complex geometry, and can be adapted rapidly as ground conditions change.
SCL is also known as the New Austrian Tunnelling Method (NATM), though this term more precisely refers to the broader observational approach to ground behaviour that SCL is typically used within.
Where Is SCL Used?
SCL is used across a wide range of underground construction projects, including:
- Metro and underground railway tunnels particularly running tunnels, station caverns, cross-passages, and adits
- Road tunnels where variable ground conditions or complex alignments make a TBM impractical
- Utility and service tunnels sewers, cable routes, and water mains in urban areas
- Mining both hard rock and soft ground underground mining operations
- Shafts and headings access shafts, ventilation shafts, and construction headings
Its ability to handle irregular cross-sections and complex junctions gives SCL a decisive advantage over segmental systems.
The SCL Construction Process: Step by Step
Stage 1: Excavation
Ground is removed in controlled increments the excavation ’round’ using a roadheader, drill and blast, or excavator depending on geology. The size of each round is determined by the ground conditions and design; soft, unstable ground requires shorter rounds to limit the exposed, unsupported face.
Stage 2: Initial Sprayed Concrete Application
Immediately after excavation, an initial layer of sprayed concrete the primary lining is applied to the exposed face. This provides rapid ground support, preventing relaxation and movement of the surrounding material. Accelerating admixtures are used to achieve fast early strength gain, often reaching 1–2 MPa within minutes.
Stage 3: Reinforcement Installation
Depending on the design, reinforcement is installed within the primary lining. This typically takes one of three forms:
- Steel fibre reinforcement: fibres mixed into the concrete batch, providing distributed crack control and ductility
- Mesh reinforcement: welded steel mesh fixed to the face before or between concrete applications
- Lattice girders: pre-formed steel frames that provide a rigid structural element within the lining
Stage 4: Secondary Lining Application
Once the primary lining has stabilised ground movement and achieved sufficient strength, a secondary lining is applied. This is typically thicker, more heavily reinforced, and designed to carry long-term loads including ground pressure, groundwater, and service loads.
Stage 5: Monitoring and Quality Control
SCL is always accompanied by a rigorous monitoring regime. Convergence monitoring measures any movement of the tunnel profile; profilometry surveys check lining thickness against design; core testing verifies compressive strength. This observational approach allows the engineer to adapt excavation sequences and lining design in real time.
Sprayed Concrete Lining Equipment
The quality of an SCL lining is inseparable from the quality of the sprayed concrete lining equipment used to construct it. Modern SCL operations typically deploy:
- Robotic spraying arms: computer-controlled positioning eliminates operator variability and reduces rebound
- High-output concrete pumps: continuous, uninterrupted supply prevents cold joints within the lining
- Accelerator dosing systems: precise admixture addition at the nozzle controls setting time and early strength development
- Digital monitoring systems: profilometry, real-time strength assessment, and thickness verification
Investment in modern, well-maintained equipment directly affects the structural quality and long-term durability of the lining.
Advantages of SCL Over Segmental Linings
For the right project, SCL offers several important advantages over pre-cast segmental lining:
- Geometric flexibility: SCL handles curves, varying cross-sections, junctions, and enlarged headings without bespoke segment casting
- Speed of response: immediate support after excavation; no waiting for segments to be delivered or erected
- Cost efficiency: particularly on shorter drives, complex alignments, or projects requiring multiple tunnel shapes
- Adaptability: lining design can be modified in response to changing ground conditions
Is SCL Right for Your Project?
SCL is a highly effective solution for many underground construction challenges but it requires the right ground conditions, an experienced contractor, and a properly resourced technical team. If you’re planning a tunnelling or underground project and want to understand whether sprayed concrete lining is the right approach, our team can provide technical input from early feasibility through to detailed design support.
Contact us today for our sprayed concrete contracting services
Sprayed Concrete Lining FAQs
What is the difference between sprayed concrete lining and shotcrete?
Shotcrete, also called sprayed concrete, is the material and the process of applying it at high velocity onto a surface. Sprayed concrete lining is the engineered tunnelling method that uses it, applying reinforced sprayed concrete in sequence to the excavated profile of a tunnel, shaft or cavern so the ground is supported as excavation advances. Put simply, shotcrete is what goes onto the face, and SCL is how the tunnel is designed and built around it.
How quickly does sprayed concrete gain strength after it is applied?
The primary lining develops early strength of 1 to 2 MPa within minutes of application. That is what allows it to support the exposed face immediately after excavation, rather than waiting for segments to be delivered and erected. Strength continues to develop over the following days, and the rate depends on the mix design, the accelerator dose and the ground conditions.
What is the difference between the primary and secondary lining?
The primary lining is the initial layer sprayed onto the exposed face to stabilise the ground immediately after excavation. The secondary lining is applied later, is typically thicker and more heavily reinforced, and is designed to carry the long-term loads on the finished structure. Reinforcement is installed between the two stages, and monitoring runs throughout to confirm the ground is behaving as the design predicted.
Is sprayed concrete lining suitable for permanent structural works?
Yes. The secondary lining is designed to carry long-term loads, so sprayed concrete lining is used for permanent tunnel and shaft structures as well as for temporary support. Because it handles curves, varying cross sections, junctions and enlarged headings without bespoke segment casting, it suits geometry that would be difficult or expensive to form with precast segments.
