Selecting the right tunnelling method is one of the most critical decisions in underground construction. Every project has unique challenges, including geological conditions, tunnel size, depth, groundwater conditions, environmental restrictions, construction timelines, and budget limitations. Among the commonly used tunnelling approaches are Sprayed Concrete Lining (SCL), Tunnel Boring Machine (TBM), Drill and Blast, Cut and Cover, and New Austrian Tunnelling Method (NATM). Choosing the correct method requires a detailed understanding of ground behaviour, project requirements, and long-term performance expectations.
Understanding the Main Tunnelling Methods
Sprayed Concrete Lining (SCL) Tunnelling
Sprayed Concrete Lining (SCL), often associated with NATM principles, is a flexible tunnelling technique where excavation is carried out in stages and the tunnel is immediately supported using sprayed concrete, reinforcement, and ground support systems. It allows engineers to adapt construction methods according to actual ground conditions encountered during excavation.
Advantages and Limitations of SCL Tunnelling
SCL provides excellent flexibility for complex tunnel shapes and variable geological conditions. It is commonly used for metro stations, caverns, junctions, and urban tunnelling projects. However, compared with TBM excavation, SCL generally has slower excavation rates and requires skilled labour, detailed monitoring, and careful ground control.
Tunnel Boring Machine (TBM) Tunnelling
Tunnel Boring Machines (TBMs) are advanced mechanised systems designed for continuous tunnel excavation. A rotating cutter head breaks the ground while the machine removes excavated material and provides tunnel support. TBMs are widely used for long-distance tunnels, including metro systems, railway tunnels, highways, water transfer tunnels, and utility projects.
Types of Tunnel Boring Machines
Different TBM technologies are selected depending on geological conditions. Earth Pressure Balance (EPB) TBMs are commonly used in soft ground conditions such as clay, sand, and mixed soils. Slurry Shield TBMs are suitable for areas with high groundwater pressure, while Hard Rock TBMs are used for strong rock formations and mountain tunnelling projects.
Advantages and Limitations of TBM
TBMs provide high excavation speed, improved safety, reduced surface disruption, and consistent tunnel quality. However, they require significant initial investment, detailed geological investigations, and are less adaptable when tunnel alignment or ground conditions change frequently.
Conventional Tunnelling Methods
Drill and Blast Method
Drill and Blast is a traditional tunnelling technique mainly used in hard rock environments. The process involves drilling blast holes, controlled blasting, removing excavated material, and installing support systems. This method offers flexibility in tunnel size and shape but may create vibration, noise, and slower excavation compared with mechanised methods.
Cut and Cover Tunnelling
Cut and Cover is commonly used for shallow tunnels where excavation from the surface is possible. It is frequently applied for underground stations, road tunnels, and utility corridors. While it is a simple and cost-effective method for shallow structures, it can cause surface disruption and is not suitable for deep underground tunnels.
Selecting the Right Tunnelling Method
The selection between SCL, TBM, and other tunnelling methods depends on several factors, including geological conditions, tunnel length, construction environment, budget, and project requirements. A detailed engineering assessment helps determine the most suitable approach to achieve safety, efficiency, and long-term tunnel performance.