Tunnels are vital structures used for roads, railways, water supply, and even underground metros. As cities grow and traffic increases, building tunnel networks has become more important than ever. But how are these underground passages actually constructed?
Two of the most commonly used methods are the New Austrian Tunnelling Method (NATM) and the Tunnel Boring Machine (TBM) technique. In this blog, we will explore both techniques in simple English, compare their advantages and disadvantages, and help you understand which method suits what kind of project.
What is a Tunnel?
A tunnel is an underground passage constructed through soil or rock, used for transport or utility purposes. Tunnels are typically used to:
- Avoid obstacles like mountains, rivers, or urban buildings.
- Create faster and shorter transport routes.
- Reduce environmental disturbance on the surface.
Why Tunnel Construction is Challenging
Tunnel construction is not just about digging underground. It involves:
- Dealing with different soil and rock conditions.
- Managing groundwater and water seepage.
- Ensuring safety for workers and future users.
- Minimising settlement and damage to nearby structures.
This is why advanced methods like NATM and TBM are needed for modern tunnel projects.
Overview of Common Tunnel Construction Techniques
There are various methods for constructing tunnels. Some of the widely used ones are:
- Cut and Cover Method – Used for shallow tunnels; the ground is dug, the tunnel is built, and then the soil is filled back.
- Drill and Blast – Traditional method used in hard rock by drilling holes and using explosives.
- Tunnel Boring Machine (TBM) – Modern, fully-mechanised tunnelling using a large machine.
- New Austrian Tunnelling Method (NATM) – Relies on surrounding ground strength and timely support installation.
This blog focuses on comparing the last two: NATM and TBM.
What is NATM (New Austrian Tunnelling Method)?
NATM is a modern and flexible method of tunnel construction that uses the strength of the surrounding rock or soil to support the tunnel. It involves:
- Minimal excavation at a time.
- Immediate installation of primary support (shotcrete, rock bolts, steel ribs).
- Continuous monitoring and adjustments based on ground behaviour.
Key Features of NATM:
- Highly adaptable to ground conditions.
- Uses observational methods (adjust the design based on actual site behaviour).
- Cost-effective in certain conditions.
What is TBM (Tunnel Boring Machine)?
A TBM is a giant circular machine that excavates the ground and installs a tunnel lining as it progresses. It consists of:
- A rotating cutter head that breaks the soil or rock.
- A conveyor system to remove muck.
- Hydraulic jacks to push the machine forward.
- Segments that form the tunnel lining (usually concrete rings).
Key Features of TBM:
- Fast and continuous tunnel construction.
- Ideal for long, straight tunnels.
- Reduces surface disruption.
- Provides a smooth, finished tunnel lining.
NATM vs TBM – A Clear Comparison
| Feature | NATM | TBM |
|---|---|---|
| Ground Dependency | Highly dependent on ground strength | Less dependent; suits mixed ground |
| Flexibility | Very flexible | Limited flexibility |
| Speed | Moderate speed | High speed |
| Initial Cost | Lower setup cost | High capital cost |
| Manpower | Requires skilled labour | More automated |
| Tunnel Shape | Custom shapes possible | Mostly circular |
| Monitoring | Real-time ground monitoring needed | Less dependency on monitoring |
Which Method is Better?
There is no one-size-fits-all answer. It depends on various factors:
- Tunnel Length: TBM is better for long tunnels; NATM is ideal for short to medium lengths.
- Soil Condition: NATM suits soft soil and variable conditions; TBM is preferred in consistent rock or soil.
- Urban Areas: TBM reduces noise and disturbance, making it ideal for city projects.
- Cost: NATM has lower machine costs but may be slower; TBM is faster but costly.
IS Codes Relevant to Tunnel Construction
In India, several IS codes govern tunnel design and safety:
- IS 5878 (Part I to IV) – Guidelines for tunnelling construction in various ground conditions.
- IS 4761 – Safety code for tunnelling work.
- IS 4091 – Design of underground reinforced concrete tunnels.
These ensure the structure is safe and meets national standards.
Advantages and Disadvantages
NATM Advantages:
- Economical in variable ground
- Flexible and adaptable
- Good for non-circular tunnels
NATM Disadvantages:
- Slower progress
- Requires experienced engineers
- Risky in poor ground without timely support
TBM Advantages:
- Fast and continuous progress
- Less surface disruption
- Safer working conditions
TBM Disadvantages:
- High initial investment
- Difficult to change direction
- Expensive maintenance
Real-Life Examples
- NATM used in the Delhi Metro Phase I tunnels due to mixed soil conditions.
- TBM used in the Mumbai Coastal Road Project and Chennai Metro, offering speed and reduced surface disruption.
Conclusion
Understanding tunnel construction techniques is vital for planning and executing modern infrastructure projects. While NATM offers flexibility and cost-effectiveness in varied soil conditions, TBM delivers speed, safety, and precision in long and consistent tunnels.
FAQs about Tunnel Construction
Q1. Is TBM always faster than NATM?
Yes, TBM is generally faster, but NATM may be more practical in unstable ground.
Q2. Which method is cheaper?
NATM usually has a lower initial cost, but the total cost depends on project conditions.
Q3. Can both methods be used in the same project?
Yes, hybrid methods are used where conditions change along the tunnel route.