Explainer: How geotechnical tunnelling is shaping infra landscape

Even as road-building and tunnelling strategies in the hills have come under the scanner, it’s the New Austrian Tunnelling Method (NATM) being used for India’s ambitious bullet train project that is grabbing the headlines as a sustainable option. The recent success on the BKC-Shillphata undersea tunnel — part of the massive Mega Airport High-Speed Rail (MAHSR) project — is an example of NATM in action.

A game-changer

In India’s infrastructure push, NATM represents a pivotal shift away from rigid, cost-heavy construction to adaptive, knowledge-driven engineering. NATM originated in Austria in the 1960s. The continuous geotechnical monitoring provides an early warning system against potential collapses, a critical feature for tunnels under waterways.

India’s tryst with this method began two decades ago, when Delhi Metro engineers embraced global practices to carve space beneath the capital’s crowded streets. In 2005, NATM shaped the intricate underground world of Chawri Bazaar station. Soon after, the Pir Panjal railway tunnel stood as a monumental railway endeavour, showcasing the method’s full potential.

The cost factor

NATM proves more economical than traditional tunnelling by harnessing the ground’s natural strength, reducing heavy supports and unnecessary excavation. Yet, cost is secondary to geology. This method thrives only where rock and soil conditions suit it. When aligned with favourable terrain and sufficient tunnel length, NATM delivers efficiency and savings unmatched by conventional approaches.

Bullet train project

The Mumbai-Ahmedabad high-speed train bullet train project includes a 21-km tunnel, with a challenging 7-km stretch going under the seabed of Thane Creek. This isn’t just about digging through solid rock. It’s about navigating a chaotic mix of terrains.

The tunnel’s path is a geological roller coaster, switching from hard basalt to soft, yielding soil, all the while contending with the immense water pressure of the creek.

While tunnel boring machines are fast, they are less versatile, especially in mixed geology. They can struggle with the constant transition from hard to soft strata. NATM, on the other hand, minimises the risk of sudden ground failure by excavating in small, controlled stages, gradually releasing stress and ensuring stability.

Delicate touch

Instead of seeing the ground as an enemy that needs to be held back with massive, pre-designed supports, NATM treats it as a partner in construction, where the earth leads and the engineers respond. In the “build-as-you-go” approach, a small section is carefully excavated and then the engineers listen to what the ground tells. They might apply a thin layer of sprayed concrete (shotcrete), use steel ribs or anchor it with rock bolts.

How it differs

Traditional tunnelling methods — “cut-and-cover” or “drill-and-blast” — usually assume a one-size-fits-all solution, placing heavy supports and linings immediately, regardless of what the ground actually requires. The design is rigid, pre-determined. NATM, in contrast, is an observational method. The design is an evolving process — continuously monitoring the ground’s behaviour, its stresses, deformations and general quality, and adjusting supports in real-time.

Testing the waters

Instead of carving out the entire tunnel at once, NATM begins by digging small drifts or segments, a cautious first step. Once a small segment is excavated, engineers apply a primary support system. This could be a quick spray of shotcrete, the placement of steel ribs or the installation of rock bolts to stabilise the immediate area.

Monitoring is at the heart of NATM’s philosophy. Instruments are strategically placed to constantly measure and record ground movement, pressure changes and deformation.

Based on real-time data, engineers can modify the support system as needed. Only after the ground and the initial support system have stabilised, often weeks or months later, is a final and thinner permanent lining placed, completing the tunnel’s structure.

Advantages and limitations

NATM’s strengths lie in its efficiency and adaptability. It’s more economical in varied geological conditions, uses less material and is much faster when the ground is cooperative. The Chenani–Nashri Tunnel (Dr Syama Prasad Mookerjee Tunnel) in Jammu and Kashmir is the longest road tunnel in India (9.28 km) built within the Himalayas.

In Himachal, Atal Tunnel (Rohtang tunnel) is the longest highway tunnel (9.02 km) at high altitude excavated using NATM.

This sophistication in tough terrains demands a team of highly skilled engineers heavily reliant on accurate monitoring instruments. It can be risky in very loose soil or waterlogged strata where the ground simply cannot provide any self-support.