NATM

The New Austrian Tunnelling Method is less a fixed procedure than a philosophy: use the strength of the ground itself as the main support. The tunnel is excavated in stages, and a flexible primary lining of sprayed concrete, rock bolts and, where needed, steel ribs or lattice girders is placed quickly, so the ground deforms a little, mobilises its own strength and forms a load-carrying ring around the opening, while not being allowed to loosen.

The method relies on observation. Convergence pins on the tunnel wall, extensometers into the ground and settlement points on the surface are measured often. If deformation slows and stabilises, the support is adequate; if it keeps accelerating, more support, a closed invert or a smaller excavation face is needed at once. The design is therefore updated during construction according to what the ground does.

Excavation sequences range from full face in good rock to top heading and bench, or side drifts in weak ground. Closing the ring early with an invert is important in soft ground. A waterproofing membrane and a final cast-in-place concrete lining usually follow once deformation has ended.

NATM is flexible in shape, so it suits station caverns, junctions and short tunnels where a TBM is not economic. Its weaknesses are reliance on skilled crews and fast reaction to monitoring; collapses have happened where warning readings were not acted on.