NATM & Tunnel Support

The idea behind NATM

The New Austrian Tunnelling Method (NATM), developed in the 1950s–60s by Rabcewicz, Müller and Pacher, is a design and construction philosophy rather than a single technique. Its central idea is that the rock mass around a tunnel is not just a load to be carried; it is the main load-bearing element. When a tunnel is excavated, the stresses in the rock redistribute around the opening and the rock deforms toward it. If the deformation is controlled, the rock can form a self-supporting ring, and only a thin, flexible support is required to control the deformation and to preserve the strength of the rock.

Convergence–confinement idea

The interaction between the rock and the support is described by the convergence–confinement method. As the face advances, the rock at the tunnel boundary moves inward (convergence) and its internal pressure on the opening decreases from the in-situ stress toward a lower value (the ground reaction curve). A support installed at some convergence has its own stiffness (the support characteristic) and the equilibrium is the point where the two curves meet.

• Too early or too stiff a support attracts a high load.
• Too late allows the rock to loosen and the load increases again (loosening pressure).
• The best support is installed at the right time with a suitable stiffness, so that the equilibrium pressure is low.

This is why NATM uses flexible shotcrete and controlled deformation instead of a rigid, heavy lining installed at once.

Excavation sequence

• Full-face in good ground; top heading and bench in fair ground; top heading, bench and invert with a short round length in poor ground; sidewall drifts in very poor ground.
• Round length (advance per round) is small in poor ground — 0.8–1.5 m — and large in good ground (3–4 m).
• Ring closure — the invert is closed within a short distance from the face (often one to two tunnel diameters) in weak ground, to stop the deformation.
• Face stability — face bolts, a face core (a wedge of ground left at the face), shotcrete on the face, forepoling or a pipe umbrella ahead.

Support elements

Shotcrete

Shotcrete is concrete sprayed at high velocity onto the rock surface, building up a layer that bonds to the rock. It gives early support, fills irregularities, seals the rock against weathering and transfers load.

• Types: wet-mix (the standard, more consistent and less dust) and dry-mix.
• Thickness: 50–100 mm for good rock, up to 200–300 mm in weak ground (built up in layers).
• Reinforcement: wire mesh or steel fibres (about 30–40 kg/m³) to control cracks and to provide toughness; synthetic fibres in some cases.
• Accelerators give a rapid set and strength gain: early-age strength (at 3–24 hours) is critical when the shotcrete is loaded soon after application.
• Properties: strength of 25–35 N/mm² at 28 days (or more); good bond to the rock; low rebound (material that bounces back).
• Application: by a shotcrete robot, in layers, with attention to the nozzle distance and angle and to filling behind reinforcement.

Rock bolts

Rock bolts reinforce the rock mass by tying the loosened blocks to the rock behind and creating a supporting zone.

• Length: commonly about a third to a half of the span for the roof; for a tunnel of span $B$ the Q-system gives $L=2+0.15B/\text{ESR}$ m.
• Spacing: typically 1–2 m in both directions; the spacing should be less than half the bolt length so that the bolts interact and form a reinforced arch.
• Pull-out tests are made on a percentage of bolts to check the capacity.

Wire mesh, lattice girders and steel ribs

• Wire mesh (welded) reinforces the shotcrete and holds small loose rock.
• Lattice girders — light three-bar trusses, embedded in the shotcrete — provide early stiffness with less material and easier bonding than solid steel ribs; the standard in NATM.
• Steel ribs (HEB sections or TH profiles) — used in weak or squeezing ground; sliding joints (TH type) allow controlled closure. Ribs are embedded in shotcrete (rib–shotcrete composite).
• Spacing: 0.75–1.5 m depending on the ground class.

Pre-support (spiles, forepoling, pipe umbrellas)

In very poor ground (fault zones, soft or loose rock, shallow cover) the ground ahead of the face is supported before excavation:

• Forepoling / spiling — steel bars or tubes driven ahead of the face in a fan, above the crown.
• Pipe umbrella (canopy tube) — a series of large steel pipes (76–150 mm) drilled and grouted along the crown ahead of the face, forming a protective umbrella and supported by the ribs.
• Face reinforcement — fibreglass bolts in the face, and face shotcrete.
• Ground improvement — pre-grouting, jet grouting or ground freezing ahead of the face.

Support classes

The support is designed as a set of support classes matched to the ground classification (RMR or Q). A typical scheme:

The class at each stage is decided by face mapping and monitoring, on the basis of the design classes in the contract.

Rock load (Terzaghi)

Terzaghi's classical approach estimates the vertical rock load on the support of a tunnel with span $B$ and height $H_t$, for descriptive rock classes:

The rock load is $p=\gamma H_p$ on the roof. It was developed for steel ribs and timber; NATM design uses the ground reaction and numerical analysis instead, but Terzaghi's values are still used to check preliminary loads.