What Is NATM (New Austrian Tunnelling Method) and How Is It Applied?

What Is NATM? A Short, Clear Definition

NATM (the New Austrian Tunnelling Method) is an observation and measurement based tunnel excavation method that treats the surrounding rock or soil mass not as a load to be carried, but as the main load-bearing component of the tunnel itself. Its core idea is to let the ground deform in a controlled manner after excavation, then arrest that deformation with a thin, flexible support shell, so the surrounding rock forms a self-supporting ring around the opening.

The method was conceptualised in the 1950s and 1960s by the Austrian engineers Ladislaus von Rabcewicz, Leopold Müller and Franz Pacher, and formally named in 1962. NATM is therefore not a single machine or a patented piece of equipment; it is a design and construction philosophy. It is usually carried out by drill-and-blast or mechanical excavation (roadheader, excavator) and relies on flexible support elements, principally shotcrete and rock bolts.

In practice NATM is an observational method in which the support and the excavation plan can be tuned to the behaviour measured on site. In this respect it differs from classical methods whose geometry and support are fixed in advance; it is a flexible, adaptable approach where engineering decisions are made as the excavation advances.

The word 'new' in its name reflects that the method emerged as an alternative to traditional 19th-century Austrian tunnelling with its rigid timber and steel sets. In the old approach the ground was always a threat and a source of load; NATM instead turns that same ground, when properly managed, into the strongest structural resource of the project. Today a large share of the world's metro, railway and highway tunnels is built on this philosophy, and the method has effectively become standard practice on many infrastructure projects.

The Core Principle: The Ground Is the Load-Bearing Element

The idea at the heart of NATM inverts classical tunnelling thinking. In the traditional approach a heavy, rigid lining tries to carry the entire ground load above it on its own. In NATM the real load carrier is the ring of ground formed around the tunnel; the job of the support is to give that ring just enough help to support itself without collapsing. In engineering terms this is the logic of ground-support interaction, or the ground reaction curve.

The key to this logic is timing. If you install a very rigid support immediately after excavation, the ground cannot deform and dumps its full load onto the support, requiring an unnecessarily thick and expensive shell. If you install no support, or install it too late, the ground loosens, loses its bearing capacity and the risk of collapse appears. NATM seeks the balance between these two by allowing controlled deformation: it lets the ground release part of its energy, then stabilises it with a thin shotcrete shell and bolts.

This is why measurement matters as much as excavation in NATM. Deformation is continuously monitored with convergence readings (inward movement of the tunnel wall), settlement plates, extensometers and stress gauges inside the support. If the movements stop and reach equilibrium within the expected limits, the design is sound; if they do not, the support is strengthened. This feedback loop makes NATM not a fixed recipe but a living engineering process.

A natural consequence of this philosophy is that NATM support is usually installed in two stages. First a thin, flexible 'primary' shell absorbs the ground's deformation energy and establishes equilibrium; then, once the deformations have stopped, the permanent 'secondary' lining is applied under a much lower and more predictable load. As a result the total volume of concrete is generally lower than in classical methods that load the full ground weight onto a rigid lining from the outset. Allowing the ground to 'work' a little produces a structure that is both more economical and more mechanically consistent.

Excavation Stages and Application Steps

In NATM a tunnel section is not opened all at once but in stages, according to the quality of the ground. In good-quality, competent rock the full face can be excavated in a single pass. As the ground weakens, the section is divided: first the top heading (crown), then the bench, and where needed the invert and side galleries as well. Splitting the section into smaller pieces preserves stability by reducing the exposed ground surface and the unsupported span at each stage.

A typical drill-and-blast cycle consists of these steps: drilling (boring the blast holes with a drilling jumbo), charging and blasting, ventilation of the fumes after the blast, removal of loose blocks (scaling), mucking (hauling out the broken rock), and immediately afterwards the initial support (shotcrete, mesh reinforcement, steel arch sets and rock bolts). This cycle is repeated with a round length usually between 0.8 and 4 metres depending on the ground; the weaker the ground, the shorter the round.

Once the initial (temporary) support has secured the ground and the deformations are confirmed to have stopped, the secondary lining is placed (the permanent inner concrete lining, often together with a waterproofing membrane). In weak ground, pre-support techniques are used to strengthen the ground around the tunnel before excavation: steel pipe umbrellas (forepoling), ground improvement by grouting, or face bolts. The sequence and scale of these steps are re-evaluated at every advance based on the measured behaviour.

Support Systems: Shotcrete, Rock Bolts and Steel Sets

The backbone of NATM support is shotcrete (sprayed concrete). Sprayed onto the excavated surface under high pressure, it sets within minutes to form a thin but continuous shell in full contact with the ground. It is typically applied 5–30 cm thick and is reinforced with steel mesh or, in modern practice, with steel fibres. Its job is not to carry the load on its own but to seal the exposed surface against weathering and to distribute the load of the ground ring around the opening.

The second main element is the rock bolt (or anchor). These steel rods, driven or grouted radially into the perimeter, tie the loosened surface rock back to the sound mass behind it and force the surrounding ground to act like a composite arch. Grouted, mechanical and friction types (Swellex, Split-Set) are all common. The bolt pattern and length are designed according to the tunnel span and the rock quality.

In weak and swelling ground, steel arch sets (lattice girders or I/H section steel ribs) are added to these two; embedded in the shotcrete, they carry early load and shape the section. In squeezing ground where large convergence is expected, yielding elements (sliding connections) are used to release structural stress in a controlled way. Together, all these elements form an integrated, flexible support system that adapts to the behaviour of the ground.

Ideal Ground Conditions and Limits

NATM can be applied across a wide range of ground, but it is strongest in rock and stiff soils that can hold themselves up for a while, that is, ground with a certain stand-up time. This is the time the unsupported opening can survive without collapsing after excavation; it gives the engineer the window needed to install the support. Fractured rock, weathered formations, marl, claystone and dense soils are typical NATM ground.

The real advantage of the method is its flexibility. The cross-section geometry can change (horseshoe, circular, wide station profiles), the staging can be adapted to the ground, and the support class can be switched as the geology changes along the same tunnel. For this reason NATM is often the only practical solution for the large, irregular sections of metro stations, at junctions and crossover zones, in short tunnels and in variable geology.

Its limits should be just as clear. Loose, water-charged sand and gravel, flowing ground and formations with almost zero stand-up time are risky for NATM; in such conditions pre-support, ground freezing or a closed-face (pressurised) TBM may be required. NATM also demands disciplined monitoring and an experienced team: if the measurement data is misread and the support is not installed on time, the method's flexibility turns from an advantage into a risk. NATM is therefore a method that rests on engineering culture more than on equipment.

To classify ground conditions, geotechnical indices such as the RMR (Rock Mass Rating) or the Q-system are typically used on site. These classifications feed into design tables that set the support class for each tunnel segment (bolt pattern, shotcrete thickness, arch spacing and round length). When the geology changes along the route, the engineer assesses the new ground by face mapping and switches to the next appropriate support class. The strength of NATM lies precisely in this seamless transition between classes: in a single tunnel it can manage conditions ranging from competent rock to a weak, water-bearing fault without halting the project.

NATM or TBM? Comparison and Selection Criteria

NATM and TBM (Tunnel Boring Machine) are not rivals but two methods that answer different problems. TBM excels in long tunnels with a fixed, usually circular section: a cylindrical machine cuts the ground continuously, erects the lining with precast segments behind it, and advances very fast and very safely over long distances. But manufacturing, transporting and assembling the machine takes months and requires a large upfront investment.

NATM, by contrast, offers flexibility and a low initial cost. Where the section geometry varies, where the tunnel is short, where the geology changes along the route, or where wide, bespoke sections such as metro stations are needed, NATM is usually more economical and feasible, because the staging and support can be adapted on site without being tied to a single giant machine. The trade-off is that the advance rate is generally lower than TBM, and blasting, if applied without control, carries the risk of surface settlement and vibration.

The practical selection criteria come down to a few questions: How long is the tunnel and how fixed is its section? Is the ground uniform or variable? What is the water pressure? Are we in an urban area sensitive to surface settlement? On most large metro projects the two methods are used together: the running lines (bored tunnels) are driven with a TBM, while the stations and connecting structures are excavated with NATM. Field experience gained on metro and railway tunnels from Kyiv to New Delhi shows that choosing the right method for the right section directly determines a project's cost and risk.

The decision must weigh not only technical suitability but also the scale and schedule of the project. A TBM only amortises its high upfront investment over sufficiently long tunnels; over short distances the machine cost can strain the entire budget. NATM, although it needs less capital, is labour-intensive and slower, so on very long tunnels it can extend the overall schedule. A mature engineering approach therefore treats the two methods not as an 'either-or' choice but as complementary tools adapted to different parts of the same project.

Common Field Mistakes and Quality Control

Flexibility, NATM's greatest strength, becomes its greatest weakness when applied without discipline. The most common field mistake is delayed support: when the time between excavation and the first shotcrete grows too long, the ground loosens, loses its bearing capacity and the load becomes greater than designed. Its twin is the neglect of measurement; if convergence readings are not taken or interpreted regularly, the method's essential feedback loop breaks and the engineer flies blind.

Other typical mistakes include: damaging the ground with excessive or faulty blasting and reducing its stand-up time; failing to close the invert in time and, by not completing the ring, allowing deformation to continue; leaving insufficient thickness, voids or poor bond in the shotcrete; and skipping pre-support (umbrella, face bolts) in weak ground. Each of these can grow from a small issue into a fast-escalating stability problem.

Quality control is therefore an inseparable part of NATM: shotcrete strength and thickness tests, bolt pull-out tests, geological face mapping, systematic deformation monitoring, and emergency action plans tied to predefined threshold (trigger) levels. A systematic quality and monitoring regime run under ISO 9001 directly improves both the safety and the budget and schedule predictability of NATM works. This discipline is exactly the priority of teams such as KMB Metro Altyapı, who run NATM and TBM methods side by side in the field.