The New Austrian Tunnelling Method (NATM) and sequential excavation rely on the ground itself to carry part of the load, which means the permanent lining is placed later, after the sprayed concrete shell and monitoring data confirm stability. That delay shapes everything about the formwork. Unlike a TBM drive, where the lining geometry rarely changes, a sequential excavation tunnel may shift profile, pass through portals, niches, and widening sections, and advance in a stop-start rhythm governed by convergence readings. The formwork must therefore be mobile, hydraulically adjustable, and precise enough to deliver a structural lining that meets tolerance without slowing the excavation cycle. This article explains what NATM formwork must do and how to specify it.
What NATM and Sequential Excavation Demand from Formwork
The primary lining in NATM is sprayed concrete, often reinforced with lattice girders or steel ribs. It is a flexible, deformable shell. The secondary lining, normally cast in place, is what provides the final structural capacity, watertightness, and the smooth interior surface required for ventilation and drainage.
That division of labour creates a specific set of demands on the formwork:
- Movement between pours: the formwork must be repositioned quickly by its own traveling mechanism, because every hour spent moving is an hour not spent casting.
- Adjustability to variable geometry: horseshoe, circular, and multi-centred profiles are all common, and transitions between them are frequent.
- Tolerance control: lining thickness must remain within design limits around the entire circumference, otherwise the structure loses cover and load path integrity.
- Safe access: workers need stable platforms for reinforcement fixing, waterproof membrane installation, and concrete placement.
In practice, these requirements converge on one machine type: the hydraulic travelling tunnel formwork system, usually supplied as a full-section or invert-and-arch configuration.
Anatomy of a NATM Lining Formwork System
A modern system is not simply a curved steel shell. It is an integrated machine, and each element exists because of a constraint in the excavation cycle.
Formwork skin and structural frame
The skin panels define the finished surface. They are typically fabricated from heavy-gauge steel with machined joints so that pour lines stay tight and grout loss is minimal. Behind the skin, a ribbed frame carries the hydrostatic pressure of fresh concrete — which, for a full-section arch pour, can be considerable. Frame stiffness determines whether the lining arrives at the designed radius or bulges under load.
Hydraulic adjustment and stripping
Hydraulic cylinders handle three motions: radial collapse for stripping, vertical jacking for grade and level, and lateral shifting for alignment. The stripping sequence matters more than most specifications admit. If the arch cannot retract cleanly away from the concrete, operators risk tearing the fresh surface or damaging the skin during withdrawal.
Traveling carriage and rail system
The carriage moves the entire assembly forward on rails set to the tunnel centreline. For long drives, the rail system must be robust enough to resist repeated loading and accurate enough to keep the formwork on line without constant re-surveying. Suppliers such as Gangda Intelligent typically design the carriage and rails as a matched set for this reason.
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For more detailed information on NATM and sequential excavation tunneling formwork, please click here: https://www.gdtunnel.com/a/blog/tunnel-formwor_f44f3.html
