Tunnel Formwork for NATM and Sequential Excavation

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.

Working platforms and access

Multi-level platforms let crews fix reinforcement and install waterproofing membranes before the formwork closes. Guardrails, toe boards, and ladder access are not optional extras on a machine of this size.

The Lining Cycle: Where Productivity Is Won or Lost

Sequential excavation sets the rhythm, but the lining operation sets the pace over the life of the project. A typical cycle runs as follows:

1. Prepare the invert and survey the profile.
2. Install waterproofing membrane and reinforcement.
3. Advance and align the formwork to the design line.
4. Close the formwork and verify geometry.
5. Cast concrete in controlled lifts, with vibration and pressure monitoring.
6. Allow the required curing period.
7. Strip, clean, and move to the next pour.

Steps 3 and 7 are where formwork design pays for itself. A system with independent hydraulic control of each section can be aligned in a fraction of the time taken by manual screw jacks, and clean stripping reduces the surface repair work that otherwise eats into the following shift. On projects with hundreds of pours, a saving of two hours per cycle translates into a substantial schedule gain.

Handling Variable Cross-Sections and Curves

Sequential excavation rarely produces a single constant profile from portal to breakthrough. Designers should anticipate three situations.

Profile transitions and widening sections

Emergency bays, cross passages, and turning niches interrupt the main tunnel section. Formwork that can be partially dismantled, or supplemented with purpose-built panels, avoids the cost of a second machine. Modular skin segments allow the same carriage to serve more than one section.

Horizontal and vertical curves

Formwork travelling on a straight rail in a curved tunnel will drift off line. Tapered panels, articulated carriages, or short pour lengths with careful surveying all address this. The correct choice depends on the minimum radius of the alignment.

Portals and shallow cover

Near the portal, overburden is thin and ground conditions are often weathered. Formwork here must be stable against lower confining pressures and usually shorter, so it can be struck and repositioned with less clearance.

Selection Criteria: A Practical Checklist

When comparing proposals, evaluate each system against the actual excavation sequence rather than the specification sheet alone:

• Profile range: can the machine cover all sections, or does each require a dedicated unit?
• Pour length: longer pours reduce cycles but increase concrete pressure and formwork stiffness requirements.
• Cycle time: ask for the supplier's assumed strip-and-move duration and check it against site logistics.
• Hydraulic redundancy: can the system operate safely if one circuit fails?
• Surface finish standard: panel joint quality and skin condition determine whether the lining meets the specified finish class.
• Support and spares: local service capability matters when a cylinder seal fails mid-drive.

Concrete Quality and Final Appearance

Formwork cannot compensate for a poor mix, but it can prevent avoidable defects. Tight panel joints limit grout loss. Adequate vibration access ports reduce honeycombing. Controlled pour rates keep pressure within the formwork's design envelope, protecting both the machine and the lining geometry. On tunnels where the lining remains exposed, these details determine whether the finished surface is accepted without remedial work.

Frequently Asked Questions

Can the same formwork be used for both the invert and the arch?

Yes, in many configurations. Some systems cast the invert separately with a dedicated travelling invert form, then close the arch section onto it. Others use a full-section design that includes the invert. The choice depends on groundwater control and the excavation sequence.

How long should a pour be?

Typical arch pours range from 6 to 12 metres. Longer pours improve productivity but raise concrete pressure and demand a stiffer, heavier machine. The decision should be based on the formwork's rated pressure and the concrete supply rate.

Is hydraulic formwork justified on short tunnels?

Not always. Below roughly a few hundred metres of lining, a simpler mechanical system may be more economical. Above that, the labour and time savings of hydraulic operation usually dominate.

Conclusion

Formwork for NATM and sequential excavation is a scheduling tool as much as a mould. It must tolerate variable geometry, move quickly between pours, and hold tight tolerances under full hydrostatic load. Specifying it against the real excavation cycle — not just the drawing — is what separates a lining operation that keeps pace with the heading from one that becomes the bottleneck. Working with an experienced manufacturer such as Gangda Intelligent early in the design phase allows profile data, pour length, and cycle targets to be built into the machine before fabrication begins, which is far cheaper than correcting them on site.