Tunnel Formwork for Drill and Blast Tunneling

Tunnel formwork for drill and blast tunneling is a specialized lining system engineered around an excavation method that is cyclic, overbreak-prone and geologically unpredictable. Unlike a TBM bore, a blasted profile rarely matches the theoretical line, and the formwork has to absorb that variation while still delivering a lining that meets tolerance. The choice of system directly affects cycle time, concrete consumption and the long-term integrity of the finished tunnel. This article explains how these systems are built, what to specify, and where the real cost drivers sit.

What Makes Drill and Blast Different for Formwork

Drill and blast excavation advances in rounds: drill, charge, blast, ventilate, muck, scale and support. Each pull typically removes two to four meters of ground depending on section size and rock mass quality. The resulting profile is a product of blast geometry rather than a machine's fixed cutterhead path.

Overbreak reshapes the concrete envelope

Blasting almost always removes more rock than the theoretical profile. Overbreak of roughly 10 to 30 centimeters is common in jointed or laminated ground, and in weak or heavily fractured zones it can be considerably larger. The formwork defines the inner face of the lining; the excavated surface defines the outer bound. Every extra centimeter around the full perimeter becomes a measurable volume of concrete, so a system that allows tight, repeatable setting-out pays for itself over a long drive.

Standing support and irregular surfaces

In many drill and blast tunnels, initial support — shotcrete, rock bolts, lattice girders or steel arches — is already in place before the final lining is cast. That means the formwork is not working against a clean rock surface but against an irregular, sometimes protruding support layer. The outer edge of the form panels and the carriage clearance must be designed around this reality, not around a nominal diameter.

Anatomy of a Tunnel Formwork System

A modern lining system for drill and blast work is essentially a mobile, hydraulically actuated steel mold. Its performance depends on how well four subsystems work together.

Traveler carriage and support frame

The carriage carries the full weight of the form panels, the concrete load and the hydraulic actuators, and it advances the whole assembly to the next pour block. Two configurations dominate:

• Rail-mounted travelers running on a track set in the invert, which give excellent alignment control and repeatability.
• Self-propelled, invert-riding travelers with rubber tires or crawler assemblies, which avoid the cost of laying and maintaining rail but demand a well-prepared, reasonably flat invert.

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