The vital role of formwork in tunnel construction

Formwork is an ancient engineering technique. How ancient? Take a trip to the Pantheon in Rome and have a look-see. Yes, like us today the Ancient Romans also used molds to prefabricate concrete segments of larger construction projects. Civil engineers have been in demand ever since. But some may consider building things on top of a solid foundation to be child’s play compared with tunneling into soil for huge distances while still accounting for the immense subterranean pressure of tons of earth, water, and sometimes urban sprawl pushing down from on high. Given all that, tunnels are some of the most magnificent feats of civil engineering in the world.

Major tunnels today are built rapidly because of advances in tunnel-boring technology, but also because of because of formwork – reinforced concrete segments can be assembled with millimeter precision. Before these segments can be laid, huge Tunnel Boring Machines (TBMs) must drill through the ground and clear enough room for the work to be done. Once the TBMs move debris out of the way using a conveyor system, the prefab concrete segments can be brought into the tunnel behind the drill head on flatbed rail cars. Produced in factories above-ground and shipped to the construction site, these trapezoidal segments are gently curved because each of them makes up part of the tunnel ring which lines the perimeter of the tunnel bore. Each segment will be lifted into place (again, with millimeter precision) using a vacuum attached to a rotating arm or “feeder,” and pressed against the segments of the previous ring with hydraulic cylinders before being bolted down. All of this happens after the rotating drill head advances through the ground far enough to allow enough space to position new segments (perhaps two meters).

Given that most modern tunnels are built to accommodate the transportation needs of expanding urban centers, we know that each tunnel ring can’t be advancing in a straight line. Tunnel formwork will use several different molds, each with marginally different dimensions, so that each complete tunnel ring will be slightly conical in shape. Adjusting how each ring fits against the last one will gradually result in a directional change in any of 360 degrees, depending on where engineers want the tunnel to go.

Having the concrete formwork take place off-site in a controlled manufacturing environment allows for a much more streamlined construction process and ensures quality control. These segments have to match requirements exactly, and so expert formwork is vital to quality tunnel construction and relied upon by tunnel engineers.