Diagram of air-formed tunnel

A circular-shaped air-formed tunnel compared with a similar square-shaped tunnel. The curved structure can be thinner and stronger without sacrificing room or utility.

Dave Collins

A cut-and-cover underground tunnel is built by excavating a trench, building the tunnel structure, and backfilling soil around and over the tunnel. Using a tube-like Airform and spray-in-place concrete, a tunnel can be formed with less labor and overall cost.