Steel Shell Tunnel

(1) Single Shell: Term applied to a tunnel consisting of elements where an outer structural steel membrane (the shell) is constructed first, very much in the manner of a ship. The steel plate also acts as a waterproofing membrane. Elements are usually designed to leave a green field site before the structural concrete is placed, though this may not be the case when other types of fabrication facility are used. Depending upon floating stability requirements, keel concrete may or may not be placed prior to launching. In this condition, draft is usually less than 3 m, making long tows relatively easy while afloat. Nevertheless, transport on barges is not uncommon. The shell plate acts as the exterior form plate for the structural reinforced concrete with which it is designed to act compositely. While stability and strength requirements may require some of the structural concrete to be placed before transportation, it is usual for this concrete to be completed during outfitting, close to the final location. Ballast may be located inside, but more usually outside on top.