Steel Shell Tunnel

(2) Double Shell: An outer steel plate, usually octagonal in shape, is added to a single shell tunnel element to act as an external form plate for tremie concrete placed as permanent ballast. The tremie concrete protects the inner shell plate from corrosion, while the outer form plate is left as sacrificial. Behaviour of the inner shell plate, and the compositely acting reinforced concrete within it, is similar to a single shell tunnel element except that the stiffening elements are usually placed outside the inner shell plate.