A low-level diversion sewer. Engineered by Alexander Binnie, the Blackwall Tunnel was built to provide a road link under the Thames between Greenwich and Poplar. When the tunnel opened fully in 1897 it was the longest underwater tunnel in the world, at 6200 feet. Construction, using the tunneling shield technique pioneered by Marc Isambard Brunel in the building of his Thames Tunnel, was difficult. The tunnel had to be excavated through sand, and in places ran les than 6 feet beneath the river bed. A second prallel tunnel was opened in 1967 to ease traffic congestion.
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