With the fourth and last 10,000-tonne concrete element sunk, the U.K.’s new, 1.5-km-long River Tyne Tunnel crossed a major milestone late last month, but a missed dredging window, imposed to protect salmon migration, will cost a delay of two months. Photo: New Tyne Tunnel. Four sunken tube elements were built in dry dock by the contractor. Photo: New Tyne Tunnel. All four sunken tube segments have been floated into place on the Tyne River. Related Links: Tyne Tunnel Sunk Work now focuses on the final connections between the four 400-m-long concrete elements of the sunken tube tunnel (STT) near Newcastle.