a tunnel built under a riverbed or some other water obstacle, such as a strait, as a passage for vehicular and railroad traffic and pipelines. Subaqueous tunnels usually run perpendicular to the riverbed and, in part, the coastal zones. The cross section of such tunnels features two sloping surfaces. Its shape is determined by geological conditions and by the relief of the bottom and shore.
The depth at which the tunnel is driven and the tunnel’s structural design depend on the construction method used. Round, prefabricated, metal, or reinforced-concrete lining is used if the tunnel is driven using the shield method. If the tunnel is built by submerging individual, prefabricated sections from the surface of the water to the specified depth, the sections are made of cast-in-place reinforced concrete with metal or polymer waterproofing. Such sections can be as long as 150 m. The cross-sectional shapes differ; they can be circular, polygonal (usually rectangular), or a combination of several shapes.
In comparison to bridges, tunnel passages have a number of advantages, particularly where the body of water is large and the shores are low. Tunnels do not interfere with navigation, and they are protected from wind, waves, and ice. The crossing distance is less for tunnels than for bridges when the required clearance for passing ships is high and the floodplain is wide, and in densely populated areas, access to a tunnel is more convenient. A disadvantage of tunnels is the need for artificial ventilation, illumination, and drainage.
The first subaqueous tunnel was constructed in Great Britain in 1843; since that time, more than 200 such tunnels have been constructed in various countries. The construction of very large subaqueous tunnels has begun under the Tsugaru Strait in Japan (36 km long) and under the English Channel (more than 50 km long).
V. P. VOLKOV