Water Tunnel

an underground water conduit closed in cross section, with water moving freely or under pressure, constructed in the earth’s crust without an open excavation. Water tunnels are built where a water conduit runs deep, where an open trench is not economically feasible, or where an open trench would cross steep, slide-prone slopes or densely populated and built-up territory. Water tunnels are classified according to basic water use: power generation, irrigation, navigation, timber floating, water runoff, water conduit, construction (for temporarily diverting river water in building a hydroengineering complex), and combination (meeting various water-use requirements).

The form and dimensions of a water tunnel depend on the nature of the movement of the water in surrounding rock and the values for vertical and lateral rock pressure. The most widespread forms of cross section in free-flow tunnels are oval, rectangular, trough-shaped, and horseshoe-shaped; pressure tunnels are usually round. The main design element in the cross section of a water tunnel is the lining, which guarantees that water will not permeate the hydraulic tunnel and protects excavations from cave-ins and deformations of rock; it also makes the tunnel walls smooth. The lining can be concrete, reinforced concrete, or metal (in the case of pressure tunnels).

In mountainous regions, where hydroengineering complexes depend on high pressure, water tunnels are often built in several tiers (the number depending on altitude), forming a single complex of underground hydroengineering installations that are linked by secondary tunnels for reaching underground machinery rooms of hydroelectric power plants, the control rooms of hydraulic water gates, ventilating and air shafts, and so on. The routing of a water tunnel is usually determined through an economic comparison of several variants that take geological and working conditions (tunneling work) into consideration.

Large water tunnels have been built by Soviet hydraulic engineers for the Aswan Dam (Arab Republic of Egypt; diameter, 15 m; length, 282 m), the Charvak Dam (USSR; diameter, 12 m; length, 774 m), and other hydroelectric complexes.