If a bridge over a river is destroyed by warfare, or a natural disaster such as an earthquake, a tunnel made of ice may be a low-cost replacement for the bridge. An ice tunnel can be used as a way to cross a river temporarily, until a new bridge is built. To make an ice tunnel, an array of coolant pipes is lowered into the river by a crane mounted on a boat. This array of coolant pipes is long enough to span the entire river. After the coolant array is installed on the bottom of the river, the coolant array is connected to a refrigeration plant. The refrigeration plant pumps liquid nitrogen (or some other coolant) through the coolant array. When the liquid nitrogen is pumped through the cooolant array, the water around the array freezes, and forms an arch-shaped structure that can be used as a tunnel. After the tunnel's shell is frozen, the water inside tunnel is pumped out. After the water is pumped out, the tunnel is ready to use.
The illustration above shows a cut-away view of a river, with the ice tunnel on the bottom of the river. A bus is shown, so the size of the tunnel can be illustrated.
Above: Another cut-away view, showing the ice tunnel.
Above: A cut-away view of the arch-shaped ice tunnel itself, showing some of the coolant pipes.
Above: A cut-away view, showing the ice tunnel, with it's coolant array at the bottom of a river.