There is one thing, however, that even these amazing machines have a hard time doing: changing direction. In the past, vertical shafts had to be built before the tunneling machines were lowered underground to start digging out the horizontal tunnel. Digging these deep vertical holes cost a lot of time and money.
A rotating shield tunnel machine. (C)Taisei Corporation
This problem has been solved thanks to the development of the "rotating shield tunneling machine." At the front end of this machine is a sphere housing the cutter. The sphere is able to rotate so the machine can make 90-degree turns, just like a mole. A rotating shield machine can start digging at ground level, make its own vertical shaft, and then turn 90 degrees to start moving and boring sideways. This makes it much easier to avoid existing underground structures like pipes, other tunnels, and the foundations of buildings.
The rotating shield tunnel machine digs a vertical hole, turns 90 degrees, then continues digging a tunnel sideways.
Shield tunneling machines are being used to build a subway under the Bosporus strait in Turkey. (C)Taisei Corporation
Japanese shield tunneling machines are at work all over the world. They were used in the construction of the Channel Tunnel under the Strait of Dover, the sea linking Britain and France. And today they are being used to build a subway tunnel under the Bosporus strait in Turkey.
(Updated in August 2010)