4. Other components

4.1 Pushbuttons

Tunnel segments are annular reinforced concrete segments used to form the structural lining of circular-section tunnels. They can also be used to create access shafts.

The lining is composed of rings, themselves made up of voussoirs. A small trapezoidal voussoir, called a "key", generally closes the stack forming the ring.

The dimensions of the rings formed by voussoirs can range from a few meters to almost 10 m in diameter. The dimensional tolerances of these elements are particularly strict, to ensure that the rings run smoothly and are correctly positioned.