The top of the tunnel is called the crown. This can be formed in different ways depending upon the method used during construction of the tunnel. Most tunnels today are dug using a TBM, or tunnel boring machine. The machine is a reinforced steel cylinder that mechanically digs through soil and rock. In this case, the top of the hole created becomes the crown. In what is called a cut-and-cover tunnel, the tunnel begins as a long, deep, trench. The "crown" is then constructed later.
The bottom part of the tunnel is called the invert. If the tunnel will be used for road or railway, it is laid here. The invert of a tunnel can be flat, or it can form a continuous curve with the tunnel's arch. If the tunnel is poorly constructed, heave or buckling of the invert can result. Invert heave is a serious danger if the tunnel is to accommodate pedestrian or automobile traffic. It also can cost millions of dollars in remedial and maintenance work.
The entrance and exit of a tunnel are called its portals. The material surrounding the tunnel portal may not be the same as that of the rest of the tunnel, as most cities will try to take aesthetics into account. A tunnel is not limited to two portals. Often tunnels will have four or more portals to accommodate different directions of traffic. It is important that the portal of a tunnel be above flood level for obvious reasons, particularly if the tunnel itself is on a decline.
The shield of a tunnel provides support at its face. It is a temporary structure used when the tunnel is dug through softer materials such as clay, silt or sand. Sometimes the native soil is so unstable that a strong support system is needed immediately for the newly-bored hole to remain open. The shield holds the new tunnel open before it can be lined with the permanent cast iron, concrete or steel. The first shields were designed specifically to protect workers from cave-ins. The lining of the tunnel gradually replaces the shield. This lining, the permanent wall of the tunnel, will most likely be made of concrete or cast iron, though steel is sometimes used as well.