________________ When installing segments, the tunnel is lined with ring-shaped steel-reinforced segments immediately after the drilling process under the protection of the shield. Geometrically different segment forms, which in most cases are temporarily screwed in place, are used for this purpose. The segments come in watertight or permeable versions.
Segments in top form
The segments are moved into position by an erector, a remote-controlled crane arm, which picks them up mechanically or by suction and then screwed in place by qualified personnel. The annulus between the segment´s external diameter and the construction ground is continuously filled with mortar or pearl gravel in order to bed and stabilize the tunnel tube. Jacking stations supported by the last segment ring to be installed then carry out the next stroke. A complete segment ring is usually cone-shaped in order to accommodate curve radii of up to around 150 meters. The narrowest spot on the segment ring is usually at the so-called key segment, which, as the last wedge element, closes and braces the ring. The key elements of the segments can be adjusted by means of groove and tongue or cam and plug systems. By contrast, segments with flat sides are of less complicated design. Flexible pieces of wood provide additional protection for the segment sides.
A segment ring is usually installed as a single, watertight shell, but it can also have a dual-shell design with an additional inner shell filled in situ with concrete. The dimensions of the key segments depend on the structural conditions in the construction ground or the mechanical engineering conditions. The single shell tunnel lining requires prefabricated reinforced concrete components of the highest degree of dimensional accuracy. This makes exceptional demands on prefabricated component manufacturers and particularly on the manufacturers of formwork moulds. The readiness for use and durability of the tunnels excavated using shield excavation depends critically on the quality of the segments. This is because, with single shell linings, load transfer and watertightness are provided solely by the tunnel lining, which consists of these prefabricated reinforced concrete components. With two shell linings, the functional separation between load transfer and water-tightness is spread over the segment's outer shell and the inner shell filled in situ with concrete.