Ring by ring till breakthrough.
In segment installation the tunnel is extended immediately after drilling under the protection of the shield by ring-shaped steel-reinforced lining segments or lining segments, as they are also termed.
Variously shaped lining segments, which in most cases are temporarily screwed in place, are used for this purpose. The lining segments come in watertight or permeable versions.
The lining segments are moved into position by an erector, a remote-controlled gibbet, which picks them up mechanically or by suction. The segments are then screwed in place by qualified personnel. The annulus between the lining segment´s external diameter and the ground is continuously filled with mortar or pearl gravel in order to bed and stabilize the tunnel. Thrust cylinders 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 about 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 lining segments can be adjusted by means of groove and tongue or cam and plug. By contrast, lining segments with flat sides are of less complicated design. Flexible pieces of wood provide additional protection for the lining segment sides.
A segment ring is usually installed as a single, watertight shell, but it can also be in two shell form with an additional inner shell filled in situ with concrete. The dimensions of the key segments depend on the static conditions in the ground or the mechanical 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 fitness for use and durability of the tunnels excavated using shield excavation depends critically on the quality of the lining segments. This is because with single shell linings load transfer and watertightness are provided solely by the tunnel lining, which consist of these prefabricated reinforced concrete components. With two shell linings the functional separation between load transfer and watertightness is spread over the lining segment's outer shell and the inner shell filled in situ with concrete.
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