Civil Engineering Reference
In-Depth Information
11.5.7.2 Tunnel boring machines with shield (TBM-S), Shield machines
(SM, DSM, KSM)
With tunnel boring machines with shield, or shield machines, the final support is installed
in the protection of the shield skin or else the shield machine is operated at the front of a
jacked pipe.
Precast elements installed in the shield tail (segments) serve to support the surrounding
ground and as the abutment for the thrust force. The structural bond between the lining and
the ground is created by grouting of the shield track as continuously as possible.
Segments and pipes are often used as a single-layer lining.
Concrete and reinforced concrete segments
The prefabricated elements used today are mostly precast concrete or reinforced concrete
segments. The loading on the segment during transport and installation is often sufficient
to require the installation of steel bars as reinforcement. Alternatively, segments with steel
fibre reinforcement or a combination of rebars and fibres can be used. Steel fibres are
particularly useful for the strengthening of the edges and corners, which are difficult to
reinforce sufficiently with rebars.
SGI lining and steel segments
Spheroidal Graphite Iron (SGI) lining are now scarcely ever used because of the cost and
fire protection problems. When the ground conditions are especially difficult, particularly
if the bedding is poor, there is a danger of high deformation (convergence) or ring offsets.
Welded steel segment rings, which are stiff in bending, are also often used to cope with the
unusually high and asymmetrical loading at crosscuts, niches and other openings.
Hybrid segments
The hybrid segment is a combination of reinforced concrete and steel segments and offers
an economic alternative to the use of full steel segments. These can be welded steel com-
partmented constructions filled with concrete or conventional reinforced concrete seg-
ments with integrated steel boxes bolted into the longitudinal or ring joint. This increases
the stiffness of the system and the deformations are reduced.
Extruded concrete
Extruded concrete is a concrete tunnel lining, which is placed in a continuous process
as unreinforced or fibre-reinforced concrete behind the tunnelling machine between the
shield tail and a travelling inner formwork. The extruded concrete thus already sup-
ports the surrounding rock mass in the wet state. The use of extruded concrete is also
possible below the groundwater table. Elastically supported face formwork, which is
pushed forwards by the wet concrete pressure, ensures constant support pressure in the
wet concrete.
Timber lagging
In ground without water, the primary support can consist of timber or steel lagging, which
is installed between steel profiles in the protection of the shield tail (ribs and lagging).
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