Civil Engineering Reference
In-Depth Information
After the steel profiles have left the shield tail, they are braced against the ground by
hydraulic cylinders, thus providing support. The tunnelling machine can be pushed for-
ward against this braced support.
This method of support is not used in Europe due to the lack of fire protection during the
construction period.
Pipes
Pipe jacking is a special process, in which reinforced concrete or steel pipes are jacked
forward from a shaft and serve as support and final lining. These are normally circular but
the use of rectangular sections is also possible.
Reinforced concrete
As with shotcrete, in-situ reinforced concrete can also be used with tunnelling machines
to support the sides of the tunnel. As no thrust force can be transferred to the support,
this type of support is only used with blade shields. The reinforced concrete is placed
conventionally with a travelling formwork unit in 2.50 to 4.50 m wide sections in the
protection of the following blades, which are still in contact with the last section to be
concreted.
This process is no longer used in Central Europe on grounds of cost.
11.5.7.3 Advance support
The use of advance support measures, which are installed in the ground from a shield
tunnelling machine, should only be used for short sections in emergency, since the im-
plementation is technically laborious due to the poor accessibility and uneconomic due to
the interruption of tunnelling advance. All possibilities of providing ground improvement
from the surface should be exhausted first.
It is generally possible with current technology to implement rock bolts, pipe screens,
drilled grouting and inclined or horizontal high-pressure grouting.
In order to make this possible, the tunnelling machine should be supplied with the nec-
essary equipment, since later installation of drilling equipment is very expensive. Drill-
ing booms are usually mounted on the segment erector and can drill through inclined
pipes passing forwards through the shield skin (minimum angle to the shield centreline:
about 8°).
Holes can also be drilled into the face through sealed openings in the pressure bulkhead. It
should, however, be noted that broken drilling rods, which cannot be recovered, will lead
to a severe obstruction of further advance.
The production of closed grouting bodies from the machine should not be provided, as this
process is not practical for geometrical reasons. There is a basic risk in grouting that the
grout can penetrate uncontrolled into the annular gap or the excavation chamber and thus
lead to a failure of the face. When consolidation of the face is required, the excavation
chamber should therefore be filled with a soil substitute first.
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