Civil Engineering Reference
In-Depth Information
The soil is excavated from the full face by a cutting wheel fitted with tools (open-mode or
rimmed wheel) and removed hydraulically. Subsequent separation of the removed suspen-
sion is essential.
If it is necessary to enter the excavation chamber, for example to change tools, carry out
repair work or to remove obstructions, the support slurry has to be replaced by compressed
air. The support slurry then forms a low-permeability membrane on the face, which how-
ever is of limited durability (risk of drying out). The membrane permits the support of
the face by compressed air and may need to be renewed regularly. The support slurry
can be completely (empty) or only partially (lowering) replaced by compressed air. The
maximum partial lowering is limited particularly by the requirement for sufficient work-
ing space. This should be chosen so large that safe working is possible at all times and an
adequately large space is available for the workers to retreat.
If an open cutting wheel is used, it should be possible to mechanically close the face with
shutter segments in the cutting wheel or with plates, which can be extended from behind,
in order to protect the personnel working in the excavation chamber while the machine is
stopped, which is also sensible due to the limited duration of the membrane effect.
Stones or rock benches can be reduced to a size that can be removed by discs in the cutting
wheel and/or crushers in the working chamber.
In stable ground, the slurry shield can also be operated in open mode without pressurisa-
tion, with water being used for muck removal.
Any additional mechanical support of the face provided by the cutting wheel or support
plates should be regarded solely as additional security and it is not permissible to consider
the supporting effect in calculations to verify the stability of the face.
Face with earth pressure support (SM-V5)
Tunnelling machines with earth pressure balance support provide support to the face
through remoulded excavated soil. The excavation chamber of the shield is closed from
the tunnel by a pressure bulkhead. A cutting wheel, fitted with tools and more or less
closed, excavates the soil. Mixing vanes on the back of the cutting wheel (rotors/back
buckets) and on the pressure bulkhead (stators) assist the remoulding of the soil to a work-
able consistency. The pressure is checked with earth pressure cells, which are distributed
on the front of the pressure bulkhead. A pressure-tight screw conveyor removes the soil
from the working chamber.
The support pressure is regulated by varying the revolution speed of the screw conveyor
or through the injection of a suitable conditioning agent controlled according to pressure
and volume. The pressure gradient between excavation chamber and tunnel is provided
by friction in the screw conveyor. If the soil material in the screw conveyor cannot ensure
the sealing of the discharge device, an additional mechanical device has to be installed.
Complete support of the face, in particular the upper part of it, is only successful if the
soil acting as a support medium can be remoulded to a soft or stiff-plastic mass. This is
particularly influenced by the percentage content of fine grains smaller than 0.06 mm. The
scope of application of earth pressure shields can be extended using soil conditioners such
as bentonite, polymers or foam, but attention needs to be paid to the environmentally ac-
ceptable disposal of the material.
Search WWH ::
Custom Search