mohring engels.indd - Keramo Steinzeug

The application range generally covers nominal sizes
between DN 200 and DN 1000 and jacking distances up
to max. 100 metres in unconsolidated material. In cohesive
soils of firm consistency extraction and conveying
of the soil can be facilitated by adding water at the face.
In water-bearing soils supplementary measures are
necessary. For use in groundwater up to max. 1.50 but
not more than 2.50 metres the face can be stabilised
with compressed air. The first conveying pipes are for
example fitted with modified augers with a shorter lead,
so that soil plugs form which prevent the compressed air
from escaping. Another possibility is to provide the
starting and target shafts with locks and operate with
compressed air. For this purpose a combined personnel
and materials lock has proved effective which enables
persons and materials to be got into the working area in
separate pressurised chambers. The control console is
installed over the jacking machine and outside the compressed-air
chambers, so that working under atmospheric
conditions is possible here. This compressed-air
alternative is very costly and reserved for special cases.
The method does however have two advantages: firstly,
conveying with augers makes the separation system redundant,
so that no special action is required in severe
frost. Secondly, any recovery necessary at the face of
obstacles to thrust boring and associated operations on
the control head are possible with the compressed-air
equipment available at the site, without need for elaborate
dewatering.
In view of the very successful two-/three-stage jacking
systems using pilot rods and expansion boring in constructing
house connections and nominal sizes for
smaller collectors up to DN 400 a new thrust-bore pipe
jacking system, first demonstrated by Bohrtec at Bauma
’98, should be mentioned.
It is based on two-phase thrust boring and is in initialcost
terms an economical alternative to thrust-bore pipe
jacking used hitherto to install sewers of nominal sizes
between DN 400 and DN 800.
In the first stage of the process a steel casing tube of external
diameter 420 mm with internally fitted augers is
jacked from a starting shaft with unobstructed width 320
cm up to max. 60 metres to a target shaft. The augers
have – like the pilot pipes – a hollow axis and, at the
point of the screw line, a taper. Depending on the position
of this oblique plane relative to the soil at the face,
control movements can be executed during jacking.
This principle of the so-called ”controlled auger” permits
soil conveying also to be optimised with respect to embedded
stone, since the entire cross-section of the steel
casing tube is available for the purpose; the optical
channel runs in the hollow axis of the revolving auger
line, no longer above a separate conveying pipe in the
clearance from the casing tube. Directional accuracy is
again monitored by means of a theodolite with CCD
camera and the monitor in the starting shaft and of the
diode target panel in the control screw. The first stage in
the process is carried out with the same equipment for
all nominal sizes to be jacked and uses a clockwise-rotating
screw line to transport the soil. When the control
auger and first casing tube arrive at the target shaft, the
second stage begins: an expansion stage with directly
driven cutting head is docked on to the end of the line of
steel tubes and, now turning anti-clockwise, conveys the
soil encountered through the steel tube line of the first
process stage to the target shaft. The size of the expansion
stage depends on the nominal size of the
product pipes (Fig. 13).
Shield pipe jacking
Micro-tunnelling Page 13
Fig. 13: Two-phase thrust boring with control
screw, augers and expansion stage, rotating
clockwise and anti-clockwise
Here thrust boring of the product pipes is accompanied
by simultaneous all-over soil removal at the mechanically
and fluid-assisted face by a directly driven cutting
head which is rotating both clockwise and anticlockwise.
In contrast to the machines using auger conveying, with
slurry-conveying machines the soil is transported by a
closed fluid circuit. As a rule the conveying medium
used is water. For loosely bedded, non-cohesive unconsolidated
material use of a bentonite suspension to
prevent uncontrolled soil removal is appropriate. This
type of machine thus lends itself well to coarse-grained
soil types at any groundwater level.
To operate the slurry system a feed pump, normally installed
at ground level, and suction pump in the starting
shaft are needed. By appropriately controlling the two
pumps any pressure required at the face can be set and
thus any water pressure counteracted. Crushers are incorporated
in all slurry systems, so that extracted material
is comminuted to an extent which ensures its subsequent
blockage-free transport through the conveyor