Civil Engineering Reference
In-Depth Information
tests. Of further extreme importance, was that the ground between both
tunnel tubes would not be disturbed. Based on this too, the choice was
made for using a Double Head Boring Method ('Doppelkopfbohrverfahren').
This method makes it possible to bore extremely accurately, while the back-
flow of soil and water is controllable (prevention of ground deformations)
and the boring can be sealed off watertight. Besides 22 freeze-borings, also
carried out per cross connection were, 2 borings for the temperature meas-
urements and 1 boring for the drainage of the soil within the circular-shaped
ice mass. In any case, there was a possibility to carry out a 'reserve' boring,
to be able to place an extra freezing pipe.
Fig. 13.11
Boring of the
freezing pipes
An important characteristic of the freeze-borings, is that the boring tubes,
which were installed up to the grout layer of the westerly tunnel tube, were
not extracted after the boring; after the end of the boring tube was closed by
means of a seal and this seal was tested, piping was fitted into the boring
tubes in which the cold salt solution was transported. The boring tube thus
acted as a freezing pipe during the freezing process in which the salt solu-
tion circulated.
At each boring process the horizontal and vertical deviations from the ideal
line were measured by means of a double inclinometer.The measuring data
was then converted directly into co-ordinates and then processed so that
the exact location of the borings (also in respect of each other) were known.
Particularly the knowledge of the precise distances between the borings on
behalf of the temperature measurements and the adjacent freeze-borings,
were of extreme importance for the controlling of the frozen soil.
Freezing installation; salt - keeping safety in mind
In order to freeze safely, a freezing generator was developed with a minimum
amount of ammonia - approximately 80 kilograms - to supply a power of
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