Civil Engineering Reference
In-Depth Information
freezing took much longer than was assumed beforehand. Especially the
major heat transporter to the freezing soil at the location of the steel seg-
ments, was detrimental.
Besides, readings indicated that the water to be frozen was more salty at
some points. Perhaps a factor played a role here whereby salt is driven out
of the freezing water during the freezing process thus leading to a higher
salt concentration in the surrounding water, which is then obviously more
difficult to freeze.
In order to achieve an optimum connection quicker, the choice was made
to actively cool the lowest steel segment in the westerly tunnel tube.
This occurred at the first cross connection by placing blocks of dry ice (solid
carbon dioxide at a temperature of
80°C) onto the segment. Besides, a
100 mm thick insulation layer was also applied onto the lining in the easterly
tube.
Frozen soil in clay continues to expand
At the second cross connection the connection of the frozen soil onto the
tunnel tube was also very difficult to realise. Other than at the first cross
connection, here the freezing occurred in Boom clay. An aspect which plays
a role here - and as was already proven in studies and tests - was that the
longer the freezing took place, so too would the expansion of the frozen soil
parallel to the freezing pipes continue to increase. A factor that could have
played a role here, was that the water-sucking action at the border between
frozen and non-frozen soil is much greater in the case of clay than with sand.
The result is that the frozen soil continues to expand evenly. Although the
connection of the frozen soil was not realized at some places, the ice con-
tinued to press harder against the tunnel tube at other places, whereby the
deformations increased; freezing for a longer period is certainly not the
solution for achieving a proper connection when it comes to dealing with
Boom clay.The deformations threatened to increase to such a degree that it
was immediately decided to undertake active cooling with dry ice. Once the
connection was achieved, the access could be broken open and the pres-
sure decreased.
In order to no longer be confronted with the problem of an inadequate insu-
lation, at the remaining cross connections (three up to and including
1. 2
Fig. 13.17a
Radial and axial
deformation test
versus time of Boom
clay; test is done
before freezing
Dilatation
1. 0
0.8
0.6
0.4
0.2
Radial
deformation
Contraction
0
0
12
24
36
48
Time (hours)
0.06
Dilatation
0.04
0.02
0.00
0.02
0.04
Axial
deformation
Contraction
0.06
0
12
24
36
48
Time (hours)
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