CROSS CONNECTIONS - The Westerschelde Tunnel: Approaching Limits - page 191

pores away. Yet it was of importance that the water in the core of the frozen
soil was drained in order to avoid that this caused problems.
The three-dimensional analysis of the main tunnel tube indicated that defor-
mations of 10 mm as a result of the freezing would cause no problems,
whereas sections of the tunnel tube during a deformation of 30 mm would
be too heavily loaded.
The engineering company, Ingenieursbureau Westerscheldetunnel (IBW)
who were responsible for the design of the cross connections, provided for
an additional view that, taking into consideration the failure safety of the
segments, a deformation of approximately 20-25 mm was acceptable. Due
to the unfamiliarity of the expected deformations of the tunnel tube and the
associated risks, it was decided to extend and to intensify the measuring
schedule. The acceptable deformation of 20 mm as calculated by the IBW
acted as the 'point to look out for'.
Besides, in order to achieve a greater insight into the behaviour of freezing
ground and its effects on the adjacent structure, under the auspices of the
Centre for Underground Construction (COB) an extra investigation was
started which is also known as the COB project F100.

Deformation of tunnel tubes concurs with research

During the construction phase the charting of the deformations was a con-
tinuous point of attention. As expected, it was clear that the deformations of
the tunnel tube that occurred at cross connections situated in sand, were
small (maximum 4 mm). However, at the cross connections which were sit-
uated in Boom clay there were deformations up to 20 mm at most. Although
the degree of deformation was difficult to forecast beforehand, the magni-
tude of the measured deformations concurred well with the expectations
regarding the expansion of the frozen soil.

The creation of the frozen soil

In order to be able to create frozen soil, 22 freezing pipes had to be bored
from within the easterly tunnel tube - where the boring had been started a
few months earlier - in such a fashion, that a closed circular shape at least

Minimal thickness of
frozen soil is 2 metres

Fig. 13.9
The configuration of
the freeze-pipes
around the cross
connection

Freezing-pipes

Outer lining

Inner lining

Steel segment

2 metres thick could be made. Inside this, the excavation of the cross con-
nection from the westerly tube takes place.
Freezing agents such as fluid gases like liquefied nitrogen, or extremely
cooled salt solutions could be considered. Due to evaporation, liquid nitrogen