CROSS CONNECTIONS - The Westerschelde Tunnel: Approaching Limits - page 192

can take up a great deal of heat. Although, a disadvantage is, that due to the
evaporation it can only be used once. Thus freezing with nitrogen is an
extremely expensive process. Besides, using nitrogen - which is an odourless
and hazardous gas (when higher concentrations than normal are present in
the air) - requires extremely stringent safety measures. When using liquefied
air, less radical safety measures suffice, as was evident in the elaboration
of an alternative for the last cross connection. Except, then the consequences
of the released (extreme) cold air in the tunnel tubes is a special point of
attention.
In the application of a salt solution which circulates in the freeze piping and
freezing pipes, the freezing agent can continually be brought to the desired
(low) temperature by means of cold-generators (chillers). Although this
technique calls for longer freezing periods, this method was nevertheless
chosen for the construction of the cross connections of the Westerschelde
Tunnel. On the one hand there are less stringent safety measures necessary,
whereas on the other hand the costs are also lower.

Laboratory research necessary

In order to determine the dimensioning of the freezing installation, the estab-
lishing of the growth of the frozen soil and the constructive dimensioning of
the frozen soil, it was necessary to have precise knowledge about the build-up
of the soil, the heat transfer soil parameters and the relevant constructive
soil parameters of the frozen and non-frozen ground.
Furthermore, the chloride content of the water was of particular importance;
salt water freezes at a lower temperature than fresh water.
Extensive laboratory research was undertaken at Professor Jessberger and
Partners' institute in Bochum, in which the (soil) parameters for frozen ground
at temperatures of respectively 5 °C, 10 °C and 20 °C were determined.

Preparatory measures

Already in the design stage of the actual tunnel tubes and at the production
of the circular tunnel segments, all relevant structural consequences of the
construction of the cross connections had to be examined.
Next to the tunnel ring which contains the access door, up to the cross con-
nection to be constructed, four 'adjoining rings' (at either side of the 'access
ring' two tunnel rings each) were designed as 'non-standard rings'. During
the production of the segments the ducts for the freezing pipes were built
into these rings in accordance with the geometry of the frozen soil.
In the determining of the positioning of the ducts in the tunnel rings, mat-
ters that obviously should be taken into account are the position of the
joints, the facilities to be built into the segments and the reinforcement of
the segments. Besides, the design provided for a possible deviation in the
positioning of the access doors to the cross connection measuring up to 1
metre. Larger deviations could be compensated by placing special tunnel
rings measuring 1 metre wide in the longitudinal direction.

Emergency partition as the utmost safety precaution

The boring of the tunnel and the construction of the cross connections
occurred parallel (in time).That also meant that unfortunately major leakages
as a result of the activities on the cross connections could also endanger the
'crew' on the tunnel boring machine.