CROSS CONNECTIONS - The Westerschelde Tunnel: Approaching Limits - page 189

Principle of ground freezing

The principle of ground freezing lies in the artificial cooling of the soil to
below freezing point. The pore water present in the soil freezes and acts as
a sort of 'binding agent' for the soil. The low temperature required for the
realization of the frozen soil is achieved by means of freezing pipes fitted
into the ground. An extremely cold medium/freezing agent (for example a
salt solution at a temperature of 37 °C) circulates within the freezing pipes

so that heat can continually be extracted from the surrounding soil. The
result is frozen soil around the pipes which grows in time. After a while the
frozen soils grow together around the various pipes - which, in the case of
the Westerschelde Tunnel were fitted more or less in a circular shape - into
a large circular-shaped sealed and watertight body. After the required thick-
ness has been achieved (for the taking up of forces), the necessary excava-
tions for the cross connections can commence within this frozen soil.

Fig. 13.6
Diagrammatic
representation of the
principle of freezing

(a) Individually frozen soil bodies (b) Closing of frozen soil

(c) Circular shaped frozen soil (d) Placing of concrete lining

Ground freezing deforms the tunnel tubes

An important aspect in the design of the cross connections was the behav-
iour of the ground when being frozen: the water which is present in the
ground expands and when such volume-increase is hindered, major forces
could occur.Then the question is how great such forces could become, what
direction they take and to what degree the tunnel tubes are affected by it?
The expansion of frozen ground had - as appeared in literature - already
been a subject of research and measurements many times. However, the
attention was always directed towards the expansion in the direction of the
temperature gradient (so-called Frost Heave tests) and thus, 'transformed'
for the Westerschelde Tunnel, perpendicular to the cross connection. The
expansion of the ground parallel to the freezing pipes (the isothermals) until