SEPARATION PLANT AND BORING SPOIL - The Westerschelde Tunnel: Approaching Limits - page 172

Introduction
During the boring of the two tunnel tubes a total of about 1.3 million m 3 of
soil became available. The composition of this excavated product varied,
dependent on the various soil deposits in which the boring machines were
situated. Where boring occurred at the dividing line between two soil
deposits, various soils inevitably got mixed with each other. That meant that
across the full length of a single tunnel tube, about 40% of the soil consisted
of a mixture of various soil deposits. In the excavation chamber of the bor-
ing machine the excavated soil was also mixed with bentonite suspension.
Piping transported this bentonite soil mixture to the separation plant at sur-
face level. Here the various fractions were separated and the bentonite and
water were recycled for reuse. The remaining (separated) spoil components,
if it complied with the necessary quality, could be reused for various pur-
poses or was spread over the Westerschelde.
With this, the separation plant served two purposes:
- the recycling of water and bentonite for reuse in the boring process;
- making reuse of the excavated soil possible.
In the first section of this chapter the operations of the separation plant
are explained, whereas the following section broaches on the manner in
which the cuttings are dealt with.
The bentonite mixing installation
Fresh bentonite suspension was produced from water and bentonite
powder in the bentonite mixing installation. The powder was pumped from
a silo to the installation and was continually stirred while adding water.
The storage of the fresh bentonite suspension took place in a storage basin
with a contents of approximately 2,000 m 3 near the separation plant. When
the storage took too much time, the suspension was pumped around
through the separation plant and was mixed there with the 'loaded'
('contaminated' with soil particles) suspension. Bentonite is a tuff which
consists of tiny clay particles which swells into a type of 'gel' when it
absorbs water. The fluid penetrates into the pores of the soil around the
excavation front and thus forms a plaster layer. Mixed with water and soil,
bentonite not only acts as a supportive fluid, but also as a transportation
medium to remove soil. Bentonite has a special (thixotropic) property
which, when stagnant, it behaves like a solid, whereas when it is in motion,
it behaves like a fluid.
In order to fulfil the supportive function at the excavation front, it is import-
ant that the bentonite suspension maintains the thixotropic properties. The
more often that the bentonite is reused, the greater the contamination of the
substance with other soil particles, which is to the detriment of the thixotropic
properties.The degree of contamination thus determines the possibilities of
reuse of the suspension. The properties which are of importance for the proper
functioning of the bentonite suspension are loss of filtration water, liquid limit
and density.