Civil Engineering Reference
In-Depth Information
Fig. 12.4
Hydro-cyclones in the
separation plant under
construction
sieve. The remaining mixture, the bentonite suspension with soil particles
smaller than 100
m, went on to the next hydro-cyclone stage.
Second hydro-cyclone
Of the remaining suspension, the matter between 100
m was sep-
arated in the second hydro-cyclone stage. In this hydro-cyclone stage the low-
est limit of 30
m and 30
m was a theoretical limit; in practice the second hydro-cyclone
stage could separate up to 40
m.
The matter which was separated in the three separation stages, was removed
via a conveyor belt.The remaining mixture (the fractions
m) went back
to the excavation front via a buffer tank, or was (partially) replaced by fresh
bentonite suspension.
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Clay looks like bentonite
In the boring trajectory where there was no clay, the sand trajectories, the
suspension which was to be reused again (fractions
m) was almost
free of soil particles after the three separation stages. On the other hand, in
the clay trajectories, a portion of the clay dissolved in the suspension. Due
to the fact that these clay particles, in the sense of its properties - like the
size - displays many concurrences with the bentonite particles, it was not pos-
sible, as expected during the design stage, to remove all these clay particles
from the suspension within the abovementioned separation stages. That
meant that the bentonite suspension that was to be reused again, gradually
became 'loaded' (contaminated) with clay particles. However, these parti-
cles do not have the bentonite properties which are necessary to keep the
excavation front stable. When the suspension was too loaded, there was a
major increase of the specific mass of the suspension and it became difficult
or impossible to pump it around, and could also not absorb soil particles in
the excavation chamber adequately. A portion of the loaded suspension
then had to be removed and be replaced with fresh bentonite suspension, in
order to bring the specific mass of the suspension to be used, back to an
acceptable limit again.
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