Civil Engineering Reference
In-Depth Information
Multiple hydro-cyclones
During the boring process the suspension could be guided through a fourth
separation stage for further cleansing. This occurred by means of multiple
hydro-cyclones which in principle, acted similarly to the hydro-cyclones.
On the one hand the multiple hydro-cyclone was an extra separation step for
the regaining of bentonite and on the other hand, the multiple hydro-cyclone's
purpose was to thicken and separate a portion of the loaded suspension
before transportation took place. This meant a substantial reduction of the
quantity of bentonite suspension which was no longer reusable.This step was
of particular importance during the boring into clay, because then a substan-
tial quantity of dissolved clay particles came about in the suspension.
The non-reusable suspension to be discharged (fractions
m also
known as discharge flow), was not removed via a conveyor belt, but was
removed separately through piping to the Westerschelde.
30
Clay chopper
During the boring of the tunnel tubes the separation plant functioned very
well. Although during the 'trip', at the time that the boring machines were sit-
uated in the Boom clay, a so-called 'clay chopper' was applied as an extra
facility in order to regulate the 'inflow' of clay better. Clay stuck in the ben-
tonite piping into large chunks which could lead to a blockage of the sieve
causing it to overflow. With the clay chopper, it was prevented that chunks of
clay that were too large (and difficult to process in the plant) would get into
the plant at one time. Due to the separation plant functioning better by this,
the boring process was also confronted with less delays. Aside from that,
design changes were also carried out prior to the boring process.This did not
particularly concern the separation principle, but the manner of removal in
view of the possibilities for reuse of the matter that became available.
Boring spoil issues
Dumping of excavated soil requires closer investigation
Originally it was assumed that the soil that became available during the bor-
ing (naturally based on a permit to be applied for) could be dumped into the
Westerschelde. The contractor drew up a design for the separation plant
based on that assumption.The idea was that the excavated soil from the two
tunnel tubes would be removed to a ship with a single conveyor belt system.
The mooring jetty of the ship was situated just outside the site in the outer
harbour of Terneuzen.
However, the original plans had to be adjusted because, in 1996 the govern-
ment - also in the light of a project in which the important fairway for
Antwerp would be deepened - made a statement that the dumping of exca-
vated soil on locations in the Westerschelde had to be examined more
closely. This meant that, in view of the Pollution of Surface Waters Act it no
longer sufficed to apply for a permit via the usual procedure; an extensive
investigation, being an Environmental Impact Assessment (EIA), had to be
set up, and that fact put the planning of the boring project under pressure.
In the spring of 1999 the boring had to start and if the correct permits were
not granted in good time, it would cause delays. That is why a separate
subproject group (EIA - cuttings) was started in order to carry out the inves-
tigative procedure within the time available.
Search WWH ::
Custom Search