Civil Engineering Reference
In-Depth Information
busy for quite a while. Diving operations were carried out for five consequen-
tial days and fatigue can play an important role in the coming about of the
decompression illness. Besides that, the high temperature in the excavating
chamber and the cooling down during the decompression, more than likely
also played a role. All cases of decompression illness were treated and
cured.
Welding at 3.6 bar; reducing the fire hazard
An exceptional operation was the welding of injection nozzles onto the
cutting wheel. These nozzles were necessary to bore through the rigid
Boom clay easier.
Worldwide there was hardly any experience with welding at a pressure of
3.6 bar; at such pressure everything burns at an explosive rate. Experience
with welding activities at the construction of the Elbe Tunnel in Germany
had shown that a rubber shoe sole burns away in mere seconds. A solution
from the offshore industry is the reduction of the oxygen content being worked
under. At the WesterscheldeTunnel this was not possible and that is why the
precautions were concentrated on the prevention of fire. The welder wore
special non-combustible clothing and a breathing mask, while another diver
guarded his activities with a fire extinguisher at hand. Besides, there were
extra smoke extractors placed around the 'work place'. The door to the decom-
pression chamber was also almost completely closed, an over pressure
prevented any smoke from entering there.
Greatest danger: excavation front instability
One of the greatest (real) dangers when working in the excavation chamber
and in front of the cutting wheel was that of an excavation front instability
(blow-out). Such an instability happened twice in the preparation of diving
activities. Fortunately, in both cases, no divers were at the excavation
front. If that were the case, then the situation would certainly have been life-
threatening. However, up to a certain point, the divers were - through their
training and experience with other projects - in a position to detect the com-
ing about of such a situation: for example, the leaking away of the air in
which activities take place during operations is of influence to the behaviour
of the bentonite level in the excavation chamber. Besides, the pressure in the
excavation chamber is permanently monitored during the diving operations.
A change in pressure could indicate the collapse of the excavation front.
Special gas mixture for working at greater depths
At pressures greater than 3.6 bar the danger of 'nitrogen narcosis' became too
great - certainly for lengthier operations. That is why use was made of
a special gas mixture which divers can inhale via a helmet. The helmet itself
was cooled with compressed air to prevent the visor from misting over. At pres-
sures up to 5.1 bar a special gas mixture was used. This consisted of 25% oxy-
gen, 50% nitrogen and 25% helium. In order to determine the reliability of the
decompression schedule used, the divers underwent so-called 'bubble' tests.
Saturation work
At the deepest point of the boring route the pressure was a little higher than
7 bar.The decompression of 7 bar to atmospheric circumstances takes many
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