Civil Engineering Reference
In-Depth Information
The production of the deviating segments in any case meant, that the entire
production line - from the 'converting' of the formwork to the eventual
're-measuring' of the segments - called for adaptation.
Reasonable cause: depth and glauconitic sand
It seems justified to conclude that a relationship exists between the 'area'
where the boring machines were positioned and the coming about of deform-
ations. Important clues for this were:
- both boring machines were confronted with the problem;
- the problems were at their severest at the deepest point (60 metres
NAP)
when the boring machines were situated in glauconitic sand;
- from the moment that the boring machines changed course upwards to
soil lying less deep and they left the glauconitic sands, the deformations
lessened.
In designing the boring machines, attention was particularly geared towards
possible sticking problems in the Boom clay. However, the problems didn't
occur in the Boom clay, but at the time that the boring machines were situ-
ated in the glauconitic sand.
A lesson to be learnt from the deformations issue is that, (multi-disciplinary)
research actually undertaken prior to such risky projects can never be too
abundant; it is traditionally known that soil always contains 'surprises'. A
remedy is provided by a good interaction between geo-technicians, struc-
tural engineers for steel and machinery manufacturers and the taking into
consideration of ample safeties in the design of the boring machine. For
such risky projects it is of vital importance that all possible available know-
ledge is brought together.
Leakages due to the bentonite valve breaking
off (westerly TBM)
In the boring shield various injection openings were applied with which the
bentonite suspension could be injected around the shield. This bentonite
served as a lubricant to reduce the friction between the shield and the soil,
and thus also reducing the required jack forces for forward propulsion.
From the time of the coming about of the shield deformations, these injec-
tion points have all been used.
After the shield was measured off to establish the development of the deform-
ation after the building-in of ring 547, the fitting out of the next ring was pre-
pared. The preparation also comprised the cleaning of the underside of the
shield and the pumping away of leak water and cleansing water which col-
lected there. However, on withdrawing the jacks in order to be able to place a
new segment, the clearing of one pump had been forgotten.
The receding jack pairs dragged this pump along over an injection pipe which
then broke below the level of a valve. A mixture of water, sand, grout and
bentonite immediately squirted into the tunnel boring machine and into the
tunnel at a pressure of approximately 6 bar - at that point in time the TBM
was positioned at the deepest point of the boring route.
Consequences
Attempts to close the hole with wood and fabric didn't work and the TBM
(and the tunnel behind) slowly filled up with the incoming 'muddy mixture'.
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