Civil Engineering Reference
In-Depth Information
Introduction
At the time that the Westerschelde Tunnel project started, not much knowl-
edge had been gained about boring techniques under the soils of the
Netherlands. The first bored traffic tunnel - the Second Heinenoord Tunnel -
was not even completed at the time. During the preparations however, les-
sons could be learnt from the experience gained in the construction of this
tunnel, also because a similar type of boring shield was used here. Yet there
could be no question about copying the solutions: the circumstances during
the boring of the Westerschelde Tunnel were of a totally different order. For
instance:
- the length to be bored being 6,600 metres;
- the tremendous depth at which activities had to take place (more than
60 metres
NAP) and the associated massive ground and water pressures;
- the large diameter of the tunnel tubes;
- the use of 2 tunnel boring machines, in parallel drives;
- the soil conditions which are quite unusual for the Netherlands;
- the necessity to construct cross connections;
- the complex logistics as a result of the simultaneous boring and (civil)
fitting out of the tunnel.
Briefly: these were circumstances which not only made the construction
of the Westerschelde Tunnel an unprecedented and unique project for the
Netherlands, but also for Europe. The unprecedented and revelatory char-
acter also involves a certain degree of uncertainty: during the implementa-
tion, situations could occur of which the possible eventuality was not known
beforehand.
In this chapter, a number of experiences gained during the boring of the
twin tubes of the Westerschelde Tunnel are briefly dwelled upon.
Boring through the impermeable block;
a toilsome course
Originally it was assumed that the eastern tunnel boring machine - Sara
was the first heading for Zuid-Beveland - would only need a few days
for boring through the impermeable block. It happened differently to what
was envisaged: shortly after commencement stagnation occurred, which
eventually resulted in a delay of some weeks. The cutters appeared to be
worn out.
The impermeable block, which was made at the front end of the starting
shaft, consisted of a layer by layer compacted sand-cement stabilization within
the construction pit. In the seven month period prior to the commencement
of the boring this layer had attained a greater strength than was intended.
Besides, it appeared that the impermeable block was built up of relatively
fine material. Due to this the cutting teeth of the cutting wheel were not able
to break coarser sections out of the block, in other words, it was more a mat-
ter of 'grinding' the impermeable block than excavating, and that resulted in
serious wear and tear of the cutting teeth.The contractor found the solution
to the problem by using cutting teeth with claws which could excavate
larger pieces. The switching of the cutting teeth happened in the excavation
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