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concrete covers were applied and it was implicitly assumed that with that,
the required 100 year life span could be achieved. When it was decided to
construct bored tunnels in the Netherlands, the life span requirement took
up a prominent position. The social benefit and the high construction costs
were the basis of this requirement.
Fig. 6.1
Storage of segments
DuraCrete provides relief
That the Westerschelde Tunnel had to last at least 100 years, was also a
requirement set for the design. However, in the contract no specification
was included in what manner this had to be proven. In other words, what
limit state could not be exceeded in 100 years and how big was the chance
allowed to be that this happened?
The ultimate limit state would obviously be exceeded if the tunnel collapsed.
Yet at an earlier stage the limitations could also be exceeded, for example in
cases of leakage or when concrete pieces fall off the tunnel wall. This could
give the users of the tunnel an unsafe feeling. In such cases it's not about
the maximum (failure) limit state, but about the service limit states or usable
service limit states. Of course it is possible to repair damages and thus post-
pone the exceeding of limit states. However, the question is whether that is
an optimum situation in an economic sense: if the tunnel has to close every
15 years for the undertaking of maintenance and repair work, that also
means a loss of income, because during the maintenance period no tolls
can be charged.
The requirement of a life span of 100 years was also prompted by this
economic interest: the tunnel may only close when absolutely necessary,
and that is why the design had to be geared so as to limit the maintenance
substantially.
The absence of a sound method to determine the life span, and the vague-
ness about what limit state it concerned, has lead to extensive discussions
between the client and the contractor. Both parties decided to call in the
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