A LIFE SPAN OF AT LEAST 100 YEARS - The Westerschelde Tunnel: Approaching Limits - page 65

assistance from the Dutch companyTNO Bouw (construction) and the German
engineering company Schiessl/Raupach. These companies were closely
associated in the development of a method - DuraCrete - which can be used
to make firm statements about the feasibility of the desired life span. On the
basis of this method, which assumes a stochastic approach, it was possible to
prove by means of calculations, that the design of the tunnel complied with
the requirement of a life span of 100 years.

Chances of failure and acceptance

In view of the costs, it is almost impossible to design a structure in such a
manner that the chances of exceeding all the limit states over a period of
100 years, is ruled out. On the other hand, the employment of an average
life span of 100 years is not acceptable; after all, an average implies that
there is a 50% chance that no limit state will be exceeded in 100 years, as
opposed to a 50% chance that this will happen. The smaller the chance that
a limit state will be exceeded in 100 years, the further the average value for
the life span (as seen in time) lies ahead.
The requirement of a life span of at least 100 years applies for all permanent
sections of the Westerschelde Tunnel. Important elements here are the rein-
forced concrete access ramps, the prefabricated segments of the bored tun-
nel including the special steel segments in which the access doors for the
cross connections are incorporated, the rubber seals around the segments
and the cross connections. Following on to the requirements of the 'Buildings
Decree', there is a reliability index requirement (

) of 3.6 for the exceeding of
the ultimate limit state (the failing of the tunnel). This means a chance of
1/1,000,000, that the tunnel will fail within 100 years is accepted.
For the exceeding of a service limit state, has been set at 1.8. That means,

a chance of 1/100, that the service limit state will be exceeded within 100
years is accepted.

- reference period of 100 years,
- reliability index 3.6 for the ultimate limit state (ULS),

- reliability index 1.8 for the service limit state (SLS)

Example of a life span
spread

Fig. 6.2

Probability Density

- Sta ndard Deviation

Reliability Index

Service Life

0 L 1

Design Service Life Mean Service Life

In brief, the designing for a life span of 100 years thus means: the acceptance
of only a small chance (1%) that a practicable limit state will be exceeded
within the 100 years, caused by an increase of the loads (more and heavier
traffic for example) and a decrease of the strength of the structure.The average
life span is considerably higher: around 180 years.The chance of failure - the
exceeding of a limit state such as the corroding of the reinforcement steel -
during that period, is however also considerably greater: 50%.