SAFETY - The Westerschelde Tunnel: Approaching Limits - page 38

level of safety, but does not define what actually happens or respectively should
happen, when a disaster does indeed occur. Thus it was strongly argued that,
besides the probabilistic approach with which the design of the Westerschelde
Tunnel had been started, a way of thinking had to be dealt with which was
more deterministic in nature: the scenario analysis. In this an analysis is made,
based on a limited number of described accident types, how road users bring
themselves to safety in cases of disaster and whether the emergency services
are in a position to provide assistance in an adequate manner. With this, the
scenario analysis gears itself specifically towards the 'correction (self-rescuing
ability)' and 'repression' links in the safety chain.

Fig. 4.1
Relationship of the
development of a fire
and the available
evacuation time

Development of
smoke, heat and
toxic substances

Intolerable climate

Tolerable climate

Detection phase t

Reaction phase
Evacuation phase

Total available time

Aside from differences, also concurrences

The quantitative risk analysis (probabilistic approach), in fact is also a
scenario analysis with the specific characteristic that the total spectrum
of incidents is viewed in general terms and in very little detail, purely in
terms of chances and the associated quantitative consequences (number of
casualties). The fact that the discussion about the level of safety of the
Westerschelde Tunnel in the first instance, was concentrated on the differ-
ences between both approaches instead of the concurrences, is not surpris-
ing anyway: the quantitative risk analysis provides risk figures which an
emergency provider cannot use very well. Simultaneously, it is awkward for
the designer to adopt an extreme scenario as a starting point for the tunnel
design, if it is not actually realistic that such a scenario would ever become
reality. In the meantime it is clear that the one approach is not better than the
other: both will do justice to each other within an integral concept on safety.

Adjustments

In brief, there were vital criticisms from the emergency services on
the design of the Westerschelde Tunnel, in the way that it formed part of
the contract which was signed in 1996. The result was, that the design once
again had to be reconsidered critically by an independent institution, and
the level of safety was analysed in respect of a 'self-rescuing ability' (cor-
rection) and 'possibilities for emergency services' (repression).
Eventually - after various investigations and reports - the decision was made
to take additional measures valued at approximately 45 million euro. The
most important conclusion was, that the number of cross connections had
to be doubled from 13 to 26.The total number of safety precautions that were