SAFETY - The Westerschelde Tunnel: Approaching Limits - page 37

Introduction
In designing the Westerschelde Tunnel, a lot of attention was paid to the
safety aspects. The design therefore provided for a major number of
measures, of which the costs amounted to approximately 30% of the
entire construction budget. At the time that the design was already at a
well-advanced stage, intensive discussions about the level of the tunnel's
safety led to an even further increase in precautions and to the setting up
of an integral safety plan. The principle of the over-all safety philosophy
which formed the brief for the design right from the start, is the so-called
'safety chain'. Basically the chain contains the following links:
- pro-action : designing in such a manner that, the occurrence of a dis-
aster is ruled out as far as possible;
- prevention : reducing the chances and the consequences of a possible
disaster;
- correction (also preparation): offers people the possibility of rescuing
themselves when there are no emergency services present as yet;
- repression : take precautions which make adequate emergency assist-
ance possible;
- aftercare : ensure a proper winding up and recovery to the original situation.
Development of the considerations about
safety in tunnels
Up to about the middle of the 1990s, the development of the considerations
about safety could tag along 'rather gently' at the rate in which new traffic
tunnels were being designed and built. In a relatively trouble-free fashion
and to the satisfaction of the various interested parties such as the emer-
gency services, the requirements in the field of safety were met for each tun-
nel project.
In the initial years of tunnel construction in the Netherlands, the manner in
which the entire tunnel design was envisaged, was particularly a question of
practical insight: the emphasis in particular, lay in the taking of precautions
which were known or which were assumed as being contributory to the safety
level of the tunnel.
In the 1970s the probabilistic approach underwent major development. In
this, the safety risk in a tunnel is based on a quantitative risk analysis. The
key question to which the risk analysis must find an answer is: 'given a set
of facilitating safety precautions, what may still be the chance of a certain
type of accident and what consequences can be expected?'This approach is
geared towards an extensive analysis of series of events and mistakes.
Discussion on the level of safety for the
Westerschelde Tunnel
In the second half of the 1990s the considerations about safety in tunnels
gained momentum; not in the least due to profound discussions about the
level of safety of the Westerschelde Tunnel, of which the design was already
at a well-advanced stage. The opinion arose that a consideration about
chances (the probabilistic approach) does say something about the expected