SAFETY - The Westerschelde Tunnel: Approaching Limits - page 39

already provided for was valued at more than 102 million euro (included in a
contract price of 494 million euro), and was thus increased to approximately
147 million euro.
The extra measures involved:
- a reduction in the distance between the cross connections from 500 to
250 metres;
- wider emergency doors;
- the introduction of a system in which the emergency doors open imme-
diately in the incident tube as well as the non-incident tube;
- optimization of the use of the escape possibilities;
- the automatic start-up of the ventilation system on detection of an acci-
dent; though having the possibility to interrupt this start-up process;
- a greater layer thickness of the heat-resistant cladding;
- replacement of the 'New Yersey' profile on the side of the road by a so-
called 'Step barrier' with which the chance of a vehicle tilting when collid-
ing with the tunnel wall is reduced;
- an overtaking prohibition for all the non-passenger vehicle traffic;
- the establishing of flexible maximum speeds (80 to 100 km/h, dependent
on the traffic);
- the set-up of an integral safety plan: it is the first time that an integral safety
plan has been drawn up for a mega project such as the Westerschelde
Tunnel, in which for the construction, as well as for the operational phase,
agreements have been recorded between all the relevant parties about
what to do during serious and less serious accidents in the tunnel.

Probabilistic approach - definition framework

The probabilistic approach considers safety in terms of risks. In order to be
able to make a statement about safety, a large number of accident scenarios
are worked out on their chances and associated consequences. Risk can be
seen as the chance of an unwanted event in combination with the conse-
quences of that event. An actual differentiation here, is the one between
'individual risk' (or localized risk) and 'group risk'.The individual risk, shows
the chances of a person suffering a certain detrimental effect as a result of
being exposed to a risky situation (expressed in chances per annum). The
group risk, shows the chances at a single point in time, of a group being of
at least a certain size, becoming the victim of a disaster. Further, 'external

Fig. 4.2
Accepted societal risk
for the Westerschelde
Tunnel

Proposed standard

10 Gradient downward

Horizontal part
Gradient upward
Total tunnel
Averaged per km

1

10 1

10 2

10 3

Road users
Anyone in the
vicinity of the road

10 4

10 5

10 6

10 7

10 8

10 9

1 10 100 1,000 10,000

Number of fatalities