MAINTENANCE ASPECTS - The Westerschelde Tunnel: Approaching Limits - page 272

Introduction
The Westerschelde Tunnel is the first bored tunnel in the Netherlands with
a large diameter to be constructed at such great depth and in a corrosive
environment (salt water). Due to the visibility onto the concrete tunnel
walls being eliminated because of the application of the road foundations
and the heat-resistant cladding, it is not easy to assess the condition of
the lining. However, inspection and monitoring is of vital importance: a life
span requirement of 100 years has been set on the design, but does the
tunnel also comply with this in practice; does the reinforcement, particu-
larly at the repair spots for example, corrode sooner than expected? What
maintenance is necessary, should repairs be carried out and what are the
costs involved in this? The contractor is responsible for the maintenance
of the tunnel in the first 10 years. Yet, to ensure adequate maintenance,
the first requirement is to have a good picture of the processes which -
being out of sight - play a role in the lining. This is particularly important
because specifically during the starting phase of the construction of the
tunnel, damages came about to the tunnel lining.
However, at the start of the project the monitoring of bored tunnels was
still rather virgin territory. The specific design requirements in combination
with the exceptional implementation aspects and their consequences,
therefore made it necessary to initiate new developments in the field of
inspections and monitoring. The client therefore actually decided to have
a 'pilot' carried out which incorporated various detection technology tests
in order to examine how a monitoring system could be designed.
Firstly, this chapter deals with the development of a monitoring system for
the Westerschelde Tunnel, thereafter the maintenance system is dealt with.
Development of monitoring system
The importance of inspections and monitoring
In 1999 the importance of inspections and monitoring was again clearly
emphasized due to a serious incident which occurred in a tunnel in Japan: as
a consequence of negligent maintenance a heavy piece of concrete (1,800 kg)
fell out of the tunnel wall in a train tunnel. Monitoring and inspecting must be
geared as such, that every possible damage factor is recognised timeously, so
that the tunnel can be optimally managed and maintained on this basis.
As mentioned before, the WesterscheldeTunnel was designed for a life span
of 100 years, in which the corrosion of the reinforcement of the tunnel wall
has been taken as the normative service limit state. Corrosion comes about
in places on the inner surfaces and joints of the segments due to the pene-
tration of CO 2 (carbonatation) and on the outer surfaces due to the penetra-
tion of chloride which comes from the salty ground water.The inner surfaces
and joints are also exposed to the penetration of chloride due to leakage of
the salty ground water and thawing salts. The limit state is a service limit
state in which a reliability index of at least 1.8 applies.This means accepting
a 1% chance that reinforcement could corrode within 100 years.
Damage during the implementation
In calculating the design of the segments on life span, no consideration was
taken into account of damage occurring to the lining during the construction