The Westerschelde tunnel in The Netherlands has recently opened for road traffic. The construction of the longest (in The Netherlands) and deepest shield tunnel was finished in March 2003. Two slurry-shield TBMs have drilled the two tunnel tubes with a length of 6.6 km at a maximum of 60 m below the Westerschelde river. The tunnel has a 100 year design lifetime. The probabilistic design of the lining has been based on the Duracrete model. During construction a number of the concrete segments and rubber seals were damaged. Where possible, these damages have been repaired using the latest technology, but the durability of the repaired spots cannot be assessed easily. It is not likely that all repairs will be as durable as the undamaged concrete and seals. On the tunnel wall a fireproofing has been sprayed, thus limiting the possibilities for visual inspection. Although the probability of any major defect occurring in relation to a repaired spot is very small, the consequences could be enormous. Therefore the tunnel owner decided to start a research program into: (1) the most possible and likely deterioration mechanisms that could lead to structural damage; (2) the options for inspection and monitoring of the initiation and propagation of these possible mechanisms, in order to get an early warning; this research is focussed on a sensor based monitoring system; (3) if possible, a demonstration with a trial system, to prove the technical feasibility; (4) a proposal for a system spread that will cover both tunnel tubes sufficiently. The three most likely deterioration mechanisms that have been identified are: (1) Reinforcement corrosion may occur in relation to the damaged and repaired concrete spots; (2) The damaged and repaired concrete spots and rubber seals may in time lead to leakage of water; (3) And the concrete repairs above the road may disbond and eventually fall down. In advance of the installation of the demonstration system, measurements have been performed to assess a baseline and to get insight into the variation of the electrochemical potential of the reinforcement steel. This information is also used in the selection of the locations for sensor installation. The trial system consisted of (1) wireless strain gauges, placed at the inner concrete surface on the interface between original concrete and repair spot, to detect differential movement, (2) leakage detectors, to detect the presence of water in the joints between segments, (3) multi-ring electrodes, to assess the water content of the inner cover of the concrete and (4) reference electrodes, for electrochemical potential measurements. After installation all instrumentation was covered with the fireproofing material. The system was completed with a recording of the discharge of the drainage pumps, to know any significant change in the overall leakage of the tunnels. The trial system is operational since December 2002 and the performance is satisfactorily. The system is producing sensible and credible results, all in line with the expected absence of any deterioration at this moment. Should any of the deterioration mechanisms indeed start to develop, it is still expected that the first significant activity will not be recorded for several years. As a result of the satisfactorily performance of the trial system, an extended system is now in preparation. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.