Regular inspections of tunnels are necessary for documenting their actual conditions, clarifying the evolution rate of detected problems and planning repairs. According to the traditional inspection procedure for tunnels belonging to the Swiss railways, (SBB 2007a, b) all the identified problems should be mapped in a specific inspection drawing once each 6 years. This means that according to the tunnel size, length and conditions a visual inspection may often require a quite long time to be completed. Today, in spite of an increasing traffic and demand for safe operations the time for performing inspections and maintenance of the infrastructure is constantly reduced. Therefore, it is necessary to develop new systems for tunnel survey and maintenance. Those systems should be able to obtain more accurate results in a shorter time.
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