The RER E tunnel, which has a diameter of 6.40 metres, was built between 1992 and 1995, and consists of cladding with arch liners in 1.4-metre thick reinforced pre-stressed concrete. Since the construction of the structure, longitudinal hairline cracks on keystones are present in several sections and appear to be spreading. Fearing that the cracks may spread, the client wished to examine the phenomenon that created them. LIRIS standalone Optical Strands were therefore positioned on cross sections, on the keystones, the haunches and the sidewalls at the level of the cracked arch liners. During the readings taken in accordance with the client's wishes, results revealed that the deformations revealing compression or decompression of the arch remained within a safety range. The client could thus be reassured regarding the behaviour of the arch and was able, if required, to carry out further investigations ahead of schedule.