Borehole imagery is a very useful method to obtain discontinuity properties but its limits should be taken into account. We have worked on the investigations realized on a tunnel project in the French Pyrenees. Two horizontal core drillings were made and optical camera loggings were performed. We worked on portions of the boreholes to compare the discontinuities obtained on loggings imaging with manual measurements realized on core samples. Our study shows that even when borehole imagery is very well performed, biases are encountered: on one of the core drilling under study 32% of fractures measured on borehole samples were not identified on borehole images, and nearly 60% of the discontinuities seen in images were stratifications that have no mechanical role. Handmade characterization is quite long and needs high quality samples that are not easy to obtain. We focused on differences revealed by these methods and their consequences on tunnelling conditions.

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