Monitoring rock reinforcement works with ambient vibrations: La Bourne case study (Vercors, France)

- Innovative monitoring of an unstable rock column bolting with ambient vibrations
- Rock column's resonance frequencies increase during bolting.
- The rise is related to the additional stiffness provided by the steel rebars.

This study uses ambient vibrations to monitor rock bolting works, the efficacy of which is usually difficult to estimate. The test site is a 760 m3 unstable limestone column located in the Bourne valley (Vercors, France). The rock column's resonance frequencies (fX) were identified, corrected for reversible thermal effects and monitored over time. We observed clear increases in fX up to ~ 17% resulting from the additional stiffness provided by the steel rock bolts. Numerical modeling helped to confirm that the rebar elements were grouted on both sides of the fracture. The major column's resonance features, such as the resonance frequencies and mode shapes, were successfully simulated, including their evolution with bolting. The amount of fX increase depends on the mode considered, likely controlled by bolt location in comparison with the modal shape. This study confirms the potential of ambient vibrations to provide global in-depth information on the stability evolution of rock compartments, with versatile applications for monitoring potential rockfalls or reinforcement works.