Sensitivity to solid volume fraction of gravitational instability in a granular medium

We report experimental results on incipient dynamics of gravitational destabilization in an immersed granular medium. Two protocols have been used to reach instability: a progressive loading and a dam-break situation. The instability triggering, analyzed by high-speed imaging and CIV post-processing, reveals distinct dynamical behaviours depending on the initial packing fraction of the medium. Two destabilisation modes have been observed: a surface avalanche and a deeper phenomenon that we called circular collapse.