Fragile granular jamming*

We demonstrate experimentally that the route to a jammed state for a set of bi-dispersed frictional disks, subjected to uni-axial compression from a random initial unjammed state, consists of a consolidation state, a fragile jammed state, and finally a rigid jammed state. In the consolidation regime, the pressure on the sides increases very slowly with the packing fraction $\phi$, and there are no detectable stress chains. In the fragile jammed state, stress chains are visible, the pressure increases exponentially with $\phi$, and the fraction of moving disks drops exponentially. Eventually, a final regime where particle displacements are below our resolution and the pressure varies approximately linearly with $\phi$ is reached. We argue that this scenario is generic for athermal frictional compressed particles.