  We investigate the evolution of multi-scale mechanical properties towards the
macroscopic mechanical instability in frictional granular media under
multiaxial compressive loading. Spatial correlations of shear stress
redistribution following nucleating contact sliding events and shear strain
localization are investigated. We report growing correlation lengths associated
to both shear stress and shear strain fields that diverge simultaneously as
approaching the transition to a dense flow regime. This shows that the
transition from quasi static to dense flow regime can be interpreted as a
critical phase transition. Our results suggest that no shear band with a
characteristic thickness has formed at the onset of instability.
