  We experimentally study the statistics of force-chain evolution in a
vertically-tapped two-dimensional granular packing by using photoelastic disks.
In this experiment, the tapped granular packing is gradually compacted. During
the compaction, the isotropy of grain configurations is quantified by measuring
the deviator anisotropy derived from fabric tensor, and then the evolution of
force-chain structure is quantified by measuring the interparticle forces and
force-chain orientational order parameter. As packing fraction increases, the
interparticle force increases and finally saturates to an asymptotic value.
Moreover, the grain configurations and force-chain structures become
isotropically random as the tapping-induced compaction proceeds. In contrast,
the total length of force chains remains unchanged. From the correlations of
those parameters, we find two relations: (i) a positive correlation between the
isotropy of grain configurations and the disordering of force-chain
orientations, and (ii) a negative correlation between the increasing of
interparticle forces and the disordering of force-chain orientations. These
relations are universally held regardless of the mode of particle motions
with/without convection.
