  When a rod is vertically withdrawn from a granular layer, oblique force
chains can be developed by effective shearing. In this study, the force-chain
structure in a rod-withdrawn granular layer was experimentally investigated
using a photoelastic technique. The rod is vertically withdrawn from a
two-dimensional granular layer consisting of bidisperse photoelastic disks.
During the withdrawal, the development process of force chains is visualized by
the photoelastic effect. By systematic analysis of photoelastic images, force
chain structures newly developed by the rod withdrawing are identified and
analyzed. In particular, the relation between the rod-withdrawing force
$F_\mathrm{w}$, total force-chains force $F_\mathrm{t}$, and their average
orientation $\theta$ are discussed. We find that the oblique force chains are
newly developed by withdrawing. The force-chain angle $\theta$ is almost
constant (approximately $20^{\circ}$ from the horizontal), and the total force
$F_\mathrm{t}$ gradually increases by the withdrawal. In addition,
$F_\mathrm{t}\sin\theta$ shows a clear correlation with $F_\mathrm{w}$.
