  We present both experimental and numerical investigations of compaction in
granular materials composed of rods. As a function of the aspect ratio of the
particles, we have observed large variations of the asymptotic packing volume
fraction in vertical tubes. The relevant parameter is the ratio between the rod
length $\ell$ and the tube diameter $D$. Even the compaction dynamics remains
unchanged for various particle lengths, a 3d/2d phase transition for grain
orientations is observed for $\ell/D = 1$. A toy model for the compaction of
needles on a lattice is also proposed. This toy model gives a complementary
view of our experimental results and leads to behaviors similar to experimental
ones.
