  Architectural structures such as masonry walls or columns exhibit a slender
verticality, in contrast to the squat, sloped forms obtained with typical
unconfined granular materials. Here we demonstrate the ability to create
freestanding, weight-bearing, similarly slender and vertical structures by the
simple pouring of suitably shaped dry particles into a mold that is
subsequently removed. Combining experiments and simulations we explore a family
of particle types that can entangle through their non-convex, hooked shape. We
show that Z-shaped particles produce granular aggregates which can either be
fluid and pourable, or solid and rigid enough to maintain vertical interfaces
and build freestanding columns of large aspect ratio (>10) that support
compressive loads without external confinement. We investigate the stability of
such columns with uniaxial compression, bending, and vibration tests and
compare with other particle types including U-shaped particles and rods. We
find a pronounced anisotropy in the internal stress propagation together with
strong strain-stiffening, which stabilizes rather than destabilizes the
structures under load.
