  It is possible to build free-standing, load-bearing structures using only
rocks and loops of elastic material. We investigate how these structures
emerge, and find that the necessary maximum loop spacing (the critical spacing)
is a function of the frictional properties of the grains and the elasticity of
the confining material. We derive a model to understand both of these
relationships, which depends on a simplification of the behavior of the grains
at the edge of a structure. We find that higher friction leads to larger stable
grain-grain and grain-loop contact angles resulting in a simple function for
the frictional critical spacing, which depends linearly on friction to first
order. On the other hand, a higher bending rigidity enables the loops to better
contain the hydrostatic pressure of the grains, which we understand using a
hydroelastic scale. These findings will illuminate the stabilization of dirt by
plant roots, and potentially enable the construction of simple adhesion-less
structures using only granular material and fiber.
