  Granular media (GM) present locomotor challenges for terrestrial and
extraterrestrial devices because they can flow and solidify in response to
localized intrusion of wheels, limbs, and bodies. While the development of
airplanes and submarines is aided by understanding of hydrodynamics,
fundamental theory does not yet exist to describe the complex interactions of
locomotors with GM. In this paper, we use experimental, computational, and
theoretical approaches to develop a terramechanics for bio-inspired locomotion
in granular environments. We use a fluidized bed to prepare GM with a desired
global packing fraction, and use empirical force measurements and the Discrete
Element Method (DEM) to elucidate interaction mechanics during
locomotion-relevant intrusions in GM such as vertical penetration and
horizontal drag. We develop a resistive force theory (RFT) to account for more
complex intrusions. We use these force models to understand the locomotor
performance of two bio-inspired robots moving on and within GM.
