  RoboClam is a burrowing technology inspired by Ensis directus, the Atlantic
razor clam. Atlantic razor clams should only be strong enough to dig a few
centimeters into the soil, yet they burrow to over 70 cm. The animal uses a
clever trick to achieve this: by contracting its body, it agitates and locally
fluidizes the soil, reducing the drag and energetic cost of burrowing. RoboClam
technology, which is based on the digging mechanics of razor clams, may be
valuable for subsea applications that could benefit from efficient burrowing,
such as anchoring, mine detonation, and cable laying. We directly visualize the
movement of soil grains during the contraction of RoboClam, using a novel
index-matching technique along with particle tracking. We show that the size of
the failure zone around contracting RoboClam, can be theoretically predicted
from the substrate and pore fluid properties, provided that the timescale of
contraction is sufficiently large. We also show that the nonaffine motions of
the grains are a small fraction of the motion within the fluidized zone,
affirming the relevance of a continuum model for this system, even though the
grain size is comparable to the size of RoboClam.
