Engineering analysis on periodic pedal motion of clams during foot penetration

Clams are bivalve species that burrow into substrate using a characteristic organ termed 'foot'. Their burrowing process involves a repetitive sequence of movements termed the digging cycle. This study focuses on the initial phase of the digging cycle: foot penetration into substrate, as a step toward clarifying the burrowing mechanism from robotics and soil mechanics perspectives. We proposed that the periodic pedal motion plays a key role in facilitating foot penetration through reduction of substrate resistance. To identify the essential characteristics, this motion was decomposed into three directional components focusing on the physical interaction between the biological structure and the substrate. To examine these components, two hypotheses were proposed: firstly, that longitudinal substrate agitation facilitates foot penetration; and secondly, that lateral and transverse agitation contributes to this process. Three experiments using 3D-printed foot models were conducted to test these hypotheses, defining the penetration (longitudinal) direction as vertically downward. The first experiment examined the effects of longitudinal agitation by measuring vertical resistive forces during penetration into saturated sand, both with and without vertical reciprocating motion. The second experiment evaluated the effects of the lateral and transverse agitation by measuring the penetration displacement of the model resulting from the application of horizontal reciprocating motion under a constant vertical load. Finally, the third experiment further tested the second hypothesis to examine the effect of lateral agitation under conditions that more closely mimic a biological foot, by incorporating a deformable foot model into the experimental setup of the second experiment. The results indicate that lateral motion is a key factor in facilitating foot penetration under reduced force, whereas the longitudinal component contributes little when acting alone. Based on fundamental theories in soil mechanics, the primary mechanism of this effect was attributed to a reduction in effective stress caused by factors such as liquefaction.