DEM modelling of root circumnutation-inspired penetration in shallow granular materials

Plant root growth is often accompanied by circumnutative motion consisting of downward helical movement of the root tip. Previous studies indicate that circumnutations allow roots to avoid obstacles that would impede root growth, while other studies show that circumnutations can reduce the penetration resistance mobilised during root growth. Discrete-element modelling (DEM) simulations were performed on probes that employ circumnutation-inspired motion (CIM) to penetrate granular assemblies at shallow depths to evaluate the reduction in penetration resistance. These simulations investigate the effect of the ratio of tangential to vertical velocity of the circumnutative motion (i.e. relative velocity) and of the probe geometry (i.e. tip tilt angle and length). The results indicate that CIM penetration reduces the penetration force and work relative to non-rotational penetration (NRP) by changing the soil fabric and diffusing the force chains around the probe tip. However, the circumnutative motion leads to an increase in torque and associated rotational work. An optimal relative velocity and probe geometry exist for the simulated CIM probes, resulting in a smaller total work than that required for NRP. CIM penetration also mobilises smaller penetration forces and work than rotational penetration (i.e. with a straight tip), particularly at smaller relative velocities. The reduction in penetration forces induced by CIM could facilitate site investigation and monitoring activities.