Plant Root Circumnutation–Inspired Penetration in Different Soil Types and Its Engineering Application

Soil penetration is a critical aspect of geotechnical site investigation, sensor installation, and foundation construction that often face challenges due to the large vertical reaction forces required to penetrate stiff or deep soil layers. This study adapts a plant root soil penetration strategy, called circumnutation, to reduce penetration resistance in soil types ranging from sands to clays. The circumnutation-inspired (CI) probe is similar to a cone penetration test (CPT) probe, but it has a bent tip at an angle (α). The CI probes are rotated at an angular velocity (ω) while simultaneously advanced at vertical velocity (v) to obtain a target ωR/v and ωDc/2πv, where R is the tip’s offset from the axis of rotation and Dc is the diameter of the probe. As ωR/v or ωDc/2πv is increased, the vertical penetration force (Fz) decays exponentially, the torque (Tz) rises then plateaus, and the total work done (WT) remains stable before increasing for all of the different soil types tested. CI probes with α=10° or 30° reduce Fz by 86%–93%, while rotational CPTs (α=0°) achieve a reduction of 21–75%. Fz and Tz show linear correlations with the CPT penetration force (Fz,CPT). The slope of the correlation between Fz,CPT and Fz depends on ωR/v or ωDc/2πv, while the correlation with Tz does not. These slopes are independent of soil type for CI probes with α=10° or 30°. However, the slope between Fz,CPT and Tz for rotational CPTs depend on soil type, with clayey soils having higher Tz than sands. This study highlights that CI penetration significantly reduces the vertical penetration force and suggests that CI measurements can be used to estimate CPT qc, which can enable evaluation of soil properties from CI soundings.