A horizontal multiple-tip penetrometer for on-the-go soil mechanical resistance and acoustic failure mode detection

Soil mechanical resistance can be used as an indicator of soil compaction. For on-the-go mapping of spatial variability in soil compaction, single and multiple-tip horizontal penetrometers have been developed and used to measure the soil mechanical resistance. However, it has been reported that the measured soil resistance in different soil layers depends not only on the degree of soil compactness but also on soil failure mode induced by the shank of the sensor. It was hypothesized that the differences in sound signals collected by microphones during penetration tests could be used to differentiate the failure modes. In this research, an acoustic multiple-tip horizontal penetrometer was developed, with three 30° prismatic tips attached horizontally to S-shape load cells and worked at depths of 10, 20 and 30 cm. The tips working at 10 and 30 cm depths were also fitted with microphones. The sensor was tested in a field with a clay loam soil. The sound signal was first de-noised using wavelet method, and then frequency spectrum and power spectral density of the signals were obtained by fast Fourier transform and Welshʹs method, respectively. When the prismatic tips were operated below the critical depth of the sensor (tips at depths of 20 and 30 cm), there was a significant relationship between horizontal resistance index (HRI) and the cone index measured by a vertically-operated cone penetrometer; whereas for the shallower depth (10 cm) the relationship was not significant. The power of the sound recorded of the tip passing through the disturbed soil above the critical depth (10 cm) was much lower than when the tip was penetrating the undisturbed soil located below the critical depth (30 cm). The increase in power of the acoustic signal with depth was in line with the increase in the measured HRI. It can be concluded that the developed combined acoustic penetrometer can both detect soil failure mode and measure soil horizontal resistance.