Soil failure mode in front of a multiple-tip horizontally operated penetrometer

Authors: ABBAS HEMMAT, AZAR KHORSANDI, VAHAB SHAFAIE

Abstract: Mechanical resistance of a soil to failure has been widely used to estimate the degree of soil compaction. Our previous study showed that the magnitude of force measured by the horizontally operated penetrometer depends on the soil failure mode in front of the sensor tip. In this research, 2 horizontally operated penetrometers were developed to investigate whether the critical depth (dc) at which the soil failure transition from brittle to compressive occurs is dictated by the aspect ratio (working depth/width ratio) of the tip or of the shank of the sensor. In the first sensor, the width of the shank (18 mm) was the same as the base dimension of the prismatic tip. In the second sensor, the width of the shank (36 mm) was twice as wide as the base dimension of the tip. The sensors were tested in a field with silty clay loam texture and gravimetric water content of 4% to 18% at a forward speed of 0.5 m s^{-1}. The results showed that the dc for the 36-mm-wide shank sensor was approximately 210 mm, whereas the dc was at a depth of 110 mm for the 18-mm-wide shank sensor. In both sensors, the aspect ratio was about 6, and the dc was dictated by the aspect ratio of the shank. The results indicated that dc increases with an increase in the width of the shank. There was no interaction between the adjacent 100-mm-distance tips as long as they operated below the dc. Visual observations showed that, in some cases, when the prismatic tip was located slightly above the dc, a trace of the tip in undisturbed soil could be vividly seen. This revealed that the tip was moving ahead of the soil rupture planes that radiated from the shank shin to the surface.

Keywords: Brittle failure, compressive failure, critical depth, horizontal cone penetrometer, precision farming, soil compactiontor

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