In‐situ Measurement of Near‐surface Soil Strength by the Fall‐cone Device

Abstract This study explored the use of the Swedish fall‐cone device for insitu measurements of shear strength near the surface of soils. A laboratory fall‐cone device was adapted for field use by replacing the original base and lengthening the cone stems. In‐situ strengths were measured on selected soils in southwestern Iowa and southeastern Nebraska. A small increase in water potential resulted in a large decrease in shear strength. A heavier cone gave a lower strength value than a lighter cone. The near‐surface strength of Sharpsburg soils in winter wheat fields was measured throughout the year. The strength immediately after fall planting was 10 to 20 kPa at about −3 kPa water potential. In the spring of 1980, largely because of consolidation from rainfall in the immediate post‐thaw period, the strength was 30 to 50 kPa. In contrast, the spring of 1981 was dry and consolidation did not occur; consequently, there was no increase in strength from that immediately after planting.