Topsoil shear strength – Measurements and predictions

Abstract

Distortion of soil from shear stresses during field traffic may be even more damaging to soil functions than compression related to isotropic stresses. A torsional shear test was applied to undisturbed topsoil samples drained to either of six matric potentials (range -30 to -300 hPa) and tested at either of six normal loads, NL (range 30–180 kPa)(Series A). Soil clay and organic matter (SOM) content ranged 0.038−0.157 and 0.021−0.033 kg kg−1, respectively. Bulk density (BD) varied from 1.19 to 1.71 g cm-3. The preload suction stress, PSS, was calculated as the product of soil pore water saturation and the water suction in hPa. PSS ranged from 17 to 234 hPa. Data from two independent data sets with contrasting soil texture (Series B: loess soil) or management (Series C: long term fertilization) but tested with the same methodology were included for validation of prediction equations established from Series A soils. Multiple regression revealed that soil cohesion taken as the intercept term in a linear regression of shear strength, τ, and applied NL correlated positively to PSS, BD and SOM, while the internal friction estimated as the slope in regression was poorly explained by soil properties. A model combining NL in tests with PSS, clay, BD and SOM accounted for more than 90 % of the variation in τ and is suggested as a pedotransfer function (ptf) for prediction of τ for given soil and loading conditions. The model predicted well τ measured in Series B and C soils. Also subsoil τ measured for similarly textured samples was reasonably predicted by the suggested ptf. In accordance with theory, PSS seems to be a key driver of soil mechanical strength. More measurements of soil shear strength and further studies of the effect of PSS are encouraged.