Estimation of surface shear strength of undisturbed soils in the eastern part of northern China's wind erosion area

Shear strength of surface soil that is unsaturated and undisturbed is an important soil mechanical property to predict its resistance against the shear force created by the wind. However, measurement of this property was always complicated and time-consuming. In this work, we obtained unsaturated (soil moisture <13%) and undisturbed surface soil samples, collected in the eastern part of northern China's wind erosion area, and determined the relationships between surface soil shear strength (SSS) and soil properties (root density; the moisture, SM; gravel content, GC; clay, organic matter, OM; calcium carbonate contents, CaCO3; and bulk density, ρb) to provide a function for predicting surface shear strength. Although roots generally strongly influence surface shear strength, the root density was not a proper soil property to reflect the relationship between plant roots and soil surface shear strength. For future study, we suggest that a new definition of root density which merely describe the quantity of the roots penetrating through the shear surface, to replace the conventional one. Surface shear strength decreased logarithmically with increasing water content. Several soil properties were strongly correlated with surface shear strength: the contents of gravel (r = 0.793, p  < 0.001), organic matter (r = 0.771, p < 0.001), and calcium carbonate (r = 0.669, p < 0.05) and the soil bulk density (r = 0.793, p < 0.001). Based on these results, we developed a prediction equation for surface shear strength of surface soil: SSS = -(0.103 GC + 7.76 ρb − 8.88) × ln(SM) + 0.131 GC + 1.655 OM + 2.192 CaCO3 + 19.467 ρb − 27.39. The predicted and observed values were strongly and significantly correlated (r = 0.83, p < 0.001) and fell close to the 1:1 line, with relatively small root-mean-square and mean absolute errors (1.32 and 0.97, respectively).