Field investigations of different techniques for measuring surface soil shear strength

Abstract

Field measurements of surface soil shear strength were made in three different Sicilian soils (a sandy loam, a silty loam and a clay soil) using four devices (two torvanes, one hand vane tester and one pocket penetrometer). The pocket penetrometer always gave the highest strength values while in most cases the two torvanes gave the lowest. This result was expected because a torvane applies a torsional shear to a soil layer which is much thinner than in the case of the hand vane tester, while the pocket penetrometer generates a compressive and shear type failure at the same time. Some slight differences can also occur between very similar devices as was the case for the two tested torvanes. Spatial variability at field scale can make it difficult to determine a single representative value of soil surface shear strength. As expected, soil surface moisture content played a major role in the performance of shear strength devices. Generally, a decrease of shear strength with increasing moisture content was observed and exponential regressions showed the best fit to the data. The values of cohesion at saturation obtained with a torvane for a highly compacted sandy loam top-soil and for uncompacted silty loam and clay loam soils were similar to those proposed as guide values for use as input data to the EUROpean Soil Erosion Model (EUROSEM). Correction factors, which vary with soil type, would be needed to values obtained with other instruments before the data could be used in EUROSEM. Since, in EUROSEM, soil cohesion is used to represent the resistance of the soil to detachment by runoff, the torvane and the hand vane tester are the most appropriate instruments to obtain input data. This is because they express the kind of shear involved in the detachment of soil particles by flow.