Measurements of shear stress distribution on the edges of a shear vane blade

The primary measurement carried out is that of torque. Soil strength is derived from this torque. The effects of the anisotropy and progressive failure, although they have been investigated, are largely ignored. This note discusses the distribution of shear stress measured by laboratory tests in Leighton Buzzard sand and field tests in over-consolidated, saturated, brown London clay. A rectangular vane 120 mm wide and 240 mm long was made so that foil strain gauges could be used to measure the loads on various parts of the vane blades. Equivalent shear stress distributions are plotted for various angular rotations of the vane shaft together with least squares regression curves fitted to normalised versions of the data. Normalised distributions from the vertical blade edge had weak correlations due to the scatter arising from the layers in the soils. However the distributions from the horizontal blade edge had much stronger correlations as the stresses were mobilised in a uniform horizontal layer of soil. It was found that shear force measurements gave values within 10 per cent of the torques measured at the top of the shear vane shaft indicating that most of the shearing resistance arises at the blade edges. Little torque arises from soil distortion in the sectors between adjacent blades. This concentration of shear distortion at the blade edges was confirmed radiographically. (TRRL)