ABSTRACT: The paper investigates the anisotropic yielding behaviour of dense Toyoura sand from very small to large strain in a p'-constant shear plane by means of hollow cylinder torsional shear tests. Efforts were made to draw yield surfaces in the stress space. To pay special attention to anisotropic yielding due to solely the shear process, p' was kept constant to avoid yielding caused by the change of p' . s'₁-direction from the vertical in the shear process was fixed to be from 0° to 90° at 11.25° interval. It was found that the sand starts yielding progressively from the strain level as small as 0.002%. The simple way of interpreting elastic behaviour below the large-scale yield surface, which is determined to be at much larger strain level, can not explain such behaviour. Two subyield surfaces, Y₁ and Y₂, were identified inside the large- scale yield surface, Y₃. It is shown that these surfaces can explain nonlinearity and degradation of stiffness moduli.
Anisotropy and mobility of the subyield surfaces were also studied in the p'-constant shear plane. When the initial stress state was at the origin of the shear plane (isotropic stress state), the shape of the yield surface was approximately circular with the center shifted toward compression in the vertical direction. Anisotropy increased with the progress of shearing. Y₁ and Y₂ surfaces were mobile and moved with the current stress point in the p'-constant shear plane, though Y₃ surface was comparatively immobile. The shapes of yield surfaces also changed with the movement of current stress point.

Key words: anisotropy, sand, shear modulus, small strain, special shear test, yield (IGC: D6)