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Under simple or combined stresses, homogeneous sand specimens exhibit localization of deformation followed by shear band formation. This phenomenon has been extensively studied under plane strain loading conditions. This paper studies the initiation and propagation of the shear bands that develop in thin long hollow cylinders of sand subjected to a combination of axial torsional and spherical stresses. Digital image processing techniques are used to observe and measure the displacements and the initiation and development of the shear bands. The effects of the inclination of the principal stresses on the axis of symmetry, the relative density, the con®ning pressure and granulometry are examined. Strain measurements within the shear bands open the door to a more complete description of the mechanical behaviour of the material, in particular beyond the peak, in the so-called strain softening zone.

A new technique involving sample preparation, video imaging, and image analysis has been developed to observe the kinematics of shear bands when geomaterials are subjected to a general state of combined stress. The technique provides an effective, low-cost, and non-invasive way to monitor the development and measure the deformations inside and outside the shear bands. Its capabilities are demonstrated through a series of drained tests in which thin hollow cylinders of sand are subjected to combinations of hydrostatic, axial, and torsional stresses. It is shown that the deformation within the shear band is different from the global one and the one in its vicinity. For sand specimens with the same configuration, density, and confining pressure, the initiation, orientation, and thickness of shear bands depend on the loading path.