Experimental study of soil anisotropy using hollow cylinder testing

Most sedimentary deposits are inherently anisotropic due to their natural deposition in horizontal layers. This inherent anisotropy highlights the fact that the response of soils to loading is depending on both stress magnitude and its direction. Most of the field problems in geotechnical engineering are three-dimensional, and a soil is more likely to subject an anisotropic stress state (σ1 ≠ σ2 ≠ σ3), together with rotation of the principal axes. It is therefore essential that the soil behaviour under such realistic and general loading conditions is to be well understood, so that engineers can devise appropriate geotechnical design and analysis in practical situations.\n\nThe Small-Strain Hollow Cylinder Apparatus (SS-HCA), developed by GDS Instruments Ltd. has been used to study drained anisotropic behaviour of sand under generalized stress conditions. In particular, the anisotropic stress-strain-strength characteristics, volume change behaviour, non-coaxiality and combined effects of α and b are studied. Three testing programs composed of two main types of stress paths (e.g. monotonic loading with different inclinations of the major principal stress and cyclic rotation of principal stress axes) were conducted.\n\nInherently anisotropic behaviour of sands is clearly illustrated by deformation response that is strongly dependent on the loading direction in the monotonic shear tests. For a given loading direction, the mechanical response of sands is affected by the material density, the particle properties and the loading history. Non-coincidence of principal directions of stress and strain increment is observed and shear band inclinations in hollow cylindrical specimens follow the theoretical predictions. Results also clearly show the effects of intermediate principal stress on the deformation response of sand. This is seen in variation of stress-strain response and peak friction angle with differing b-values.\n\nA significant plastic deformation is induced during rotational shear despite the magnitudes of principal stresses remaining constant. Volumetric strain during rotational shear is mainly contractive and the amount of the volumetric strain increases with the increase in the stress ratio. Most of the contractive volumetric strain occurred during the first 20 cycles and its accumulation rate tended to decrease as the number of cycles increases. When principal stress rotation continues, the sand samples appear to be stabilized and the strain trajectory in the deviatoric plane approaches an ellipse. The test results also demonstrate that the mechanical behaviour of sand under rotational shear is generally non-coaxial, and the stress ratio has a significant effect on the non-coaxiality. The larger the stress ratio, the lower degree of non-coaxiality is induced. It was also observed that parameter b is not a negligible factor for the sand deformation during rotational shear, but has significant impact. The larger the b-value, the more the volumetric strain is accumulated.