Study of the yielding of sand under generalized stress conditions using a versatile hollow cylinder torsional apparatus

The hollow cylinder torsional apparatus (HCTA) is particularly suited to studying the mechanical behavior of cross-anisotropic test specimens since both the magnitudes of the three principal stresses and the orientation of the major-minor principal stress axes can be independently controlled. Novel features of the University College Dublin HCTA, particularly the instrumentation and calibration procedures necessary to achieve accurate generalized stress path testing, are presented. The plastic yield behavior of fully saturated Leighton Buzzard sand was studied under generalized stress conditions. Physically identical test-specimens (35.5 mm inner radius, 50.0 mm outer radius and 200 mm in length) were prepared using a water-pluviation technique that reproduced the inherent cross-anisotropic fabric of many natural sand deposits. A series of stress path tests systematically probed the stress space in order to locate segments of yield loci. The deviator stress at yield was found to vary in a well-defined pattern that depended on the magnitude of the intermediate principal stress parameter. However, the deviator stress at yield was largely independent of reorientations of the major principal stress that had occurred during the initial anisotropic consolidation stage of the tests. The experimental yield data was used to show that the Matsuoka-Nakai and Lade yield criteria provide satisfactory predictions of the onset of plastic yielding in sand deposits under generalized stress conditions for routine geotechnical engineering design.