Study of the yielding of sand under generalized stress conditions using a versatile hollow cylinder torsional apparatus Brendan C. O’Kelly a, * , Patrick J. Naughton b,1 a Department of Civil, Structural and Environmental Engineering, Museum Building, Trinity College Dublin, Dublin 2, Ireland b School of Engineering, Institute of Technology, Sligo, Ballinode, Sligo, Ireland article info Article history: Received 15 June 2007 Received in revised form 8 October 2008 abstract 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 full saturated Leighton Buzzard sand was studied under generalized stress conditions. Physically identi- cal 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 inter- mediate principal stress parameter. However, the deviator stress at yield was largely inde- pendent 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. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Geomaterials behave pseudo-elastically up to about 10 3 strain. In elastic–plastic modeling, the transition with increasing stress from pseudo-elastic to elastoplastic behavior occurs at the yield point. Hence, the estimation of the yield stress is important in engineering design since it defines the onset of larger plastic deformations with fur- ther increases in applied stress. In most ground engineer- ing problems, the principal stresses rotate as the foundation load is applied. For example, Fig. 1 shows the stress conditions at five points (labeled A–E) along a poten- tial rupture surface in the ground. Since most sedimentary deposits are inherently anisotropic, the stiffness and strength responses of the ground foundation will vary from point to point along the rupture surface and are functions of the stress history and both the magnitudes and the directions of the principal stresses. At point D, the major principal stress (r 1 ) has rotated through an angle (a r ) to the vertical direction. The hollow cylinder torsional apparatus (HCTA) is un- iquely capable of independently controlling the magni- tudes of the three principal stresses and the orientation of the major–minor principal stress axes. Hight et al. (1983) and Saada and Townsend (1981) summarized the evolution of HCTA type devices in studying the constitutive response of geomaterials, and in particular sedimentary sand deposits. 0167-6636/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.mechmat.2008.11.002 * Corresponding author. Tel.: +353 18962387; fax: +353 16773072. E-mail addresses: bokelly@tcd.ie (B.C. O’Kelly), naughton.patrick@ itsligo.ie (P.J. Naughton). 1 Tel.: +353 719155489; fax: +353 719155390. Mechanics of Materials 41 (2009) 187–198 Contents lists available at ScienceDirect Mechanics of Materials journal homepage: www.elsevier.com/locate/mechmat