ABSTRACT: A series of thick-walled hollow cylinder tests was conducted on a synthetic shale. The tests were performed under both hydrostatic (essentially isotropic) and plane strain (with elevated initial axial deviatoric stress) conditions. The purpose of the tests was to experimentally examine the differences in the hollow cylinder behaviour under the two test conditions. The experimental results showed that considerable differences could be generated by the two conditions. The axial stress (intermediate principal stress, s2) has a significant stiffening and strengthening effect on the borehole. Numerical analyses were conducted which showed that a failure criterion incorporating s2 was required to model the observed strength and deformation behavior of the hollow cylinders.

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