Fully Softened and Residual Shear Strengths of Midra Shale

Abstract

This paper presents a comparison of the fully softened shear strengths measured using the triaxial and torsional ring shear devices for a shale that possesses noticeably high plasticity and induration. The residual shear strength measured using the torsional ring shear device was also introduced for a supplementary comparison. It was shown that the mode of shear (e.g., the triaxial mode or the torsional ring mode) slightly affects the measured fully softened shear strength of Midra shale. The fully softened strength failure envelopes associated with the two shearing modes exhibit a moderate degree of nonlinearity that is in turn higher than that of the residual shear strength envelope. The measured classification indices of Midra shale are significantly sensitive to the sample preparation procedure. As a result, care should be taken when predicting the fully softened and residual shear strengths of Midra shale using relationships that correlate shear strength to such indices.