Effects of stress ratio on small-strain stiffness during triaxial shearing

Abstract

A comprehensive series of triaxial tests were performed on granular materials to study the stress-dependence characteristics of small-strain stiffness (i.e. the quasi-elastic Young's modulus). The tests were performed on large prismatic specimens measuring both axial and lateral strains locally. The small-strain stiffness at a given stress state was measured from the stress–strain response obtained by applying very small-amplitude cyclic normal stresses, in which the single-amplitude major principal strain was less than 0·002%. The stress state of a given specimen was varied along various stress paths comprising both ‘small shear’ and ‘large shear’ stresses in both triaxial compression (TC) and triaxial extension (TE) stress space while applying very small stress cycles. At small shear levels (defined as the principal stress ratio within the range between 0·5 and 2·0), small-strain stiffness defined for a given direction was a function of the principal stress in that direction. The unique relationship did not hold at larger shear stress levels when the major-to-minor principal stress ratio was approaching around 3 to 4. In the latter case, both quasi-elastic Young's moduli, defined for both major and minor principal strains, during TC and TE experienced a decrease due to damage to the initial fabric that was triggered by a high rate of plastic straining as well as creep deformation.