Drained deformation and non-coaxiality of granular materials under pure principal stress rotation: role of particle morphology

Abstract Understanding the deformation and non-coaxiality behavior of granular materials under pure principal stress rotation (PSR) is a highly challenging topic in soil mechanics. However, there has been limited research investigating the influence of particle morphology on these behaviors. In this study, drained torsional shear tests with PSR stress path were performed on four glass beads with different particle morphologies, under various intermediate principal stress coefficients and stress levels. Microstructural analysis using X-ray CT scanning of dummy cylinder specimens was employed to support the interpretation of the experimental data. Results reveal that as particle shape becomes more irregular, the deformability and shear modulus of granular materials initially increase and then decrease, while non-coaxiality consistently increases. A moderate increase in the shape irregularity leads to an increase in the mean coordination number $$\overline{Z}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mover> <mml:mi>Z</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> </mml:math> in particle packing, while a highly irregular particle shape results in a decrease in $$\overline{Z}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mover> <mml:mi>Z</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> </mml:math> and a looser fabric, which is detrimental to shear modulus. The contact normal directions in non-spherical materials are more concentrated, exhibiting a stronger initial fabric anisotropy that potentially promotes non-coaxiality. Additionally, the proportion of deviatoric strain that is non-coaxial with deviatoric stress tensor significantly increases as particle shape becomes more irregular.