Fabric evolution and strain localisation in inherently anisotropic specimens of anisometric particles (lentils) under triaxial compression

Inherent anisotropy is often observed in natural soils and other granular materials formed during gravity deposition. Fabric characterisation and evolution have been mainly studied using numerical simulations, given the difficulty of retrieving the contact network and the kinematics of particles in physical experiments. This work presents the results of five triaxial compression tests on inherently anisotropic lentil specimens imaged with x-ray tomography. Each specimen is prepared in a way such that all the particles present a unique mean orientation, as results from gravity in natural soils deposits - this can be called "bedding effect ". Particles are identified and tracked from the first image all the way through the test using a novel tracking algorithm, enabling the measurement of particle and contact fabric evolution, as well as strain localisation within the specimens. The results reveal that the deformation processes take place essentially in a planar scenario, both at a micro and mesoscale. Additionally, it is observed that under deviatoric loading two mechanisms are responsible for fabric evolution: rotation of the main orientation of the fabric tensor, and increase of the anisotropy. Finally, the orientation of the shear band is found to be independent of the initial orientation of the particles.