How contact stiffness and density determine stress-dependent elastic moduli: a micromechanics approach

To investigate the origin of the stress-level dependency of soil elasticity, a series of stress-path experiments were simulated for loose and dense soil specimens with three different contact surfaces. In the discrete element analyses, an assumption was introduced in which the contact body had the geometry of an elastic sphere with local, axi-symmetric irregularity. To evaluate the cross-anisotropic elastic shear moduli, small-strain cyclic shear tests were simulated under stress conditions along four stress-probing paths. For dense specimens with high coordination numbers, the internal structure was represented by the degree of fabric anisotropy and the coordination number remained unchanged during shearing, thus leading to the coincidence of the sum of the exponents in the contact stiffness model. For the loose specimens with low coordination numbers, the fabric structure evolved continuously during shearing, which resulted in the increase of the exponents in the power function of the elastic modulus. The rearrangement of particles and the transition of contact-force chains, along with the evolution of the fabric, manifested as increasing dependency of the elastic moduli on the stresses in such loose specimens.