Mechanical characteristics and microstructure of saturated sand under monotonic and cyclic loading conditions

Abstract

To gain a deeper understanding regarding the intrinsic properties and correlations of macro mechanical responses, including critical states and large flow deformations under monotonic and cyclic loading conditions, with a view to revealing the fundamental characteristics of internal forces and deformations in granular materials. A series of undrained triaxial compression, triaxial extension and cyclic triaxial numerical tests are performed using discrete element method (DEM) software PFC3D. Under both monotonic and cyclic loading conditions, a comparable deformation mechanism of strain softening and strain hardening is observed, and the effective stress skeleton curves are in good agreement. As the progression from high stress ratio to the critical stress ratio, flow deformation and strain hardening are observed in saturated sand, where the mechanical coordination number gradually increases and the sliding ratio, suspended particle ratio decrease. The fabric anisotropy tends to increase in the flow deformation stage, decreases slightly in the strain hardening stage but remains highly anisotropic, and decreases rapidly in the stress decay (unloading) stage. As the cycle number increases after liquefaction of the sample, the percentage of deviatoric strain in the flow deformation stage to the total unidirectional shear strain increases, while the percentage in stress decay (unloading)/hardening stages decreases, which means that the strain development of samples is more fully developed in the flow deformation stage. The results serve as a reference for studying soil dilatancy and modifying constitutive models.