Microinsights into Failure of Two-Dimensional Granular Materials during Isochoric Cyclic Shearing

This paper investigates the degradation of microstructure leading to different cyclic failure modes, i.e., residual deformation failure, cyclic mobility, and cyclic instability, observed during isochoric cyclic shearing of dry granular material. A “primary” network, defined by interparticle contacts with normal contact forces exceeding a threshold derived from the initial liquefaction criterion, is introduced. For comparison, an “effective” network, excluding contacts of rattlers, is also examined. The mechanical states of both networks are characterized by using fabric anisotropy and mechanical coordination number. The initiation of failure is identified as the knee point on the curve of peak pore-water pressure curve versus cycle number. The results from discrete element simulations demonstrate that the “primary” network more effectively predicts the onset of cyclic failure because both fabric anisotropy and coordination number converge to limiting values at the point of cyclic instability or initial liquefaction. By contrast, the “effective” network provides a more reliable description of the progressive degradation of mean effective stress or, equivalently, pore-water pressure evolution. Furthermore, all tests exhibit a unique failure-initiation state corresponding to an identical peak in the pore-water pressure ratio or effective stress ratio degradation. These findings provide a microstructural perspective for understanding cyclic failure modes and for developing micromechanical constitutive models for saturated granular materials once pore-fluid coupling effects are incorporated.