Structural degradation of sands during cyclic liquefaction: Insight from DEM simulations

The macro stress-strain responses of sands are closely related to the evolution of the force transmission network. In this paper, variations of the mechanical stability and reversibility of the force transmission network during the cyclic liquefaction process are explored using discrete element method (DEM) simulations. It is shown that sands degrade gradually from hyperstatic states to isostatic states during cyclic loading and become hypostatic when approaching liquefaction. During this process, the number of excessive contacts decreases gradually and the sample becomes unjammed. An effective and resilient force transmission network should contain enough mechanically stable particles to spread throughout the entire sample. Structural degradation during cyclic loading towards liquefaction is associated with decreasing reversibility of the force transmission network that is characterized by increasing mean squared displacement, increasing fraction of broken contacts and decreasing size of the largest force transmission network. Flow deformation and the development of significant double-amplitude axial strain are attributed to the inability of a large proportion of the broken contacts to re-form post-liquefaction. The size of the largest force transmission network (S-L) decays linearly with mean effective stress and there exists a critical value of S-L below which liquefaction will occur.