Critical State Soil Mechanics for Cyclic Liquefaction and Postliquefaction Behavior: DEM study

Discrete-element method (DEM) simulations of three-dimensional (3D) assemblies of ellipsoid particles were used to evaluate the critical state (CS) for both drained and undrained (constant volume) conditions. A series of conventional triaxial cyclic liquefaction tests with symmetrical cyclic deviatoric stress (${\sigma }_d$) with initial $q=0\mathrm{kPa}$ were simulated to develop a relationship between the cyclic stress ratio ($CSR={\sigma }_d/2{\sigma }_0^{\prime }$) and the number of cycles required for initial liquefaction ($N_L$), where ${\sigma }_0^{\prime }$ is the mean effective normal stress at the end of consolidation. Both cyclic mobility and instability type behaviors were observed depending on the initial void ratio ($e_0$) and ${\sigma }_0^{\prime }$. The micromechanics quantities, i.e., the coordination number ($CN$), von Mises fabric ($F_{vM}$), fabric anisotropy intensity (${\alpha }_c$), and stress-strain behavior, suggested that cyclic mobility and instability may depend on the phase transformation and instability state, respectively. The cyclic resistance ratio ($CR{R}_{15}$), i.e., $CSR$ at $N_L=15$, showed a unique relation with the initial state parameter (${\psi }_0$), irrespective of $e_0$ and ${\sigma }_0^{\prime }$. Two series of postliquefaction monotonic simulations with and without reconsolidation exhibited a unique CS, which perfectly matched with the original CS line. The $F_{vM}$ also reached a unique, narrow range at the CS. The postliquefaction settlement during reconsolidation also showed a linear relation with ${\psi }_0$.