A DEM Investigation on the Influence of Cyclic and Static Stress Ratios and State Variables on the Pore Water Pressure Generation in Granular Materials

A granular soil may undergo liquefaction failure due to excess pore water pressure ($\mathrm{\Delta }u$) generation when subjected to cyclic loading such as an earthquake. Several past studies indicated $\mathrm{\Delta }u$ generation may be independent of cyclic stress ratio (CSR), whereas others contradicted this observation. A soil element in its natural state often remains in $K_0$ consolidation state, i.e., subjected to nonzero static shear stress ratio (SSR). Using the discrete element method (DEM), this study performs a large number of cyclic triaxial tests and evaluates various factors influencing the magnitude and rate of $\mathrm{\Delta }u$ generation under symmetric and nonsymmetric cyclic loading conditions. The study also evaluates the capabilities of several existing $\mathrm{\Delta }u$ generation models to predict the development of $\mathrm{\Delta }u$ under different failure scenarios. It was observed that the generation of $\mathrm{\Delta }u$ can be influenced by CSR for specimens subjected to symmetrical cyclic loading and by both CSR and SSR for nonsymmetric cyclic loading conditions. A new prediction model for $\mathrm{\Delta }u$ generation was developed along with an objective framework to estimate its input parameters. The model showed good agreement with the DEM simulation results.