Kosei YANAGI

Study on Liquefaction Energy Capacity during Cyclic Shear under Constant Volume Condition by Discrete Element Method

Takaaki IKEDA, Masataka SHIGA

In current liquefaction prediction, common stress-based method hardly expresses complex seismic characteristics. As a new simplified method for consistent liquefaction prediction, energy-based method has been proposed. This method determines the liquefaction potential by comparing the upward energy and the liquefaction energy capacity. The former is calculated from the seismic velocity-time history waveforms, and the latter is calculated from the liquefaction resistance ratio via some regression formulas. The concept of liquefaction energy capacity is based on previous studies , which show that the cumulative dissipated energy of ground prior to liquefaction is independent of its density and external seismic forces. However, a unified consensus on whether density and force differentially affect liquefaction energy capacity remains elusive. Demonstrating the validity of the energy-based method, both experimentally and analytically, can strengthen its foundation and potentially facilitate its future implementation in practical settings. Therefore, this study evaluates the energy dissipation during the liquefaction process and investigates the dependence of the liquefaction energy capacity on the density and external forces using Discrete Element Method (DEM) analysis, which is capable of considering macro- and micro-parameters of individual particles. In this research, the analysis was performed using an open source DEM framework, Yade. This framework can track the changes in internal energy under different the density and external forces, namely the target void ratio and the cyclic stress ratio (CSR). Given that dissipated energy occurs as changes in internal energy of granular materials, the results confirm that a significant portion of the liquefaction energy capacity and cumulative dissipated energy is due to frictional dissipation between particle contacts. In terms of excess pore water pressure ratio (ru), it is confirmed that the frictional dissipation significantly depends on both CSR and void ratio from just before ru reaches to 95%. In terms of double amplitude of shear strain (DA), the dependence on external force gradually increases throughout the liquefaction process, while the dependence on void ratio becomes significant around DA=30%. Therefore, the frictional dissipated energy during cyclic shear under constant volume condition is considered to have a dependency with void ratio and external force in both ru and DA.