- Liquefaction of saturated granular material due to cyclic loading can result in catastrophic damage to the built environment. The mechanics of liquefaction have been previously studied through field testing, physical modeling, laboratory experiments, and numerical simulations. However, significant questions remain with respect to the particle-scale behavior of granular materials during the initiation of liquefaction. In the current work, the discrete element method (DEM) was employed to study three aspects related to sand liquefaction.
In the first topic, stiff boundaries and periodic boundaries were implemented in DEM simulations of constant volume (i.e., undrained) cyclic triaxial and cyclic simple shear tests. The model response was analyzed at the specimen scale by comparing stress paths, the accumulated decrease in mean effective stress, and the number of cycles to liquefaction across simulations. Particle scale analyses, including quantification of the contact force network, entropy of the local void ratio distribution, and spatial variability in the distribution of void ratio, contact number, and normal contact force are used to provide insight into model response. Simulation results imply that periodic boundaries are more appropriate for consolidating specimens to a homogenous state and minimizing boundary effects during cyclic loading. The findings from this work will have implications for the future modeling of cyclic element tests using DEM.
In the second topic, DEM is employed to study the effects of global void ratio, initial confining stress, overconsolidation ratio, and particle friction on liquefaction resistance. Mechanical coordination number, entropy of the local void ratio distribution, fabric anisotropy, and average rotational velocity are quantified in each assembly to study the particle-scale physics governing liquefaction resistance. Results show that lower global void ratio, increased confining stress, higher OCR values, and higher particle friction all increase liquefaction resistance. LVRD entropy data show that before liquefaction initiation, particles become better arranged and start to disordering after liquefaction initiation due to fabric collapse. The mechanical coordination number shows a sharp drop right before liquefaction initiation.
Finally, DEM is used to simulate both monotonic and cyclic simple shear tests. Through comparison to published laboratory data, DEM simulations have previously been shown to reasonably predict the cyclic response of granular materials, making it an appropriate tool for studying the effects of initial static shear stress on liquefaction initiation. Mechanical coordination number and the normal force-weighted fabric tensor are used to describe the evolution of fabric after monotonic preshearing and during cyclic loading. We find that higher initial static shear stress results in lower cyclic strength in stress-controlled cyclic simple shear tests, but there is no such effect in strain-controlled cyclic simple shear tests because the normal force-weighted anisotropy is removed in the first three loading cycles. It is also found that the effect of initial static shear stress on liquefaction resistance is a combination of reduced stability before cyclic loading and accumulated shear strain during cyclic loading.