Threshold of Cyclic Phenomena

Granular materials subjected to cyclic loading are characterized by changes in their shear modulus, pore water pressure, and volume. These changes occur when the amplitude of the cyclic shear strain exceeds the cyclic threshold shear strain. To investigate cyclic phenomena, the force transmission in a system of grains and voids (sand) was modeled using the numerical discrete element method. Using a three-dimensional numerical model, the laboratory direct simple shear test (DSS) is simulated. The numerical model comprises a large number of grains (574,507). The extraction and analysis of the data from the numerical model are carried out at several levels. At the micro level, an example of determining the stability of a single force chain is provided. At the meso level, the criteria for defining force chains are verified (mean force, contact force angle, force chain length, and force chain branching). Several force chains form a strong network, the stability of which and, thus, the stability of the system, are determined based on changes in the number of force chains, mechanical coordination number, and contact force size distribution. The accumulation threshold cyclic shear strain γt=4×10−5 and stability threshold γt=3×10−4 are suggested as characterizing the cyclic behavior of granular materials at the macro level.