Abstract: The particle flow numerical simulation was conducted to investigate the undrained cyclic triaxial test of sand samples.The particle contact forces were classified into strong,medium,and weak force chains based on their strength levels.The influence of initial effective confining pressure and porosity on the behavior of these force chains were discussed.The evolution law of force chain in the liquefaction process of sand samples under cyclic loading was investigated,along with its correlation to the phase transition pore pressure ratio and average flow coefficient.The boundary force chain strength was defined and proposed as the microscopic basis for assessing the solid-liquid phase change of saturated sand,revealing the generation and evolution mechanism of the fluidity under cyclic loading.The results indicate that as the confining pressure increases,there is a gradual reduction in the number of medium and weak force chains,while the number of strong force chains significantly increases.The number of force chains will gradually decrease with the increase porosity,while the number of strong chains and weak chains is minimally affected by porosity.The presence of strong chains diminishes completely when the pore pressure ratio reaches 0.8,while the average flow coefficient of the sample exhibits a sharp increase.When the pore pressure ratio reaches 1.0,medium force chain numbers disappear,and weak force chain numbers stabilize.Finally,the validity of designating the boundary force chain strength as the maximum force chain strength corresponding to the phase transition pore pressure ratio is ultimately confirmed.