Abstract: In the southeastern coastal regions of China, granite residual soil (GRS) is widely used as subgrade fill, and its dynamic characteristics merit thorough investigation. Vehicle loads and seismic actions are inherently multidirectional, and the dynamic behavior of soils under such multidirectional cyclic loading differs significantly from that under unidirectional cyclic loading. To explore these effects, a series of horizontal cyclic direct shear tests under normal cyclic loading was conducted on GRS using a large-scale bidirectional cyclic direct shear apparatus. The tests were performed under different normal stress frequencies (0.02, 0.05, 0.1, 0.15, 0.2 Hz), and discrete element method (DEM) simulations were carried out to reproduce and further interpret the bidirectional cyclic behavior of GRS. The results indicate that as the frequency of cyclic normal stress increases from 0.02 Hz to 0.2 Hz, the dynamic shear strength of GRS decreases by 15.29%, the vertical displacement increases by 21.00%, and the energy dissipation capacity improves by 25.71%. An increase in normal stress frequency results in a 6.14% reduction in the maximum number of strong force chains within the particle assembly. Meanwhile, the total and fine contact mechanical coordination numbers increase by 0.64% and 0.81%, respectively. The normal stress frequency primarily influences the contact behavior of fine particles, thereby altering the overall contact characteristics of the soil. Additionally, sliding dissipation, rolling-sliding dissipation, and dashpot dissipation increase by 69.35%, 50.52%, and 92.79%, respectively, with increasing normal stress frequency.

Key words: cyclic loading, normal stress frequency, granite residual soil, cyclic shear characteristic, discrete element method