Frequency-domain characteristics of shear stress and flow-state transition mechanisms in granular landslides

The shear response of granular materials is closely related to the initiation and acceleration of geological hazards such as landslides, and it can be described in terms of transitions between jammed-like relaxation and more mobile, inertial rearrangement. To quantify this dynamic process from a frequency-domain perspective, this study combined Fast Fourier Transform (FFT) and Empirical Mode Decomposition (EMD) to analyze shear-stress signals obtained from ring-shear tests on glass beads with different particle sizes. The results showed that the topology of the particle contact network and local inertial effects jointly controlled the frequency distribution of the dominant shear mechanisms. As particle size increased, the energy dissipation mode of the system shifted from low-frequency global cooperative relaxation in finer particles to high-frequency local intermittent rearrangement in coarser particles. In particular, the dimensionless critical frequency increased monotonically with particle size, indicating that particle size is a key factor controlling the threshold of dynamic state transition in granular systems. These findings clarify the role of particle size in cross-scale energy migration within granular systems and demonstrate that the dimensionless critical frequency may serve as a potential physical criterion for distinguishing steady relaxation from inertial rheology. The study therefore provides a new perspective for understanding shear instability in granular materials and offers a potential basis for early warning of geological hazards.