Unraveling the Dynamic Weakening Mechanism of Earthquake-Triggered Landslides through Vibration-Induced Frictional Sliding Instability Experiments on Granular Materials

Abstract

Purpose: Strong earthquakes with greater magnitudes and longer durations can trigger numerous landslides. Several mechanisms have been proposed to explain the triggering process, such as seismic waves providing additional driving force, which increases the shear stress on the sliding plane. Earthquakes may also increase pore pressure, which reduces the effective normal stress on the sliding plane. However, the mechanisms that apply to both wet and dry conditions are not fully understood. Herein, we purposed to study the seismic response and triggering mechanisms through vibration experiments on granular materials. Methods: We used a ring shear apparatus to study the mechanical behavior of granular materials under cyclic loading. 0.2-0.4 mm glass spheres (SiO2) were placed in a ring shear box, and constant normal stress and constant shear stress were applied. Then, sinusoidal cyclic shear stress with different numbers of cycles were applied respectively. The frequency of cyclic loading was 1 Hz. We used dynamic triaxial-bender tests to monitor the evolution of the shear modulus of samples under cyclic loading. The confining pressure was 300 kPa and deviatoric stress was 380 kPa, and sinusoidal cyclic dynamic loads with amplitudes of 45 kPa, a frequency of 1 Hz and a cycle number of 200 were applied. All of the experiments were under dry conditions (room humidity). Results: The dynamic ring shear experiments show that the co-vibration and post-vibration shear displacements increased with an increase in the number of cycles, and the instability of granular materials can be triggered by a larger number of cycles while the shear stress returned to the initial value after the vibration ended. The stepwise increase curves of co-vibration shear displacement show platform segments and upward segments, the platform segments corresponded to trough segments of cyclic shear stress, and the upward segments to crest segments of cyclic shear stress. The shear displacement value of each upward segment increased with the increase in the number of vibration cycles (Fig. 1). The dynamic triaxial-bender test result shows that the specimen shear modulus decreased with the increase of the number of vibration cycles, while the density decreased slightly and the axial strain increased, roughly follow a logarithmic law during vibration (Fig. 2). Conclusions: Our dynamic ring shear experimental observations suggested that it is easier to trigger landslides with longer durations of vibration, and the triggering is closely related to the weakening of shear resistance. The results suggest that the reduction of shear modulus is an important mechanism for triggering failure of earthquake-triggered landslides. We infer that the earthquake-induced decrease in shear modulus was caused by structure changes, particle rearrangement, and slipping at the scale of micro-asperities.