Landslides commonly evolve from slow, progressive movements to sudden catastrophic failures, with saturation and displacement rates playing significant roles in this transition. In this paper, we investigate the influence of saturation, displacement rate, and normal stress on the residual shear strength and creep behaviour of shear-zone soils from a reactivated slow-moving landslide in the Three Gorges Reservoir Region, China. Results reveal a critical transition from rate-strengthening to rate-weakening behaviour with increasing displacement rates, significantly influenced by the degree of saturation. This transition governs the observed patterns of slow movement punctuated by periods of accelerated creep, highlighting the potential for exceeding critical displacement rates to trigger catastrophic failure. Furthermore, partially saturated soils exhibited higher residual strength and greater resistance to creep failure compared to nearly and fully saturated soils, underscoring the contribution of matric suction to shear strength.

On 1 September 2022, a giant loess landslide occurred in Huzhu Tu Autonomous County, Qinghai Province, China. This catastrophic event brought to light a unique loess fluidisation phenomenon. In specific parts of the landslide, the loess completely transformed into a viscous, fluid-like state, whereas other parts showed a deepseated slide that retained their structural integrity. In this case, loess with different sliding patterns exhibited varying levels of mobility and destructive potential. Based on the field investigation, electrical resistivity tomography was employed to investigate the groundwater condition of the slope. Subsequently, ring-shear tests were carried out to examine the mechanical properties of the sliding zone loess under different saturation degrees and its response to rainfall as a triggering factor. The results indicate that the natural water content in the original slope was unevenly distributed, influenced by local terrain and groundwater runoff. Following the initial slide caused by cumulative rainfall, the overlying sliding material with high degree of saturation was likely to fluidise due to the increase in excess porewater pressure caused by continued shearing, ultimately resulting in flow-like movement features. In contrast, in areas with a deeper groundwater table, the initial shear could only be sustained over a short distance. This study reveals a mechanism of multiple movement patterns that may coexist in giant loess landslides.

The rapid movement and extensive displacement of gravel-silty clay landslides result in significant property damage and loss. Following the destabilization of the Shaziba landslide in Enshi City, it transformed into a debris flow, ultimately obstructing the Qingjiang River and creating a barrier dam. This study delves into the failure mechanism, leap dynamics, and motion processes of this specific landslide by employing a blend of ring shear testing and the discrete element method. Initially, the residual shear strength of the sliding soil was assessed through ring shear tests conducted under various coaxial stresses and shear rates within the sliding region, using field surveys and aerial imagery. Building upon this foundation, the entire progression of the landslide-from sliding to settlement-was replicated using PFC3D, allowing for an examination of the landslide's movement characteristics such as speed, displacement, and trajectory. The findings indicate that the shear displacement and residual friction coefficients are higher at elevated shear rates compared to lower rates. The landslide commences with an initial acceleration phase, with the silty clay material's movement lasting approximately 757 s, reaching a maximum velocity of 32.5 m/s and a displacement exceeding 1000 m. The simulated settlement volume of the landslide (9.31 x 105m3) closely aligns with the results obtained from field investigations (1.5 x 106m3). This research offers comprehensive insights into recent Shaziba landslides, serving as a valuable resource for enhancing our understanding of the dynamics involved and mitigating the potential risks associated with such events.

Interparticle friction is an intrinsic property of particles, which plays an important role in the macroscopic and microscopic shear mechanical properties of granular materials. In this research, we investigate the shear behavior of granular materials with different friction coefficients using ring shear tests. The particle image velocimetry (PIV) technique was also used to analyze the shear flow characteristics. The results indicate that the peak shear strength of granular materials increases with the increase in shear rates, especially for granular materials with high friction coefficients. The shear stress fluctuation difference is smaller under low normal stress. Under high normal stress, the shear stress fluctuation of granular materials with high friction coefficient is higher than that of granular materials with low friction coefficient. In addition, the shear stress fluctuation shows a trend of increasing with the increase of shear rates. The range of the liquid phase flow region of granular materials decreases with the increase of friction coefficient and normal stress. This work reveals the shear flow characteristics of granular materials under different conditions, which can provide reference for the flow processes of geological disasters such as landslides and debris flows.