Largely unknown mechanisms restrain motion of clay‐rich, slow‐moving landslides that are widespread worldwide and rarely accelerate catastrophically. We studied a clayey, slow‐moving landslide typical of thousands in Northern California, USA, to decipher hydrologic‐mechanical interactions that modulate landslide dynamics. Similar to some other studies, observed pore‐water pressures correlated poorly with landslide reactivation and speed. In situ and laboratory measurements strongly suggested that variable pressure along the landslide's lateral shear boundaries resulting from seasonal soil expansion and contraction modulated its reactivation and speed. Slope‐stability modeling suggested that the landslide's observed behavior could be predicted by including transient swell pressure as a resistance term, whereas modeling considering only transient hydrologic conditions predicted movement five to six months prior to when it was observed. All clayey soils swell to some degree; hence, our findings suggest that swell pressure likely modulates motion of many landslides and should be considered to improve forecasts of clayey landslide initiation and mobility.

The generation of acoustic emission (AE) in a material relates to the rapid energy release process, which delivers important information concerning the failure mechanism for stressed geologic materials. Here we report on ring‐shear friction experiments to investigate the mechanical behavior and AEs for sheared granular materials with different particle sizes under constant normal stress and a range of shear rates. Results show that the mechanical behavior of shear resistance, sample compaction and slip displacement, and the release of acoustic energy can be affected by both shear rate and particle size. The main AEs are strongly correlated with the global mechanical failure, and the maximum absolute amplitude of AEs generally increases with increase of the magnitude of stress drops. By analyzing the event sequences, it is found that the onset of AEs precedes the beginning of stress drop, which provides more useful information on the failure mechanisms of geologic granular materials.

Gas flow in soil plays a crucial role in terrestrial ecosystems, and numerical simulation of their movement needs to know their effective diffusion coefficients. How pore structure influences the effective diffusion coefficient has been studied intensively for dry porous media, but much remains unknown for unsaturated soils. Here, we employed the X-ray tomography technique at the pore scale to directly obtain the soil structures, the geometry of their pores and the water distribution under different water saturation levels were calculated using a morphological model. The results show that pore structures including porosity, interface area of gas–solid–water and pore diameter are closely related to water saturation. The increase of mean pore diameter with gas saturation can be fitted into a power law. We also investigated the impact of pore geometry and water saturation on the effective diffusion coefficients, which is independent of the molecular mass of gas after normalization. As the normalized effective Knudsen diffusion coefficient increases with average pore diameter following a power law, with the scaling factor related to pore geometry and the exponent is a constant, we explained and proved that the Knudsen diffusion coefficient increases with gas saturation, also following a power law.