Rainfall has been recognized as a key factor in triggering landslides. However, it is not entirely clear why many landslides have been triggered by slight-to-moderate rainfall. The Mudui landslide that occurred in Sichuan Province, China, on June 22, 2020, exemplifies the evolution of landslides induced by seasonal rainfall, which can cause substantial damage to infrastructure. This landslide was a deep-seated debris slide with a volume of approximately 0.64 million m3. It occurred in colluvial deposits, which are heterogeneous soil-rock mixtures with high permeability that easily retain water. On the basis of detailed site investigations and various monitoring data-including interferometric synthetic-aperture radar (InSAR), ground-slope and subsurface-slope deformation monitoring, and hydrogeological monitoring-we investigated the landslide-triggering mechanism along with pre- and post-landslide kinematics and assessed the effects of remedial works. The results show that both the soil water content and the slope deformations have significant seasonal characteristics. The soil water content decreases during dry seasons and increases during rainy seasons. Correspondingly, the deformation rates increase with the onset of rainy seasons and decrease with the onset of dry seasons. The landslide area underwent progressive deformations linked to groundwater seepage, inducing a continuous deterioration of the soil body. Finally, prolonged rainfall triggered the landslide of the deteriorated soil mass. The results indicate that the adverse effects of long-term seasonal soil-water-content fluctuations need to be take into account in analyzing slope instabilities in colluvial deposits.

Slope failures are a significant natural geohazard in hilly and mountainous regions, often resulting in loss of life and infrastructure damage. The Muketuri-Alem Ketema road in Ethiopia is particularly vulnerable to landslides due to colluvial deposits on steep slopes from the higher northeastern plots to the lower Jemma River valley. This study investigates the characteristics of colluvial soil and evaluates the stability of slopes prone to landslides. It combines geophysical data, penetrometer tests, laboratory analyses, Google Earth images, and detailed field visits to assess the soil and bedrock composition and structure. Numerical methods, including limit equilibrium (Bishop, Janbu, Spencer, and Morgenstern-Price methods) and finite element methods, were used to analyze slope sections under various saturation conditions and simulate different rainfall patterns. The results indicate that the Bishop, Morgenstern-Price, and Spencer methods produce similar safety factors with minimal differences (<0.3%), while the Janbu method shows more significant variation (1.5%-5.6%). Safety factor differences for sections A-A and B-B range from 5.26% to 9.86% and 3.5%-4.7%, respectively. Simulations reveal that short-term saturation significantly reduces the stability of the upper slope layer by 20%-46.76%, and long-term saturation decreases the entire slope by 26.81%-46.76% compared to dry conditions due to increased pore water pressure and self-weight. Long-term saturation effects, combined with dynamic loads, can further reduce colluvial soil stability by over 50% compared to a dry static state. The finite element method predicts larger failure zones than limit equilibrium methods, emphasizing the need for accurate predictions to characterize slope behavior during failure and inform stabilization decisions. This study provides crucial data for maintaining and planning the Muketuri-Alem Ketema Road, highlighting slope performance over time and the effectiveness of stabilization techniques.

This study establishes a foundational framework addressing challenges, implications, and potential remedies related to collapsible soils. Serving as a cornerstone for global exploration, it emphasizes the importance of understanding geological, structural, and mechanical characteristics for early identification and proactive mitigation. The study underscores the significance of preventing structural damages in regions prone to collapsible soils, discussing their diverse types and origins, structural composition, and mechanical behavior. A detailed exploration highlights their prevalence in semi-arid and arid regions, emphasizing distinct geological features associated with their occurrence.