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Small-scale landslides that occur at unexpected locations and seasons can cause severe losses of life and property. Unusually, the Shidang landslide occurred prior to the rainy season in Jinkouhe County, Sichuan Province, China, resulting in 19 deaths and missing persons. In this paper, landslide characteristics and causes are revealed through a field investigation, laboratory experiments, and numerical calculations. The results show that the Shidang landslide was a small-scale landslide with a total volume of approximately 19500 m3, originating from hollow colluvial deposits. The geomorphological hollow above the landslide provided topographic conditions for the collection of subsurface runoff. Groundwater flow was transported downward along dolomite and accumulated in the area above shale, with a low conductivity. Groundwater table rise combined with the decrease in the soil strength led to landslide occurrence. Improper siting of residential areas and insufficient vigilance against post-rainfall landslides caused this tragedy. Considering the post-disaster lessons, it is strongly recommended that in the future, more attention should be given to the investigation of landslides triggered by groundwater, especially in determining landslide hydrological system boundaries. In mountainous areas where the developable space is limited, areas with hollow landforms are potentially at risk of geohazards. This paper could provide a reference for the prevention and management of similar landslides and new insights for updating landslide investigation systems and residential housing plans. [ABSTRACT FROM AUTHOR]

The preferred plane often exists in a layered slope, which has a main influence on the slope stability. Such a plane can evolve into the failure surface, due to the internal and external triggers of landslides. This paper proposes a case study of the failure of a highway slope in Sichuan, China. The site investigations were conducted using a number of methods, such as geological survey, unmanned aerial vehicle survey, slope monitoring, in-situ and laboratory tests. Through the case study, the failure mechanism of the layered slope was discussed, where the slope stability analysis was conducted using the limit equilibrium method (slip-wedge method) and strength reduction method (SRM). According to the circumstance of the landslide area and the requirement for the highway, this paper also proposes the stabilization measures, which was subsequently validated by SRM. Finally, the sliding mechanism of the landslide was discussed. [ABSTRACT FROM AUTHOR]

Landslides which occur at unexpected locations and times, and which have unrecognizable risk areas, often cause mass casualties. Despite there being few preceding landslides in the same season, the catastrophic Qijiaba landslide occurred in the new government-selected settlement of Wufu village, Sichuan Province, China, on September 5, 2021, killing three people and destroying 13 houses. In this paper, the landslide features and causes are discussed based on field investigations, remote sensing interpretations, and laboratory-based soil experiments. The results showed that it was a typical soil landslide within a geomorphic hollow of the slope with a volume of 20,230 m3, recurrence interval of at least 69 years, and runout distance of 137.5 m. This type of landslide is difficult to predict. The soil strength in the sliding surface decreased suddenly as the water content increased, and the soil's rich clay mineral and high clay particle contents determined its high mobility (Heim's ratio of 0.44). The occurrence time of the Qijiaba landslide lagged 1.3 days behind the peak rainfall, and the 2-day cumulative rainfall was a crucial factor in the landslide occurrence. Finally, the large water confluence area behind the main scarp was determined to be 3.9 times the area of the landslide itself, thereby providing abundant water conditions to induce failure. The factors underlying the casualties included inappropriate site selection, insufficient awareness of potential landslides, and failure to evacuate in time. The results of this study provide a basis for future prevention, mitigation, and early warning of similar landslides. [ABSTRACT FROM AUTHOR]

This paper reports on a typical slow-moving landslide that occurred in Shuitianba village in Wanyuan County, Sichuan Province, China, on September 16, 2017. The landslide in Wanyuan County is regionally outstanding for its features and well-developed landslide morphology. Hence, this event can provide a good case study for understanding the possible triggering conditions of the occurrences of all the landslides examined in Wanyuan County. The 225 residents of Shuitianba village did not flee the village because the deformation of the landslide was extremely slow. However, the primary emergency tasks are still necessary to reduce the landslide risk that may affect the residents. Detailed field investigations, boreholes, and exploratory trenches were performed to investigate the characteristics and triggering factors of this landslide. Rainfall and anthropogenic activities (such as informal, unplanned settlements and deforestation) were revealed to be the main triggering factors. The main engineering control measures, including anti-sliding piles, an anti-sliding retaining wall, and intercepting ditch, were applied to stabilize the landslide. However, landslide mitigation measures have not been finished, and hence, some interim measures are suggested to effectively reduce landslide risk. [ABSTRACT FROM AUTHOR]

This paper reports on a recent landslide event, which occurred in Yanyuan County at Xichang City, Sichuan Province, China, on 19 July 2018. Due to the continuous and heavy rainfall between 8 July 2018 and 18 July 2018, approximately 18 million m3 of soil detached from an ancient landslide and buried 65 buildings, the entire sliding process lasted for several minutes. Here, based on field investigations and unmanned aerial vehicle (UAV) photography, we describe the characteristics and failure mechanism of this landslide. The results showed that the landslide was triggered by the continuous and heavy rainfall. Additionally, during the first post-failure stage, the motion of this landslide involved typical successive movement, and over time, the style of movement progressively turned flow-like, which can be classified as debris slide by using the 1978 Varnes classification scheme. The landslide event described here is an outstanding case for landslide studies. In particular, this landslide represents a good illustration of the transform mechanism of landslide movement from sliding to flow-like. [ABSTRACT FROM AUTHOR]

Calculating the failure probability of a landslide is important in engineering related to geological process and geomorphological evolution. Strength parameters of a soil (i.e., cohesion and internal friction angle) are regarded as uncertain-but-bounded parameters. In this study, a new method is proposed for computing the landslide failure probability based on a convex set model and artificial neural network (ANN). In the new method, ANN is used to determine the limit state function of landslide stability, and the failure probability is determined using a simple iterative algorithm. The new method was applied to calculate the failure probability of the Gufenping landslide in Nanjiang, Sichuan, China. The results calculated using a Monte Carlo simulation (MCS) method confirmed that the new method accurately and quickly obtains the failure probability of a landslide. Additionally, compared with the two-dimensional calculation method, the one-dimensional analysis method overestimates the failure probability of the landslide. The results of single factor and global sensitivity analysis indicate that the average of internal friction angle is the main factor affecting the stability of the landslide. It is easy to calculate failure probability of landslides using the novel method than using the conventional methods. [ABSTRACT FROM AUTHOR]

The failure of the 2017 Xinmo catastrophic rockslide in Maoxian County, Sichuan, Southwest China was a combined effect of long-term and short-term triggering factors. Field investigation, historical data collection, laboratory tests, chemical and microstructure analysis and numerical simulations were adopted herein to reveal the initiation and failure mechanism of the Xinmo rockslide. The analytical results showed that this failure involved the coupling of several triggering factors. The initial cracks in the rock mass were induced by historical earthquakes due to the convex topography and the direction effect. Rock masses from the source area of the Xinmo rockslide contain water-sensitive minerals, i.e., albite and chamosite, and the easily oxidized chemical element Fe, resulting in obvious strength deterioration under the action of water. The scanning electron microscopy (SEM) experimental results indicated that the internal structure of the rock mass is conducive to weathering. The compressive and shear strengths of the rock mass were reduced due to freeze-thaw cycles and weathering. The antecedent rainfall further deteriorated the stability of the slope, and stress and deformation accumulated continually in the locked section. Finally, the locked section sheared out, and the slope failed. An entrainment effect was observed in the Xinmo rockslide due to the presence of old landslide deposits and the antecedent rainfall, resulting in an amplification of the catastrophic rockslide. A simplified three dimensional analysis model was established in this study to reveal the influence of the triggering factors on the failure mechanism of the Xinmo rockslide. [ABSTRACT FROM AUTHOR]

A large, deep-seated ancient landslide was partially reactivated on 17 June 2020 close to the Aniangzhai village of Danba County in Sichuan Province of Southwest China. It was initiated by undercutting of the toe of this landslide resulting from increased discharge of the Xiaojinchuan River caused by the failure of a landslide dam, which had been created by the debris flow originating from the Meilong valley. As a result, 12 townships in the downstream area were endangered leading to the evacuation of more than 20000 people. This study investigated the Aniangzhai landslide area by optical and radar satellite remote sensing techniques. A horizontal displacement map produced using cross-correlation of high-resolution optical images from Planet shows a maximum horizontal motion of approximately 15 meters for the slope failure between the two acquisitions. The undercutting effects on the toe of the landslide are clearly revealed by exploiting optical data and field surveys, indicating the direct influence of the overflow from the landslide dam and water release from a nearby hydropower station on the toe erosion. Pre-disaster instability analysis using a stack of SAR data from Sentinel-1 between 2014 and 2020 suggests that the Aniangzhai landslide has long been active before the failure, with the largest annual LOS deformation rate more than 50 mm/yr. The 3-year wet period that followed a relative drought year in 2016 resulted in a 14% higher average velocity in 2018–2020, in comparison to the rate in 2014–2017. A detailed analysis of slope surface kinematics in different parts of the landslide indicates that temporal changes in precipitation are mainly correlated with kinematics of motion at the head part of the failure body, where an accelerated creep is observed since spring 2020 before the large failure. Overall, this study provides an example of how full exploitation of optical and radar satellite remote sensing data can be used for a comprehensive analysis of destabilization and reactivation of an ancient landslide in response to a complex cascading event chain in the transition zone between the Qinghai-Tibetan Plateau and the Sichuan Basin. [ABSTRACT FROM AUTHOR]

Post-event Interferometric Synthetic Aperture Radar (InSAR) analysis on a stack of 45 C-band SAR images acquired by the ESA Sentinel-1 satellites from 9 October 2014 to 19 June 2017 allowed the identification of a clear precursory deformation signal for the Maoxian landslide (Mao County, Sichuan Province, China). The landslide occurred in the early morning of 24 June 2017 and killed more than 100 people in the village of Xinmo. Sentinel-1 images have been processed through an advanced multi-interferogram analysis capable of maximising the density of measurement points, generating ground deformation maps and displacement time series for an area of 460 km straddling the Minjiang River and the Songping Gully. InSAR data clearly show the precursors of the slope failure in the source area of the Maoxian landslide, with a maximum displacement rate detected of 27 mm/year along the line of sight of the satellite. Deformation time series of measurement points identified within the main scarp of the landslide exhibit an acceleration starting from April 2017. A detailed time series analysis leads to the classification of different deformation behaviours. The Fukuzono method for forecasting the time of failure appear to be applicable to the displacement data exhibiting progressive acceleration. Results suggest that satellite radar data, systematically acquired over large areas with short revisiting time, could be used not only as a tool for mapping unstable areas, but also for landslide monitoring, at least for some typologies of sliding phenomena. [ABSTRACT FROM AUTHOR]

The 2008 Wenchuan earthquake induced a large number of landslides, and a vast amount of loose landslide materials deposited on steep hill slopes or in channels. Such loose materials can become sources of deadly debris flows once triggered by storms. On 13 August 2010, a storm swept Yingxiu and its vicinity, triggering a catastrophic debris flow with a volume of 1.17 million m in Xiaojiagou Ravine. The debris flow buried 1,100 m of road, blocked a river and formed a debris flow barrier lake. A detailed field study was conducted to understand the initiation mechanisms and runout characteristics of this debris flow. Two types of debris flows are identified, namely hill-slope debris flow and channelized debris flow. The hill-slope debris flow was triggered in the forms of firehose effect, rilling and landsliding, whereas the channelized debris flow was triggered in the form of channel-bed failure. This debris flow was a water-rock flow since most particles were gravel, cobble or larger rocks and the fraction of silt and clay was less than 2%. Grain contact friction, pore-pressure effects and inertial grain collision were the three most important physical interactions within the debris flow. Such interactions yielded a smaller runout distance (593 m) compared with those of mud-rock flows of similar size. [ABSTRACT FROM AUTHOR]

The Wenchuan Earthquake of May 12, 2008 triggered large numbers of geo-hazards. The heavy rain on 13 August 2010 triggered debris flows with total volume of more than 6 million cubic meters and the debris flows destroyed 500 houses and infrastructure built after the Wenchuan Earthquake. The study area Qingping Town was located in the northwestern part of the Sichuan Basin of China, which needs the second reconstructions and the critical evaluation of debris flow. This study takes basin as the study unit and defines collapse, landslide and debris flow hazard as a geo-hazard system. A multimode system composed of principal series system and secondary parallel system were established to evaluate the hazard grade of debris flow in 138 drainage basins of Qingping Town. The evaluation result shows that 30.43% of study basins (42 basins) and 24.58% of study area, are in extremely high or high hazard grades, and both percentage of basin quantity and percentage of area in different hazard grades decrease with the increase of hazard grade. According to the geo-hazard data from the interpretation of unmanned plane image with a 0.5-m resolution and field investigation after the Wenchuan Earthquake and 8.13 Big Debris Flow, the ratio of landslides and collapses increased after the Wenchuan Earthquake and the ratios of extremely high or high hazard grades were more than moderate or low hazard grades obviously. 23 geo-hazards after 8.13 Big Debris Flow in Qingping town region all occurred in basins with extremely high or high hazard grades, and 9 debris flows were in basins with extremely high hazard grade. The model of multimode system for critical evaluation could forecast not only the collapse and landslide but also the debris flow precisely when the basin was taken as the study unit. [ABSTRACT FROM AUTHOR]