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The reactivation of ancient landslides, a persistent issue that hinders urban development and threatens human safety, is the central focus of this paper. Specifically, this paper offers a detailed description and analysis of an ancient landslide that reactivated on February 26, 2021, in Guoye town, Zhouqu County, Gansu Province, China, and has remained slow-moving since then. With a volume of approximately 21.2 million m3 of rock and soil masses involved, this landslide poses a significant risk to the safety of 1119 individuals from 282 households in Moli village and Guoye village. A combination of field investigations, remote sensing interpretation, in situ monitoring, and interferometric synthetic aperture radar (InSAR) was used to investigate the landslide characteristics and reactivation mechanisms. The results revealed that the presence of sliding-prone rock formations and tectonic movement were the primary contributors to the formation of the Moli landslide. Furthermore, the accumulation of bedrock blocks in the quarry and collapsed rock masses caused by fault activity at the rear parts, along with the additional load from buildings on the middle and rear sections, increased the sliding force. Simultaneously, the erosion of the Dashuiba River resulted in a reduction in the anti-slip force, thereby directly facilitating landslide reactivation. The notable increase in annual rainfall in recent years has played a crucial role in ancient landslide reactivation by elevating the groundwater level and softening the shear strength of the sliding zone soil. The findings of this study contribute to an enhanced understanding of the reactivation mechanisms in giant, ancient landslides situated within areas characterized by highly complex geological conditions. [ABSTRACT FROM AUTHOR]

Due to the changing mechanical environment, it is difficult to determine the evolution process of the rocky bank. In this paper, the Banbiyan dangerous rock mass (BDRM) in the Three Gorges Reservoir area was taken as an example to analyze its failure characteristics. As for this specific evolutionary environment, a new judgment system was proposed, involving the comprehensive field survey method and indoor tests. The key information about the steep areas, interior areas, and underwater areas was obtained, which overcomes the limitations of the conventional survey methods. On this basis, combined with a simplified geo-mechanical model and dry-wet cycle tests, the evolution trend of BDRM under variable water levels was predicted. The results show that the stability of BDRM depends on many factors, including the self-weight of overlying rock mass (the location of the broken area), the strength of the broken area, the water pressure of the trailing edge crack (rainfall) and the reservoir water level. According to the calculation results, the BDRM is likely to slip along the broken area after 46 dry-wet cycles. The application of related methods can provide an important reference for judging the evolution trend of unstable rocky banks worldwide. [ABSTRACT FROM AUTHOR]

On June 28, 2016, a landslide occurred in Xinlu Village, Qijiang District, Chongqing, China. The landslide volume was 854,200 m3. Exposed to continuous rainfall, the rocky part of the trailing edge slipped first, and then the soil part of the leading edge was pushed downward. Numerical simulations were used to analyze the formation characteristics of the landslide and to predict the stability of the landslide after the slip. In order to ensure the accuracy and rationality of numerical calculation, we used field surveys, laboratory tests, and back-calculation to restore the typical sections, determine the relevant mechanical parameters, and control boundary conditions. The displacement, stress, effective plastic strain distribution, and safety factor, obtained by numerical calculation, were compared to examine the action of rainfall. According to the evaluation of landslide stability and numerical calculation results, the Xinlu Village landslide remained in a peristaltic stage after the initial overall sliding, and additional sliding could occur under storm conditions. [ABSTRACT FROM AUTHOR]

The 2008 Ms 8.0 Wenchuan earthquake triggered a large number of extensive landslides. It also affected geologic properties of the mountains such that large-scale landslides followed the earthquake, resulting in the formation of a disaster chain. On 10 July 2013, a catastrophic landslide-debris flow suddenly occurred in the Dujiangyan area of Sichuan Province in southeast China. This caused the deaths of 166 people and the burying or damage of 11 buildings along the runout path. The landslide involved the failure of ≈1.47 million m, and the displaced material from the source area was ≈0.3 million m. This landslide displayed shear failure at a high level under the effects of a rainstorm, which impacted and scraped an accumulated layer underneath and a heavily weathered rock layer during the release of potential and kinetic energies. The landslide body entrained a large volume of surface residual diluvial soil, and then moved downstream along a gully to produce a debris flow disaster. This was determined to be a typical landslide-debris flow disaster type. The runout of displaced material had a horizontal extent of 1200 m and a vertical extent of 400 m. This was equivalent to the angle of reach (fahrböschung angle) of 19° and covered an area of 0.2 km. The background and motion of the landslide are described in this study. On the basis of the above analysis, dynamic simulation software (DAN3D) and rheological models were used to simulate the runout behavior of the displaced landslide materials in order to provide information for the hazard zonation of similar types of potential landslide-debris flows in southeast China following the Wenchuan earthquake. The simulation results of the Sanxicun landslide revealed that the frictional model had the best performance for the source area, while the Voellmy model was most suitable for the scraping and accumulation areas. The simulations estimated that the motion could last for ≈70 s, with a maximum speed of 47.7 m/s. [ABSTRACT FROM AUTHOR]

This paper uses the catastrophic rockslide at Sanxicun village in Dujianyan city as an example to investigate the formation mechanism of a rapid and long run-out rockslide-debris flow of fractured/cracked slope, under the application of a rare heavy rainfall in July 2013. The slope site could be affected by the Wenchuan Ms 8.0 Earthquake in 2008. The sliding involved the thick fractured and layered rockmass with a gentle dip plane at Sanxicun. It had the following formation process: (1) toppling due to shear failure at a high-level position, (2) shoveling the accumulative layer below, (3) forming of debris flow of the highly weathered bottom rockmass, and (4) flooding downward along valley. The debris flow destroyed 11 houses and killed 166 people. The run-out distance was about 1200 m, and the accumulative volume was 1.9 × 10 m. The rockslide can be divided into sliding source, shear-shoveling, and flow accumulative regions. The stability of this fractured rock slope and the sliding processes are discussed at four stages of cracking, creeping, separating, and residual accumulating, under the applications of hydrostatic pressure and uplift pressure. This research also investigates the safety factors under different situations. The double rheological model (F-V model) of the DAN-W software is utilized to simulate the kinematic and dynamic processes of the shear-shoveling region and debris flow. After the shear failure occurred at a high-level position of rock, the rockslide moved for approximately 47 s downward along the valley with a maximum velocity of 35 m/s. This is a typical rapid and long run-out rockslide. Finally, this paper concludes that the identification of the potential geological hazards at the Wenchuan mountain area is crucial to prevent catastrophic rockslide triggered by heavy rainfall. The identified geological hazards should be properly considered in the town planning of the reconstruction works. [ABSTRACT FROM AUTHOR]

Red beds are widespread in the eastern Sichuan Basin, SW China, and are often called 'landslide prone strata'. The dip angle of these red beds is commonly less than 10°, which is much less than the angle of internal friction between the bedding planes. Thus, in theory, failure will not occur along the bedding planes in such strata. We used field methods, numerical modeling, and ring shear tests to investigate the effects of rainfall on landslides in shallow-dipping red beds, using the Qingning landslide in SW China as an example. Our results suggest that the probable mechanisms leading to failure are (1) permeation of rainfall through the cracks at the back of the landslide site that increases the hydrostatic water pressure, which adds to the sliding stress and reduces the effective stress on the failure surface, gradually destabilizing the mass. In the Qingning landslide, the maximum water pressure was as high as 21.8 m before failure. When the factor of safety decreased markedly to 1.015, the sliding mass started to slowly slide. (2) As sliding takes place, the sliding zone soil is under undrained shear and its shear strength decreases sharply to a critical value, which promotes rapid sliding. In the Qingning landslide, the shear resistance of the sliding zone soil decreased markedly to 25.9 kPa as the pore water pressure increased to 118.3 kPa. This study may provide a theoretical basis for landslide forecasting in gently inclined red beds of the eastern Sichuan Basin as well as in similar geological settings worldwide. [ABSTRACT FROM AUTHOR]