Yifei Cui

Experimental study on the moving characteristics of fine grains in wide grading unconsolidated soil under heavy rainfall

The initiation mechanism of debris flow is regarded as the key step in understanding the debris-flow processes of occurrence, development and damage. Moreover, migration, accumulation and blocking effects of fine particles in soil will lead to soil failure and then develop into debris flow. Based on this hypothesis and considering the three factors of slope gradient, rainfall duration and rainfall intensity, 16 flume experiments were designed using the method of orthogonal design and completed in a laboratory. Particle composition changes in slope toe, volumetric water content, fine particle movement characteristics and soil failure mechanism were analyzed and understood as follows: the soil has complex, random and unstable structures, which causes remarkable pore characteristics of poor connectivity, non-uniformity and easy variation. The major factors that influence fine particle migration are rainfall intensity and slope. Rainfall intensity dominates particle movement, whereby high intensity rainfall induces a large number of mass movement and sharp fluctuation, causing more fine particles to accumulate at the steep slope toe. The slope toe plays an important role in water collection and fine particle accumulation. Both fine particle migration and coarse particle movement appears similar fluctuation. Fine particle migration is interrupted in unconnected pores, causing pore blockage and fine particle accumulation, which then leads to the formation of a weak layer and further soil failure or collapses. Fine particle movement also causes debris flow formation in two ways: movement on the soil surface and migration inside the soil. The results verify the hypothesis that the function of fine particle migration in soil failure process is conducive for further understanding the formation mechanism of soil failure and debris flow initiation.

The formation of the Wulipo landslide and the resulting debris flow in Dujiangyan City, China

The Wulipo landslide, triggered by heavy rainfall on July 10, 2013, transformed into debris flow, resulted in the destruction of 12 houses, 44 deaths, and 117 missing. Our systematic investigation has led to the following results and to a new understanding about the formation and evolution process of this hazard. The fundamental factors of the formation of the landslide are a high-steep free surface at the front of the slide mass and the sandstone-mudstone mixed stratum structure of the slope. The inducing factor of the landslide is hydrostatic and hydrodynamic pressure change caused by heavy continuous rainfall. The geological mechanical model of the landslide can be summarized as “instability - translational slide - tension fracture - collapse” and the formation mechanism as “translational landslide induced by heavy rainfall”. The total volume of the landslide is 124.6×104 m3, and 16.3% of the sliding mass was dropped down from the cliff and transformed into debris flow during the sliding process, which enlarged 46.7% of the original sliding deposit area. The final accumulation area is found to be 9.2×104 m2. The hazard is a typical example of a disaster chain involving landslide and its induced debris flow. The concealment and disaster chain effect is the main reason for the heavy damage. In future risk assessment, it is suggested to enhance the research on potential landslide identification for weakly intercalated slopes. By considering the influence of the behaviors of landslide-induced debris flow, the disaster area could be determined more reasonably.

Effects of particle size of mono-disperse granular flows impacting a rigid barrier

Understanding the interaction between complex geophysical flows and barriers remains a critical challenge for protecting infrastructure in mountainous regions. The scientific challenge lies in understanding how grain stresses in complex geophysical flows become manifested in the dynamic response of a rigid barrier. A series of physical flume tests were conducted to investigate the influence of varying the particle diameter of mono-dispersed flows on the impact kinematics of a model rigid barrier. Particle sizes of 3, 10, 23 and 38xa0mm were investigated. Physical tests results were then used to calibrate a discrete element model for carrying out numerical back-analyses. Results reveal that aside from considering bulk characteristics of the flow, such as the average velocity and bulk density, the impact load strongly depends on the particle size. The particle size influences the degree of grain inertial stresses which become manifested as sharp impulses in the dynamic response of a rigid barrier. Impact models that only consider a single impulse using the equation of elastic collision warrant caution as a cluster of coarse grains induce numerous impulses that can exceed current design recommendations by several orders of magnitude.xa0Although these impulses are transient, they may induce local strucutral damage. Furthermore, the equation of elastic collision should be adopted when the normalized particle size with the flow depth, δ/h, is larger than 0.9 for Froude numbers less than 3.5.

Earthquake-triggered landslides affecting a UNESCO Natural Site: the 2017 Jiuzhaigou Earthquake in the World National Park, China

On August 8th, 2017, an Ms 7.0 magnitude earthquake occurred in Jiuzhaigou County, northern Sichuan Province, China. The Jiuzhaigou Valley World National Park was the most affected area due to the epicentre being located in the scenic area of the park. Understanding the distribution characteristics of landslides triggered by earthquakes to help protect the natural heritage sites in Jiuzhaigou Valley remains a scientific challenge. In this study, a relatively complete inventory of the coseismic landslides triggered by the earthquake was compiled through the interpretation of high-resolution images combined with a field investigation. The results indicate that coseismic landslides not only are concentrated in Rize Gulley, Danzu Gully and Zezhawa Gully in the study area but also occur in the front part of Shuzheng Gully along the road network (from the entrance of Jiuzhaigou Valley to Heye Village). The landslides predominantly occur on the east- and southeast-facing slopes in the study area, which is a result of the integrated action of the valley direction and fault movement direction. The back-slope effect and the slope structure caused the difference in coseismic landslide distribution within the three gullies (Danzu Gully, Rize Gully, and Zezhawa Gully) near the inferred fault. In addition, the topographic position index was used to analyse the impact of microlandforms on earthquake-triggered landslides by considering the effect of the slope angle. The study results reveal a higher concentration of landslides in the slope position class of the middle slope (30°-50°) in Jiuzhaigou Valley. These findings can provide scientific guidance for the protection of natural heritage sites and post-disaster reconstruction in Jiuzhaigou Valley.

Back analysis of a debris landslide based on a real-time video recording: sliding process and post-slide investigation

A real-time video recording of the 2008 landslide in Nanmu, China is analyzed to reveal information regarding the format, velocity, post-failure characteristics and destructiveness of the slide. This recording indicates that the failure proceeded in three stages. Back analysis has found that colluvial deposits (residual slope deposits and cemented colluvial deposits) in a weakly cemented form were the internal factor that contributed to the failure, and that damage resulting from the Wenchuan earthquake in 2008 and focused rainstorms were the external contributing factors. A field investigation showed that the destructiveness of the slide involved integral pushing damage to the base of man-made structures and damage to buildings from the impact of rolling gravel. In the 5xa0years following the Nanmu landslide, the local landscape has evolved further with sporadic individual rock falls or local collapses along the sliding bed and boundary. Corresponding prevention practices for the secondary disasters have demonstrated that the combined measures, including the removal of accumulated gravel, installation of a passive prevention net on the sliding bed, construction of an underground diversion culvert and a retaining wall, have been extremely effective and economical during the past 5xa0years.

The Effect of Climate Change on Alpine Mountain Hazards Chain: A Case Study in Tianmo Ravine, Tibet, China

Mountain hazards behave a relatively high incidence under regional climate change condition. This compound hazard is often initiated by environment and climate change. A combination of glacial melting and rainfall-induced compound disaster event happened in Tianmo Ravine, Bomi County, Tibet, China on 25 July and 4 September 2010, separately. The debris flow with 450,000 m3 volume transported along gully and then flushed into Parlung Tsangpo River in the deposition zone, resulting in a destruction of 400 m length section of the G318 highway from Bomi County to Lhasa. A detailed interpretation of this disaster was conducted using unique and high-resolution images obtained through remote sensing. The source material, terrain condition, and climate condition were analyzed to have a comprehensive understanding of environmental background for hazard initiation. It is concluded that the occurring of this disaster event is the comprehensive result of multiple factors led by the climate change. Abundant antecedent precipitation, melting of the glacier, and the instability of soil caused by wetting-drying cycles, led to the occurrence of the Tianmo Ravine disaster. Based on taking various factors into account synthetically, the comprehensive hazards pattern is named using “Mountain hazards pattern in glacial alpine region under climate change (MH-GA-CC)”. The climate is the special factor feature that should be taken into account emphatically. The study here provides a scientific base pattern for the future risk assessment of glacial alpine areas under regional climate change background.

Correction to: Utilizing crowdsourcing to enhance the mitigation and management of landslides

The published version of this article, unfortunately, contained error. The author realized an important change of the disclaimer section, which needs to qualify that only Figure 2 requires the permission from the GEO and not the entire content of the manuscript

Utilizing crowdsourcing to enhance the mitigation and management of landslides

Landslides are mainly triggered by earthquakes and rainfall and have poor temporal predictability. Landslides pose significant threats to settlements and infrastructure in mountainous regions around the world. To mitigate this natural hazard, a new paradigm of landslide mitigation and management is required. Increasing smartphone ownership around the world, especially in developing countries, offers scientists an opportunity to embrace crowdsourcing so as to improve landslide research. This paper presents a new landslide information system (LIS) comprising a smartphone app and an administrative interface and database. The mobile app has been published for both iPhone and Android platforms. The interface of the smartphone app is powered by the highly-customizable Google Maps platform, which is overlaid with real-time landslide data. Users can choose between visualizing “known sites” and “contribution” of landslide data. The visualization option shows published landslides and areas that are susceptible. Users can contribute their GPS coordinates and multimedia to enhance landslide reports. A comparison with similar systems, potential applications, and challenges of using smartphone technology for mitigating landslides are also discussed.

The characteristics of the Mocoa compound disaster event, Colombia

A rainfall-induced compound disaster happened in Mocoa in the pre-dawn hours of 1 April 2017. More than 300 people were killed, and a large number of houses and roads were destroyed in the worst catastrophe in the history of Mocoa. To investigate this disaster, a detailed interpretation was carried out using high-resolution images. Analysis of disaster characteristics based on satellite image revealed that the disaster could be identified as a consequence of compound mountain hazards including landslides, debris flows, and mountain torrents. The mountain hazards converged in the mountain watershed, which amplified the disaster’s effects. Analysis considers that this disaster is the result of heavy rainfalls. Moreover, in-depth interpretation of rainfall data and satellite images spanning over 16xa0years reveals that the previous El Niño event (2014–2016) also played an important role, which caused reduced rainfall and vegetation coverage. The long period of drought brought by El Niño affected the growth of vegetation and reduced the ability of vegetation to cope with heavy rainfalls. The results reveal that both antecedent rainfalls and climate impact need to be taken into consideration for mountain hazard analysis.

Effect of joint type on the shear behavior of synthetic rock

The shear behavior of the discontinuities of rock is important because it is closely related to the stability of a rock mass. The scientific challenge lies in the understanding of how different types of joint are related to the failure criterion. In the current study, direct shear tests are used to investigate the shear behavior of continuous planar joints, stepped joints, and discontinuous open joints. The joints were cast in a synthetic rock made of plaster, sand, and water and tested under normal stresses that ranged from 50xa0kPa to 3.5xa0MPa. The shear behavior of both the continuous and discontinuous joints has been found to be dependent on the normal stress. At normal stresses above the magnitude of the tensile strength, continuous and discontinuous joints displayed either strain weakening or brittle behavior. Results with the combination of all joint types indicated that the shear strength of the different types of joint increases sharply at low normal stress, and then approaches a lower bound residual strength envelope at high normal stress. At normal stresses of less than the tensile strength (1.84xa0MPa), the strength is dominated by cohesion, while at normal stresses greater than the tensile strength, friction appears to dominate the shear strength. For open joints, the shear stiffness is independent of the normal stress. For closed joints, the shear stiffness will increase as the normal stress increases, particularly evident below a normal stress of 1xa0MPa. Increasing the normal stress reduces the brittleness index of rock samples from 1 to 0. A primary reason for this non-unique failure envelope was the large dilation that occurred at high normal stresses. This dilation was attributed to grain crushing, and the roughness resulting from this crushing and gouge formation as shearing occurred.