Fawu Wang

Mechanism of a long-runout landslide triggered by the August 1998 Heavy rainfall in Fukushima Prefecture, Japan

Abstract Heavy rainfall from 26 to 31 August 1998 triggered many landslides in Nishigo Village of southern Fukushima Prefecture, Japan. The Hiegaesi landslide, a long-runout landslide with travel angle of 11°, which occurred in loamy volcanic-ash/pumice layer and was deposited in a nearby rice paddy, was investigated. In an observation pit dug in the middle part of the landslide deposit, the sliding zone just above the deflected rice plants was observed, and it was confirmed that grain crushing occurred in the sliding zone. The triggering and sliding mechanisms of this landslide then were investigated by ring-shear tests in laboratory. For the triggering mechanism, one saturated naturally drained test (test A: torque-controlled test) and one saturated undrained test (test B: speed-controlled test) were conducted on the samples taken from the source area of the landslide. Even in the naturally drained test opening the upper drain valve of the shear box, a temporary liquefaction occurred. In the undrained test, excess pore-pressure was generated along with shearing, and “sliding-surface liquefaction” phenomenon was observed. The effective stress and shear resistance finally decreased to near zero. These results can explain the observed phenomenon of small friction resistance like a flow of liquid when the sliding mass slid out of the source area. For the sliding mechanism of the landslide in the rice paddy, saturated undrained test (test C: speed-controlled test) was performed on soil sample above the deflected rice plants. The apparent friction angle obtained in this test was 8°. In addition, the residual friction angle measured after test B and test C was the same value of 41°. Combining with the observation on the shear zone in the ring-shear box after test C, it is concluded that, during the sliding in rice paddy, the undrained shear strength of the soil layer itself mainly influenced the high mobility of the landslide, probably because the friction between rice plants and soils is greater than the undrained shear strength inside the soil mass.

Displacement Monitoring and Physical Exploration on the Shuping Landslide Reactivated by Impoundment of the Three Gorges Reservoir, China

The Three Gorges Dam construction on the Yangtze River in China is the largest hydro-electricity project in the world. After the first impoundment in June 2003, many landslides occurred or reactivated. Shuping landslide is one of the most active landslides among them. In this paper, the deformation of the Shuping landslide monitored by GPS, extensometers, and crack measurements are summarized. Also, for the investigation of the groundwater situation, 1 m depth ground temperature measurement was conducted, and the groundwater veins were estimated. Based on the monitoring data and exploration results, a deformation model of the landslide caused by impoundment of reservoir was proposed.

Landslide disaster mitigation in Three Gorges Reservoir, China

Regional Properties of Landslides.- Geo-hazard Initiation and Assessment in the Three Gorges Reservoir.- Bank Slope Stability Evaluation for the Purpose of Three Gorges Reservoir Dam Construction*.- Research on the Characteristics and Slope Deformation Regularity of the Badong Formation in the Three Gorges Reservoir Area.- Distribution of Dangerous Rockmasses on the High Steep Slopes in the Three Gorges Area.- An Evaluation Study of Bank Collapse Prediction in the Three Gorges Reservoir Area.- Distribution Features of Landslides in Three Gorges Area and the Contribution of Basic Factors.- Discussion on Land Use Based on Landslide Management in Three Gorges Reservoir Areas.- Case Studies for Typical Landslides.- Mechanism for the Rapid Motion of the Reactivated Qianjiangping Landslide in Three Gorges Dam Reservoir, China.- Evaluation of the Roles of Reservoir Impoundment and Rainfall for the Qianjiangping Landslide in Zigui County, Three Gorges Area.- Unsaturated Creep Test and Modeling of Soils from the Sliding Zone of the Qianjiangping Landslide in the Three Gorges Area, China.- Monitoring on Shuping Landslide in the Three Gorges Dam Reservoir, China.- The Anlesi Landslide in Wanzhou, China: Characteristics and Mechanism of a Gentle Dip Landslide.- Preliminary Study on Mud-Rock Flows Channel of the Bailuxi River, Wuxi County, China.- Stability Assessment and Stabilizing Approaches for the Majiagou Landslide, Undergoing the Effects of Water Level Fluctuation in the Three Gorges Reservoir Area.- Mass Rock Creep and Landsliding on the Huangtupo Slope in the Reservoir Area of the Three Gorges Project, Yangtze River, China.- Study on the Possible Failure Mode and Mechanism of the Xietan Landslide When Exposed to Water Level Fluctuation.- A Study of the 1985 Xintan Landslide in Xiling Gorge, Three Gorges Area, China.- Time Prediction of the Xintan Landslide in Xiling Gorge, the Yangtze River.- Back-Analysis of Water Waves Generated by the Xintan Landslide.- New Methodologies Applied in this Area.- Intelligent Optimization of Reinforcement Design Using Evolutionary Artificial Neural Network for the Muzishu Landslide Based on GIS.- The Application of Fractal Dimensions of Landslide Boundary Trace for Evaluation of Slope Instability.- Uncertainty Evaluation of the Stability of the Huanglashi Landslide in the Three Gorges of the Yangtze River.- Recognition of Lithology and Its Use in Identification of Landslide-Prone Areas Using Remote Sensing Data.- Construction and Application of a Real-Time Monitoring System for Landslides.- Entropy-Based Hazard Degree Assessment for Typical Landslides in the Three Gorges Area, China.- The Conceptual Model of Groundwater Systems in a Large-Scale Landslide - A Case Study of the Baota Landslide in the Impoundment Area of Three Gorges Project.- Bank Collapse Along the Three Gorges Reservoir and the Application of Time-Dependent Modeling.

Key factors influencing the mechanism of rapid and long runout landslides triggered by the 2008 Wenchuan earthquake, China

BackgroundThe 2008 Wenchuan earthquake triggered many rapid and long runout landslides, which directly caused great loss of property and human lives and were responsible for a large percentage of total damages caused by the earthquake. It is very important for the purposes of landslide disaster prevention and mitigation to understand the earthquake triggered mechanism of initiation and motion of rapid and long runout landslides, which can potentially be the deadliest of ground failures.ResultsIn this paper, field investigations of some highly damaging landslides caused by the Wenchuan earthquake are introduced first, and followed by data from ring shear tests used to simulate the initiation and motion of one landslide in particular, the Donghekou complex landslide.ConclusionsIt was found that groundwater and valley water played key roles in the rapid motion and long runout process of this landslide during the great earthquake.

The laboratory evidence of phase transformation from landslide to debris flow

The dominant factors initiating the transformation of landslides into catastrophic debris flow are examined. The research found that a threshold pore pressure determined from theoretical and experimental analysis was enough to initiate liquefaction type of failure in sandy materials. Loading tests to failure on sourcearea sandy soils from a catastrophic landslide location show that under definite conditions of loading, a threshold state, characterized by the equality and constancy of pore pressure and shear resistance develops in the sands at a threshold density. Sands at this density clearly define the boundary between contractive and dilative specimens under same effective normal stress. Confirmatory experiments were then conducted using silica sand. Analyses showed that samples in which the threshold pore pressure was exceeded readily liquefied while those in which the pore pressure was below the limit dilated. The concept of threshold pore pressure fills the gap created by classical theories of soils liquefaction that have identified densities at which phase transformation and steady state lines can be defined. The new concept shows there is a density at which both lines merge and it is proposed that sands transiting from dense to loose and vice versa will first pass through the threshold state. While the stability of a slope founded on sandy soils may be breached when the pore pressure exceeds a certain limit, it is possible to make estimates of the limit. Where such estimates are accompanied with adequate field measurements, the effectiveness of landslide prevention projects may be enhanced.

Landslides Induced by a Combined Effect of Earthquake and Rainfall

Many landslides are triggered by rainfall and many landslides are triggered by earthquakes. The probability that rainstorms (typhoons or hurricanes) and earthquakes attack same area is not high or rare. Combined effects of rainstorms and earthquakes have not been examined. The 2004 Mid-Niigata Prefecture earthquake (M6.8) caused twelve landslides more than one million cubic meters, and many landslide dams were formed by large-scale displaced landslide masses. While, the 1995 Hyogo-ken Nambu earthquake (M7.2) did neither cause any large-scale landslide, nor landslide dam although it has a greater magnitude and a similar depth of earthquake. One major difference is: a typhoon attacked Niigata Prefecture in three days before the earthquake, and the 1995 Hyogo-ken Nambu area was very dry before the earthquake. Combined effects of two triggering factors were examined for two cases which the authors investigated: the 2006 Southern Leyte landslide possibly triggered by a nearby small earthquake (M2.6), and the Higashi- Takezawa landslide triggered by the 2004 Mid-Niigata Prefecture landslide.

Mechanism of Landslide Causing the December 2002 Tsunami at Stromboli Volcano (Italy)

Between 29 and 30 December 2002 the NW flank of Stromboli Volcano (Sciara del Fuoco) was involved in a series of large-scale instability phenomena which culminated in submarine and subaerial destructive landslides provoking two tsunami waves with a maximum run-up of 10 m. In this paper, part of the results of a joint research between the National Research Council (Italy) and the Disaster Prevention Research Institute of Kyoto University (Japan) are presented. The activity has focused on the mechanical characterization of the volcanoclastic material forming the Sciara del Fuoco depression and the interpretation of landslide mechanisms on the basis of large-scale ring shear tests. Attention is given here to the initiation and propagation of the submarine landslide which caused the first tsunami. In order to investigate the material response to different displacement rates in terms of shear resistance, pore pressure generation and grain crushing, ring shear tests were conducted in both undrained and drained conditions. Experimental results indicate that a fully or partial liquefaction mechanism can be invoked to explain the failure of the submarine flank of the Sciara del Fuoco and the long run-out which followed, as it was suggested by comparing pre- and post-failure in situ observations.

Critical hydraulic gradients for seepage-induced failure of landslide dams

BackgroundLandslide dams formed by rock avalanche processes usually fail by seepage erosion. This has been related to the complex sedimentological characteristics of rock avalanche dams which are mostly dominated by fragmented and pulverized materials. This paper presents a comprehensive experimental programme which evaluates the critical hydraulic and geometrical conditions for seepage-induced failure of landslide dams. The experiments were conducted in a flume tank specifically designed to monitor time-dependent transient changes in pore-water pressures within the unsaturated dam materials under steady-state seepage. Dam models of different geometries were built with either mixed or homogeneous materials. Two critical hydraulic gradients corresponding to the onset of seepage erosion initiation and collapse of the dam crest were determined for different upstream inflow rates, antecedent moisture contents, compactive efforts, grain size ranges, and dam geometries.ResultsTwo major types of dam failure were identified: Type I and Type II. These were further subdivided into minor failure processes which include exfiltration, sapping, downstream toe bifurcation, and undermining of the downstream face. The critical hydraulic gradients for seepage erosion initiation varied from 0.042 to 0.147. Experiments conducted with the mixed materials indicate that the critical hydraulic gradients for collapse of the dam crest increased with an increase in uniformity coefficient.ConclusionsThe deformation behaviour of the dams was significantly influenced by particle density, pore geometry, hydraulic conductivity, and the amount of gravel and pebbles present in the materials. The results indicate that the critical seepage velocity for failure of the dams decreased with an increase in downstream slope angle, but increased with an increase in pore geometry, dam height, dam crest width, upstream inflow rate, and antecedent moisture content.

Effects of topographic and geological features on building damage caused by 2015.4.25 Mw7.8 Gorkha earthquake in Nepal: a preliminary investigation report

BackgroundThe 2015.4.25 Gorkha earthquake affected about eight million people in Nepal. Most injuries and loss of life were due to building collapse and damage. This work aims to investigate the topographical and geological effects on the severe damage caused by this earthquake.FindingsIn one-week field investigation in the earthquake-affected areas, several severely damaged areas with different topographic and geological features were surveyed, as well as the site of Kaligandaki River landslide dam failure. Some general tendency related to the building damage and landslide dam failure was obtained.ConclusionsThrough the field investigation, it was found that geological and geomorphological characteristics of a site, combined with the structure feature of the building, such as the short column effect, amplified the seismic vibration and caused severe building collapse and damages, i.e., 1) For buildings on flat area consisting of lacustrine deposit or diluvial deposit, resonance effect might be the main reason, while for the buildings on the top of hills or narrow ridges, topographic effect and sometimes, short column effect should take the main responsibility; 2) For buildings located on the gentle slopes or landslides, the settlement in the infill side caused by the strong seismic vibration can be the main reason; 3) Besides of the building failure on lacustrine deposit, failure patterns in three types of topographic and geological features, i.e., narrow ridges formed by landslides, diluvial deposits and alluvial fans, and landslides, were proposed as the possible mechanism of the building damage caused by the earthquake. For landslide dam failure, it was found that landslide dam could easily breach or collapse, when the landslide-dam-deposits were fine.

Initiation and Motion Mechanism of the Donghekou Rapid and Long Runout Landslide Triggered by the 2008 Wenchuan Earthquake, China

The 2008 Wenchuan earthquake triggered many rapid and long runout landslides, which directly caused great loss of property and human lives and were responsible for a large percentage of total damages. For landslide disaster mitigation, it is very important to understand the initiation and motion mechanism of the rapid and long runout landslides, which can potentially be the deadliest of ground failures. In this paper, results of field investigation and ring shear tests simulating the initiation and motion of the Donghekou landslide is presented. It is found that groundwater caused great drop-down of the shear resistance during the initiation and rapid motion of this landslide when it was triggered by the great earthquake.