Landslide Investigation Results in Sapa Town, Lao Cai Province, Vietnam in December 2019
Nguyễn Văn ThăngGo SatoAkihiko WakaiHoang Viet HungNguyen Duc ManhTakashi KimuraTakanari YAMASAKIShinichi TosaKazunori HayashiAkino WatanabeTakatsugu OzakiNobuyuki AsaiNanaha Kitamura
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Abstract:
Every year, especially in the rainy season, landslides occur quite often in Lao Cai – a northern mountainous province of Vietnam. Specifically, in the year 2019, several landslides were observed to occur near the Sapa Ancient Rock Field in Hau Thao commune, Sapa town, Lao Cai province. In December 2019, a landslide investigation was conducted to examine the mechanism and possible causes of the landslides. Besides that, as the landslide distribution in this area is still unclear, this study will also aim to show the landslide denseness in a 700 m × 700 m square map as well as survey results in 2019 of two main landslides in such map. According to the survey, the landslide is the main phenomenon of geomorphological development in this area, being a combination of multiple different landslides with varying sizes and dissimilar triggers. The first survey landslide is about 50 m wide and 350 m long and has still been going on in recent years, with annual horizontal displacement being around 0.8 m. Meanwhile, the second one is a typical flash-landslide caused by rainfall. Despite being quite small in scale, about 15 m × 40 m, its characteristics indicate a dangerous implication in the future. This information will be the basis for further ongoing studies.Keywords:
Field survey
Landslide mitigation
Moluocun Landslide comes from multi-stage accumulations of debris flow. Although the landslide has relatively gentle slope and sliding face,the dynamic water pressure arising out of the good catchment condition of the landslide and the underground water flow is an important factor of the landslide deformation development. Based on the different geneses,Moluocun Landslide may be divided into two landslides,wherein the Landslide 1 is a thrust load caused landslide induced by rainfall with the key to treatment lying on interception and drainage of underground water out of the landslide; the Landslide 2 is a regressive landslide caused by river souring and erosion,and required to be protected by setting anti-scour wall. It is stable now after treated.
Landslide mitigation
Debris flow
Interception
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By investigating typical earthquake-induced landslides in the field,the landslides triggered by 5.12 Wenchuan earthquake were classified into 4 kinds of basic types: slide type landslides,slide-flow type landslides,fall-slide type landslides and fall-eject-slide type landslides according to movement characteristics and modes of the landslides.Moreover,according to the characteristics of sliding and material composition of the landslides and macro disrupted degree of sliding mass,sliding landslides were classified into 3 kinds of secondary types: coherent landslides,disrupted landslides and debris landslides.On the basis,basic characteristics,especially movement characteristics and processes for all types of landslides was thoroughly analyzed by different types of typical landslides cases.It is not only helpful to comprehensively understanding the types and characteristics of earthquake-induced landslides,but also can provide a mitigation and prevention basis for future earthquake-induced landslides.
Landslide mitigation
Debris flow
Mass movement
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This paper takes a landslide in south Qinghai Province as an example,on the basis of the analysis of regional geology and landslide characteristics,analyzed the mechanism of landslide and factors influencing landslide,determined the sliding mode,selected rational parameters for landslide stability calculation,carried out analysis of landslide stability according to the calculation result,summarized the mechanism and reasons of landslide in the area,providing reference experiences for similar geological disaster management.Through the analysis and calculation it concludes: the cause for occurrence of landslide hazards is poor lithology,frozen soil depth in the area where landslide is located is larger,regional temperature difference between summer and winter is large,surface water is frozen in winter and melt in next summer,inducing muddy clay and shale as well as lower cohesion,sliding body creeps continuously and slides down.By using transfer coefficient method,the stability of landslide in different conditions is calculated.The results show that under the natural conditions,shallow sliding surface of landslide is in creep deformation condition,deep sliding surface is basically stable,in consideration of rainstorm and seismic conditions,the landslide is in unstable state.
Landslide mitigation
Lithology
Cohesion (chemistry)
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Abstract Base on the treatment project Han he landslide, the mechanism of rainfall-induced landslide was carried out. SRM was used to analyze the stability of the landslide in the natural state and the rain state. The influencing factors of the landslide and the safety and stability of the landslide were analyzed. The research results are as follow: (1) Han he landslide body is developed in Wu dong Huang dong kou Formation (∈h) mudstone and silty mudstone, so the rock is highly weathered and easily softened by water. (2) According to this survey and the stability calculation, it shows that the Han he landslide is in an unstable state during heavy rain, and the shallow landslide is in unstable state. The theoretical analysis is basically consistent with the numerical results. (3) Stress is on the front edge of the landslide concentrated in the dangerous area, the landslide may continue to slide, and the trailing edge steep wall may also gradually collapse on a small scale. It is recommended to improve the drainage system and treatment measures as soon as possible. The mechanism of the landslide can provide engineering reference for the treatment and prevention of similar landslides.
Landslide mitigation
Trailing edge
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Earthquake is considered to be one of the main factors of the generation of a landslide. Especially in high mountain gorge areas,the huge landslide body inrushes into the river and barriers the river,which may further expand the disaster areas and form a disaster chain. Taking Xiaojiaoqiao landslide for example,the three-dimensional(3D) instability process of the landslide is reconstructed by using dynamic finite element method based on detailed geologic investigation results of the landslide area. With the analytical results,the forming process and river-blocking mechanism of Xiaojiaqiao landslide can be divided into four phases:(1) earthquake triggering and progressive failure of the rock mass;(2) landslide body failure and sliding with high speed;(3) impacting the opposite slope,being disintegrated,and blocking the river and forming the landslide dam;and (4) density under the impacting. Based on the geological structure characteristics of the landslide dam and the stability analytical results under different cases,the stability of the slope of landslide dam is good under natural state,but there may induce local collapses with strong earthquake;and the most possible failure model may be the gradual erosion damage of topping. As for the internal rock structure with general stability,there is little possibility forpiping failure and dam-breaking as a whole.
Landslide mitigation
Mass movement
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Landslide processes are a consequence of the interactions between their triggers and the surrounding environment. Understanding the differences in landslide movement processes and characteristics can provide new insights for landslide prevention and mitigation. Three adjacent landslides characterized by different movement processes were triggered from August to September in 2018 in Hualong County, China. A combination of surface and subsurface characteristics illustrated that Xiongwa (XW) landslides 1 and 2 have deformed several times and exhibit significant heterogeneity, whereas the Xiashitang (XST) landslide is a typical retrogressive landslide, and its material has moved downslope along a shear surface. Time-series Interferometric Synthetic Aperture Radar (InSAR) and Differential InSAR (DInSAR) techniques were used to detect the displacement processes of these three landslides. The pre-failure displacement signals of a slow-moving landslide (the XST landslide) can be clearly revealed by using time-series InSAR. However, these sudden landslides, which are a typical catastrophic natural hazard across the globe, are easily ignored by time-series InSAR. We confirmed that effective antecedent precipitation played an important role in the three landslides’ occurrence. The deformation of an existing landslide itself can also trigger new adjacent landslides in this study. These findings indicate that landslide early warnings are still a challenge since landslide processes and mechanisms are complicated. We need to learn to live with natural disasters, and more relevant detection and field investigations should be conducted for landslide risk mitigation.
Landslide mitigation
Natural hazard
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Abstract Landslides are affected by engineering geological conditions, precipitation and rainfall. It is of greatly significance to study the causes and treatment methods of landslides to prevent and ensure the safety of people’s lives and property. Base on a case of Songmu landslide, the causes and mechanism of rainfall-induced landslide are analyzed. The stability of landslide under natural and rainfall conditions is analyzed respectively by using finite element strength reduction method and standard calculation method. The influencing factors of landslide are analyzed and the safety and stability of landslide are evaluated. The research results show that the disaster mechanism of Songmu landslide is that rainwater penetrates the clay layer, and the weathered layer is easily softened when rain seepage into landslide. The low strength of the soil induces local deformation and finally occurs sliding. In natural condition, the whole landslide is basically stable, but in saturated condition, the whole landslide should be in unstable state. The formation mechanism of the landslide can provide guidance for the prediction of the landslide deformation and treatment in the future.
Landslide mitigation
Strength reduction
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Based on engineering survey and experiments,slide mass structure and the mechanical parameters of Dagouwan landslide were obtained.FLAC3D was employed to analyze the stability and the deformation mechanism of Dagouwan landslide at different working conditions and water levels.The results of analysis show that:when the water level rose to 452 m,there showed a continuous crack at the back of the landslide from one side of gully to another,and the shear outlet of the landslide was located in the thick deposits of the slope at 580 m,but the whole slope still showed stability;when the water level rose to 488 m,another independent landslide formed from the toe of the landslide to 570 m.Interactions between two landslides caused the failure of the whole slope.The phenomenon had been confirmed during the two field explorations.The failure reasons of the landslide can be concluded as the loose structure of the landslide,high frequency of showers and the fluctuation of the water level in the reservoir,which finally led to the form of this complex retrogressive-progressive landslide.
Landslide mitigation
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Landslide mitigation
Pumice
Debris flow
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Longwangping landslide can be divided into the upper landslide and the lower landslide.Based on the engineering geological condition investigation,the deformation process is analyzed.The deformation and failure mode are determined.The upper landslide belongs to the pushing mode and the lower landslide is the pulling mode.With the unbalanced thrust transfer coefficient method,the landslide stability is evaluated both in normal natural state and the heavy rain conditions.Using the finite element method,the change regularity of the seepage field and stress field considering heavy rain is obtained.Then the influence by storm on the landslide stability is estimated.The result shows that under the normal natural condition,the landslide is stable.But under the heavy rainfall,Long-wangping landslide is in the less stable state.
Landslide mitigation
Mode (computer interface)
Stress field
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