| Slide-bending landslide is one of the geomechanical models of slope deformation and failure in Engineering geological analysis principle.In this book,it is divided into plane type and chair type according to the shape of sliding surface,and the formation conditions and evolution process of this kind of landslide are described in detail.There is such a special type of landslide in the Three Gorges Reservoir area.The initial state of deformation is the state after the failure of plane sliding-bending landslide.The slope structure and evolution process are obviously different from the original two types of sliding and bending,and also different from the common chair-like rock landslide.Here,this kind of landslide is regarded as a special kind of sliding-bending landslide.The sliding surface of this kind of landslide is chair-shaped.The middle and upper parts of the sliding surface are bedding,and the slope is generally 20 to 30°.The front rock strata are cut into layers,with a relatively slow or even nearly horizontal occurrence.Generally,it is a large platform of 5 to 10°,which is mainly developed in the middle-steep dip outer layered slope.It is commonly seen in the soft-hard interbedded rock mass of sandstone with carbonaceous shale with strong ductility.Such landslides are widely distributed in the Three Gorges Reservoir area and pose a great threat.Due to the blocking of the stable mountain of the front platform,the stability is good under general conditions,but under complex conditions(rainfall,reservoir water level rise and fall),deformation and even instability may occur.Tanjiahe landslide,Muyubao landslide,Jiuxianping landslide and Outang landslide in the Three Gorges Reservoir area are all such special sliding-bending landslides,which are all in a state of continuous deformation and seriously threaten the safety of the golden waterway of the Yangtze River.Slide-bending landslide has huge volume,strong concealment,complex formation mechanism,strange disaster mode,difficult disaster reduction and high risk.Taking this kind of landslide as the research object has strong theoretical significance and practical value.Muyubao landslide in the Three Gorges Reservoir area is one of the most typical cases of this kind of landslides.Some studies before the impoundment of the reservoir suggested that the Muyubao landslide will still be in a stable state after impoundment to 175 m,and the stability coefficient would bigger than 1.18.However,after the impoundment of the Three Gorges Reservoir,the deformation of the landslide is obvious.From 2006 to 2020,the cumulative deformation of the surface reached 1.6 to 2.6 m,and the trailing edge cracks have been connected.The cumulative deformation is still increasing,which seriously threatens the life and property of the residents in the reservoir area and the safety of the gold channel.Therefore,the type definition,classification,geomechanical model,deformation mechanism and stability of such sliding-bending landslides need to be studied systematically and deeply.Based on the analysis and summary of previous research results,this paper compares and analyzes the topography,lithology and slope structure characteristics of similar landslides,and summarizes the development environment and later evolution mode of such landslides.Taking Muyubao landslide in the Three Gorges reservoir area as an example,through detailed field investigation and long-term field inspection,the sliding boundary and strong deformation area of Muyubao landslide are clarified.Combined with GNSS surface monitoring data and reservoir water and rainfall data,the relationship between slope deformation and reservoir water and rainfall is analyzed,and the reservoir water and rainfall thresholds of landslide deformation are given.Combined with numerical methods,the deformation evolution mechanism of this kind of landslide is analyzed.Finally,the basis for early identification of such landslides is given.The purpose of this study is to provide a theoretical basis and reference for the deformation mode and stability evaluation of similar reservoir landslides,and provide theoretical support for the decision-making of disaster prevention and mitigation in the reservoir area.It has strong engineering value and scientific theoretical value.The main conclusions and results are as follow:(1)Based on the data collected and comparative analysis of slip-bending landslide,combining with the typical cases of Muyubao landslide,Outang landslide,Jiuxianping landslide in the Three Gorges Reservoir area and Vajont landslide,the formation conditions and deformation characteristics of sliding-bending landslide are clarified.On the basis of previous studies,the evolution mode of this type of landslide is further classified.According to the section shape,it can be divided into plane type(I)and chair type(II),and the later failure forms are mainly shear layer type and overall sliding type.Plane type may occur multiple.sliding-bending-shear,divided into sliding-bending-shear(I-1)and sliding-bending-collapse-shear(I-2).The chairs are further divided into two types: the front bedding type(variable dip angle)and the front cutting type.According to their failure modes,the chairs are further divided into sliding-bending-collapse-whole slip(II-1,II-2)and slip-bending-shear(II-3).According to the dominant factors are divided into reservoir-oriented,rainfall-oriented,excavation-oriented three types.(2)The subhorizontal slope structure in front of Muyubao landslide and the lithologic between soft and hard phases of quartz sandstone and carbonaceous shale control the evolution process of Muyubao landslide.The continuous cutting of the Yangtze River in front of Muyubao landslide provides good free-face conditions for its deformation.(3)Through field geological survey,macroscopic inspection and historical remote sensing images,it is clear that the landslide had local deformation before the impoundment of the reservoir in 2003.After the impoundment of the reservoir in 2003,the deformation rate increased,belonging to the reactivation of the ancient landslide.The boundary of the landslide and the range of the strong deformation zone on the right side are clearly analyzed.At present,the boundary cracks have been formed,and the rear ‘M' type tensile fracture zone has formed.With the increase of deformation,a large area of landslide in the direction of the Dalegou may occur in the strong deformation zone on the right side in the future.The overall evolution mode of the landslide is sliding-bending-shear-collapse-overall slip,which is now in the overall deformation stage.(4)Muyubao landslide has been continuously deformed since the implementation of professional monitoring in 2006.Combined with GNSS surface displacement monitoring data,reservoir water level and rainfall data of the landslide for many years,it is considered that the landslide is in the overall deformation,and the deformation during high water level has the characteristics of accelerated growth.The acceleration of landslide deformation is mainly dominated by high reservoir water level,and the threshold of reservoir water level is 172.5 m.Higher than this water level will cause obvious acceleration of the landslide,and lower than this water level,the deformation rate of the landslide will slow down.Continuous rainfall will promote landslide deformation.Cumulative-3-day rainfall more than 56 mm and cumulative-10-day rainfall more than 106 mm will accelerate slope deformation,especially during high water level.The operation days of high water level and continuous rainfall in high water level period control the annual displacement of landslide.At present,the slope is in a state of long-term uniform speed and accelerated deformation of high reservoir water level.It is considered that under the condition of long-term high water level combined with continuous heavy rainfall,the overall deformation of the slope is accelerated.(5)The Geo-studio finite element program,SEEP / W and SLOPE / W modules are used to analyze and evaluate the seepage field and stability of Muyubao landslide under two actual conditions of reservoir water and reservoir water coupling with rainfall.The calculation results show that under the rise and fall of reservoir water level,the groundwater level in the front of the slope is consistent with the change of reservoir water level.The slope stability coefficient begins to decline rapidly when the reservoir water level reaches a high level,and the stability will gradually rise under high water level.When the stability coefficient is less than 1.04,the deformation rate will increase.The stability coefficient of landslide deformation acceleration under reservoir water level coupling with rainfall is less than that of slope acceleration under the condition of reservoir water level,which decreases from 1.04 to 1.02(about 2%).During high water level operation,continuous rainfall will deteriorate the slope and lead to stability reduction.(6)The stability of Muyubao landslide under the action of high water level is analyzed by FLAC3 D numerical simulation software.The results show that the displacement difference of each part before and after Muyubao landslide is small.The reservoir water level rises from 145 m to 175 m,and the stability coefficient of the landslide decreases from 1.08 to 1.023.The high reservoir water level is the main factor affecting the overall deformation of the landslide.Because the landslide is mainly fractured rock,especially the middle and front parts are more broken,the Muyubao landslide will not appear the typical collapse deformation characteristics of strong uplift in the future.(7)According to the comparative analysis of the formation conditions and the spatial and temporal evolution characteristics of deformation and failure,the sliding-bending landslide has obvious identification characteristics in the early creep rupture,the middle front uplift or turning point is squeezed,and the later failure deformation stage,which can provide the basis for the early identification of such landslides. |
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