Study On Instability Mechanism Of Baige Large Landslide In Jinsha River

Baige landslide occurred on October 11 th,2018 and November 3rd,2018,two large-scale landslides occurred one after another 24 days apart,and two dams were formed to block the Jinsha River.Although it did not directly cause casualties,due to the rise of the water level of the dammed lake and the flood discharge of the dammed lake,some houses in the townships on the upper and lower reaches of the Jinsha River were damaged,roads and bridges were destroyed,and the cultivated land was submerged,resulting in large secondary disasters and Economic losses have had a huge social impact.In the areas around the Qinghai-Tibet Plateau,there are still many issues worthy of exploration in the study of landslide development and distribution characteristics and genetic mechanism in areas with high crustal uplift,high altitude and high seismic intensity.Especially with the successive planning and construction of Sichuan-Tibet Railway,Sichuan-Tibet Expressway and hydropower projects,research on the development characteristics and instability mechanism of regional landslides is urgently needed to provide a certain amount of slope for the construction of SichuanTibet Railway and highway Reference value.During my internship at Meishan Geological Engineering Survey Institute,I participated in the field investigation of Baige landslide.By referring to relevant literature and data,using on-site geological survey,drilling,remote sensing image interpretation,numerical simulation,and post-deformation monitoring methods,the engineering geological conditions of the grid landslide,the development process of the deformation of the Baige landslide,and the analysis of the Baige landslide are analyzed.Mechanism of instability.The knowledge gained is as follows:(1)Through a detailed investigation of the geological conditions of the Baige landslide,it is concluded that the Baige landslide is located in the deep-cut canyon section of the Jinsha River,most of the valleys are "V" shaped valleys,and the landslide is developed on the right bank(Tibet bank)of the Jinsha River.Stepped shore slope.The landslide is affected by the Polo-Mexie fault(F14)at the trailing edge.The upper part of the slope is mainly the altered and fragmented serpentine invaded by the Hualixi period,and the middle and lower part is the metamorphic gneiss of the Proterozoic Xiongsong Group,Baige The landslide is a high rock slope with a soft upper rock mass and a lower rock mass with a hard rock mass.(2)Baige landslide is a large high-level,high-speed rock landslide.The plane shape of the landslide is in the shape of a circle chair,the main slide body is in the shape of a wedge,and the cross-sectional shape is in a steep and stepped shape.There are three levels of platforms on the slope: the third level platform is the largest,with an elevation of about 3550-3450 m,the second level is next,the elevation is about 3200-3100 m,the first-level terrace(platform)is very small,the elevation is about 2970-2940 m,and is located at the landslide shear outlet.The trailing edge elevation of the landslide is 3732 m,the leading edge elevation is 2882 m,and the front and rear elevation difference is about 850 m.The shear outlet is located above the Jinsha River and the elevation is about 3000 m.The longitudinal length of the landslide is about 1600 m,the maximum width is about 700 m,the average width is about 550 m,the average thickness is 45 m,and the main sliding direction is 82° to 102°.(3)A detailed survey of the on-site deformation and damage signs was carried out.According to the landslide shape and deformation characteristics,the landslide body was divided into the source area(A1,A2),the scraping area(B1,B2),the accumulation area(C),and the surge Affected area(D),landslide affected area(K1,K2,K3).According to the difference in lithology of the sliding source area,it is subdivided into two sub-zones: the left sliding source area(A1)and the right sliding source area(A2);due to the different degree of scraping,it is divided into the main scraping area(B1)and the secondary scraping area Zone(B2);the landslide-affected zone is divided into three sub-zones: trailing edge unstable zone(K1),downstream unstable zone(K2)and upstream unstable zone(K3)according to the positional relationship.(4)Based on the on-site investigation,it is concluded that the unfavorable slope structure is the unfavorable geological conditions formed by the landslide.The undercut erosion of the Jinsha River Valley,rainfall and earthquake all have a detrimental effect on the evolution of the slope,resulting in partial damage to the slope Gradually form the slope destruction boundary.The landslide and its formation can be divided into three stages: the trailing edge slippage and the subsidence stage;the development and penetration of cracks in the slope,the formation stage of the locked section;the shear failure of the locked section-the high-speed sliding stage of the landslide.(5)According to rock physical and mechanical tests in the landslide area: the internal friction angle of the Hualixi intrusive serpentine is 31.3°,the cohesion is 3.22 MPa,and the shear resistance is low;the internal friction angle of the Proterozoic Xiongsong Group gneiss 45 °,cohesion 21.3MPa,high shear resistance,uniaxial compressive strength 45.01MPa;the difference between the mechanical properties of the two and the topographical conditions are the fundamental reasons for the formation of the locked section of gneiss evolution.Among them,altered serpentine rocks are easily softened by water,and the strength of rock mass decreases sharply.(6)The 1-1 engineering geological section of Baige landslide was selected,and the landslide was modeled using FLAC-3D6.0 software.The results showed that:(1)When the undercut of Baige landslide is not completed,the initial stress generally increases from the slope surface to the inside,and the maximum principal stress is 61 Mpa.When the river valley is cut down,the maximum principal stress is-37 Mpa.During the erosion process of the slope under the river,obvious lateral and vertical unloading of the slope occurred,and the maximum principal stress was reduced by 24 Mpa.Before the river valley is cut,the minimum principal stress is compressive stress and the minimum principal stress is-21.7Mpa.After the river valley is cut,there is both compressive stress and tensile stress in the minimum principal stress.At the slope valley,the stress is reduced,and the minimum principal stress is-14.2Mpa.It appears on the slope surface and the slope surface of the upper and middle slopes.The tensile stress area is about 0.023 Mpa.The closer to the direction of the Jinsha River,the smaller the minimum principal stress changes on the slope surface and the smaller the changes in the deep position.It shows that the unloading effect mainly occurs on the slope surface with better free space conditions,and the unloading subzone is obvious.(2)Under earthquake conditions,the deformation of the slope is exacerbated.There are obvious tensile cracks near the F14 fault on the slope.Due to the poor mechanical properties of serpentine,there are banded stress concentration zones in the serpentine.At the junction of serpentine and gneiss,local bulging occurred,and the slope body was in a stable state as a whole.(3)Under the conditions of rainfall,the collection of surface groundwater increases the bulk density of the rock and soil body.The Quaternary and fully weathered serpentine on the surface of the slope body have good permeability,which is conducive to rainwater infiltration.The serpentine rock in the upper part of the landslide was partially unstable due to rainwater loading,strength softening and hydraulic action.At the shear outlet of the landslide,the plastic zone gradually penetrated upward,but no penetration surface was formed.The gneiss on the slope serves as a "retaining wall".(4)After rainfall and earthquake,the upper serpentine is under gravity,the settlement of the trailing edge of the slope body is intensified,and the tension cracks are deepened.The gneiss in the middle of the slope body cannot bear the load on the upper part of the slope body,the plastic zone penetrates,and the landslide is unstable.After the instability,due to the good vacant conditions of the slope body,the upper steep part has greater energy in the slope body,and the Jinsha River is destabilized in the direction of N90°E,and the surface of the slope soil is scraped along the line.On the opposite bank of Sichuan,a surge zone was formed,which finally blocked the Jinsha River and formed a dammed lake.