| Yan'an area is located in the hinterland of the Loess Plateau.The Quaternary loess covers the sedimentary rock layers before the Neogene.A large number of lLoess landslides have occurred owing to the loose structure,low strength after immersed in water,and the development of joints and fissures in loess.Moreover,rainfall and river erosion also promotes the development of landslide.Loess-basement rock interface landslide is the most common.In recent years,a lot of infrastructures have been built in Yan'an area.The overdevelopment of land resources caused a large number of fossil landslides reactivation.The reactivation of fossil landslides not only endangers the neighborhood residents' life and property safety,but also restricts the economic development of Yan'an area.In order to assist the urban system construction,a number of geological hazard investigation projects focusing on the ancient landslide revival,caused by rainfall or excavation,have been carried out.Based on the above background,investigations about the loess-bedrock interface landslide in Yan'an area was processed in this paper.The results proposed herein are helpful to enrich the study of landslide and evaluate the landslide geological disaster in Yan'an area.In order to address the formation mechanism of the loess-bedrock interface landslide,three typical loess-bedrock interface landslides located at Dingjiagou,Erzhuangke and Xiaobiaogou were taken as the research object.The microcosmic and macroscopic structure characteristics of slip soil were studied through Quanta 650 environmental scanning electron microscope and strain controlled direct shear apparatus,respectively.Moreover,the relationships between the microcosmic structure and macroscopic mechanical parameters have been built.Based on the work mentioned above,laboratory model experiments and numerical simulation tests were carried out to simulate the evolutionary process and discover the formation mechanism of the reactivation of loess-bedrock interface landslide in Yan'an area.Some results were obtained as follow:(1)The mathematical statistics and analysis indicate that the loess was generally fake-integrated on the bedrock,which causes the sliding surface of loess-bedrock interface landslide always formed on the bedrock surface.This kind of landslide mainly occurs in the slopes of both sides of the river and valley or formation lithology combination site with slope combination of 25~60 degrees,slope height 40~130m and the slope direction 315 ~360 degrees.(2)The analysis about the mineral component and microcosmic structure of loess show that the main mineral existed in slip soil of loess-bedrock are quartz,orthoclase,chlorite,muscovite and illite.Furthermore,the directional frequency of slip soil was mainly distributed between 80°~100°.Good orientation was observed and the porosity tends to long and narrow.(3)High order linear mapping relationship between the microcosmic structure parameters and cohesion of soil was found.The cohesion of soil was oscillatorily decreased with the increasing of the degree of circularity or the decreasing of dispersion index of soil particle.Moreover,the cohesion of soil decreased with increasing of directional probability entropy or fractal dimension.In different geographical ranges,the cohesion of soil increased with the increasing of directional parameter value.(4)High order linear mapping relationship between the directional microstructural parameters and internal friction angle of soil was found.The relationships between the microstructural parameters and internal friction angle of soil were oscillating.For an individual landslide,the internal friction angle of slip soil was decreased as the increase of the orientation of soil particle.(5)The analysis about the mechanical strength characteristics of slip soil indicates that both strain hardening and strain softening behaviors were observed.The difference between the peak shear strength indexes is obvious while the difference between the residual strength indexes is small.As the increase of moisture content of slip soil,the cohesion of soil was exponentially decrease and the internal friction angle was linearly decreased.(6)Based on the similarity three theories,the laboratory model experiments were processed to simulate the evolutionary process of loess-bedrock interface landslide in Yan'an area under the condition of rainfall.The results illustrate that the failure mode of landslide under rainfall condition could be described as ‘tractive deformation? creep fracture caused by tension? unitary sliding'.In the process of failure,the change of pore water pressure could be classified into three stages,i.e.the formation of pore water pressure,the formation of excess pore water pressure and redistribution of pore water pressure.(7)The numerical simulation tests indicate that the deformation and failure of landslide was a gradually developed process with the rainfall duration.The deformation and failure of landslide fell into three stages,i.e.the generation of fracture,the extension of fracture and slide.The process of stress variation in landslide could be described as the generation of stress,the extension of stress and the redistribution(relaxation)of stress.(8)The deformation of landslide is mainly tensile deformation.Shear deformation exists in local region.The change of plastic zone of landslide also fell into three stages,i.e.the generation of plastic zone,the extension of plastic zone and plastic zone connection,which is corresponded with the process of deformation and failure of landslide.A variety of methods was adopted to analyze the deformation and failure mechanism of loess-bedrock interface landslide in Yan'an area.The meaningful results obtained in the present study have strong pertinence and promotion value to the investigation of the landslide in loess region.In the meantime,the work in this paper could provide theoretical support for the prevention and control of landslide in Yan'an area. |
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