Study On Mechanical Characteristics Of Loess And Mechanism Of Loess Landslide In The Southern Tableland In Jingyang County

Loess is a special structural soil,widely distributed in northwest China.With the formal implementation of the Northwest Development Strategy and the opening of the "the Belt and Road" economic zone,the engineering construction in the loess region of the western region is increasing day by day,which makes the regional economy develop rapidly.But the contradiction with the following geological problems such as landslides is becoming more and more prominent.Many scholars have made some achievements in the research of the shear strength characteristics of loess and the starting mechanism and movement mechanism of loess landslide,but the research is still immature.Supported by the national basic research program of China: Occurrence and evolution mechanism and control theory major disaster and disaster chain in loess,this paper selects South Jingyang tableland of Shaanxi province as the main research,and Lishi loess taken from this area as the research object.This paper designs pressure plate test for soil-water relationship(SWCC)and mechanical tests,including conventional triaxial test under consolidated undrained shear condition and the ring-shear test;And designs microscopic test,including mercury injection test(MIP)and scanning electron microscopy(SEM).Through the above experiments,on the one hand,this paper analyses the relationship between the microstructure change of loess and the macroscopic mechanical properties,and reveals the microscopic mechanism of loess macroscopic mechanics performance;On the other hand,analyses the starting mechanism and motion mechanism of typical loess landslide in south plateau of Jingyang.The conclusions are as follows:(1)The SWCC curves of undisturbed loess show an anti-" S " shape,and at the same time show obvious hysteresis loop phenomenon,and have obvious stage characteristics.The curve is divided into four regions according the characteristics of the two landmark points(the air intake and the residual volumetric moisture content)and the slope of the curve,that is boundary effect region,main transition zone,secondary transition zone and unsaturated residual region.(2)Under the influence of water content and confining pressure,the undisturbed loess is characterized by three kinds of stress-strain forms by CTC test: hardening type,stable typeand softening type,especially in the saturated state of loess,the softening phenomenon is particularly prominent.The results of ring-shear test show that the residual strength increases greatly with the increase of axial compression,but the residual strength of soil varies with different moisture content,dry density,sample preparation method and shear mode.The shear strength parameter-cohesion(residual cohesion)is obviously affected by moisture content,but the internal friction angle(residual internal friction angle)? is unobviously affected by it.(3)The influence of matrix suction on the shear strength,residual strength and strength parameters of them is obviously different.The influence of matrix suction on the shear strength of loess is higher than that on residual strength.The c(cohesive force)and ?(the internal friction angle)of the two intensities vary nolinearly with the influence of matric suction,of which c is obviously affected by matrix suction and there are special state points of the matrix suction,which controls the development trend of cohesive force.According to the sensitivity of soil strength affected by matrix suction,this paper divides the curve into four stages: weak sensitive area,strong sensitive area,moderate sensitive area and insensitive area.The boundary point between each region is basically consistent with that of soil-water characteristic curve.(4)After the shear test,the large and middle pores which are active pore are reduced,the inert pore is increased;And before the shear test,the edge of particle is smooth,and the contact type between of particles is mainly point-point contact and plane-plane contact;And the particles are arranged directionly,and most of them are flat,broken and angular after shearing.(5)Taking Ximiaodian landslide as a typical case,this paper,based on laboratory test results and field investigation,puts forward for the first time the starting time-dependent segmentation model of the mobile landslide in Jingyang Nanyuan,and reveals the start-up mechanism and movement mechanism of the high-speed and long-range landslide in Jingyang Nanyuan.