The Experimental Study On The Formation Mechanism Of Irrigation-Induced Loess Landslide At South Jingyang Plateau

The natural geological environment has been changed in south bank of Jing River due to the large irrigation projects. It causes the frequent occurrence of landslide disaster, which not only reduces the benefice of irrigation projects and the sustainable development of agricultural production, but also threatens the safety of the local people. Under on-site inspection, the text summarized the distribution law and analyzed the formation conditions and influencing factors of irrigation-induced loess landslide in this area. The effect of irrigation on the collapsible deformation characteristics of loess in the top of the slope was analyzed through the collapsibility test that controlled the initial moisture content of loess and the variation law of microstructure of loess that before and after tests. Mechanical properties and deformation failure law of the loess were analyzed by the related mechanical test and the change rule of the microstructure characteristic of the loess that before and after the experiment. On the basis of the above study, the formation mechanism of irrigation-induced landslide was discussed considering the effect of the loess collapsibility combined the results of the stability calculation. Finally we arrive at conclusions as follow:(1) Most of the loess landslides in southern tableland of Jingyang County are caused by the irrigation. It has the multiphase accumulation characteristics and and cluster distribution, and often occurres in the spring irrigation period and thawing period. The water that gatherd in the relative impermeable layer through dominant channel causes the rise of water level, which makes the the change of intensity and stress distribution and influences the stability of slope. What's more, steep slope caused by Jing River erosion provides conditions for the formation of the landslide.(2) The change law of pore structure after collapsibility test in different conditions reflects the characteristics of collapsible deformation.In the process of collapse, the pore changes from large and medium pore to small and tiny pore, and the complexity of pore increases gradually performanced as the change of anisotropy. The irrigation changes the discrepancy of water content in space. It appears that the moisture decreases from the center of irrigation to the surrounding places, which makes the difference of unsaturated collapse deformation. And then, some cracks are produced by the uneven settlement, which provides dominant channel for the infiltration of surface water.(3) With the increase of confining pressure and moisture content, the stress-strain behavior of Qp3 loess changes from soften type to harden type, while shows the soften type in Qp2 loess. According to the the stress-strain curve, the corresponding strain value of loess structure yield stress ?k is less than 2% in the study area. On the basis of these studies, the failure modes at different locations of the slope are obtained. Finally, the parameters of steady state line and state boundary surface in the study area are got based on the tri-axial consolidation undrained shear test. Moreover, under the assumption that the steady state converges under different confining pressure conditions, the approximate critical surface formula is obtained.(4) The soil water characteristic curve (SWCC) illustrates the moisture-absorbing and dry-absorbing characteristics of loess. It also helps getting the relationship between the moisture content and matrix suction. The stress-strain curve and deformation failure mode was analyzed through the unsaturated loess strength test. Based on the strength formula of unsaturated loess, the formula of failure envelope of loess in the study area is obtained, which provide a basis for the selection of stability calculation parameters.(5) The microstructure characteristics of loess samples before and after shearing test under different initial moisture content and different pressure are mainly manifested in the microstructure type, porosity and the development degree of micro-cracks:With the increase of confining pressure, the microstructure types of loess transformed from the overhead structure to the mosaic structure, and then to the bond structure. And the complexity of pore increases gradually performanced as the narrow of shape and the change of anisotropy. While with the increase of water content, the number of large and medium pores reduce significantly and the small and micro pores disapper gradually. They are all made by the softening effect of water on soil. In addition, the development degree of micro-cracks in the scanning electron microscopy (SEM) images can also directly reflect shear capacity.(6) The stability coefficient of the slope decreases with the increase of the water level, The growth of the crack deep has an expansionary effect on the decrease of slope stability that caused by the raise water level. The formation mechanism of irrigation-induced loess landslide is summarized as follow:Forming cracks caused by uneven settlement formation. ? Decrease of the shear strength caused by the raise of the underground water. It promotes the failure of soil at the foot of slope. ? Crack extension caused by the excess pore water pressure which is made from the limitation of the external conditions (such as confining pressure, weak permeability, etc.) ? Formation of landslide. The excess pore water pressure caused by sliding or flowing of soil on the fracture surface together with the soil liquefaction accelerate the promoting of the coalescence of fracture. The landslide appears soon and slides far after the coalescence of fracture.