Research On The Formation Mechanism And Hazards Assessment Of Large Loess Landslide On The North Bank Of Weihe River Baoji City, Shaanxi Province

The plateau slope zone is a large area susceptible to landslide hazard. The paper selects the plateau slope between the north bank of Weihe River and Fufeng, and conducted detailed studies in the formation mechanism and hazard assessment of large landslide therein. By means of extraction of spatial information of geomorphic feature, geometrical analysis of structural deformation, dynamic numerical analysis and creep and ring shear tests of sliding stratum, etc., it discusses the structural and geomorphological process of slope zone, primary slope structure, geometrical deformation gross feature and dynamic process of typical landslide, researches the creep and sliding friction mechanisms of sliding stratum that form slip soil, and evaluates the landslide hazard in the slope zone. Some new breakthroughs have been achieved as follows.1. This paper preliminarily reveals the four stages and multi-stage features of corrosion of the evolution of plateau slope on the north bank of Weihe RiverAccording to the research, the ancient landslide in the Baoji Gorge-Fufeng slope formed on0.13Ma.BP third terrace in the corrosion period, and the old landslide came into being on0.009Ma.BP second terrace in the corrosion period. From west to east, the erosion degree of the slope surface shows an identical multi-stage characteristic as the landslide density. The landslide density of the Baoji Gorge-Qianhegou section is91.0%-98.0%, while that of the Qianhegou-Caijiapo section,87.7%-95.1%, and that of the Caijiapo-Fufeng section,68.9%-82.8%. Slope erosion and revival of old landslide will be the main forms of slope deformation.2. This paper initially discloses that along the active fault of the north rim of the Weihe River, the graben structure controls the north plateau slope structure and its large deep-seated landslide deformation: Through geological survey and dynamic numerical simulation research, this paper initially reveals that the active fault of the north rim of the Weihe River controls the geomorphic feature of the north plateau slope zone and affects the deformation process of the landslide.(1) Active faults in northern margin of the Weihe River, as graben-like structures, control the terrain and physiognomy of slope zone of the plateau edge on the northern bank. The details are that principal fracture surface is S-dipping and the dipping angle is68°on average controls the angle of plateau edge in bank slope; and the assemblage of secondary faulted structure controls the slope structure and affects the structure of large loess landslide on the plateau edge;(2) The principal fracture surface of the plateau edge is consistent with the smooth wall of back scarp of many large loess landslide, and it has influenced on the configuration and occurrence of large landslide. The maximum distance of fault offset is7.1m;(3) Active faults have graben-like structure and local secondary fracture surface, controlling the structure of multi-order sliding surface of plateau edge and its formation and evolution. For instance, in the Yangjiacun Landslide, some landslides are controlled and sheared by faulted structure with shear angle reaching at72°. The toe of surface of rupture of most landslides were not affected by fracture surface and their angles are23°on average3. By calculation of balanced section, this paper researches and discloses the geometrical characteristics of deep-seated landslide and its dynamical significanceBased on the balanced section and PFCC2D numerical analysis, geometrical pattern and kinematic process of large deep-seated plateau landslide are researched in a quantitative manner. Deep plateau landslide mainly includes slope-foundation type and slope-body type, of which the former is featured by the Wolongsi Temple landslide. The dip angle of the rear fractured wall of the sliding face is57°, its shear crack is12°and its middle is approximately horizontal. Its displacement deformation concentrates in the rear edge and slope toe of the sliding body. The rear edge descends as high as58m, the front edge uplifts27m, and the horizontal displacement is about335m. The slope-body type is characterized by the Caijiapo landslide, of which the dip angle of the rear fractured wall of the sliding face is48°, and its front-middle part wedges out almost horizontally. The rear edge of the sliding body descends about36m and its front edge is sheared outwardly and rises by22m, and its horizontal slippage is about130m. Based on PFC2D numerical analysis of the above geometrical morphology, this paper verifies the correctness of analysis on balanced section and obtains that the average speed of the Caijiapo landslide in sliding is3m/s.4. This paper initially suggests the microcosmic deformation process and intensity attenuation trend of large landslide sliding zone and its sliding friction mechanismAccording to the comparative study on the fabric and deformation characteristics before and after the deformation of deep landslide sliding zone, it may be found that in transforming from primary rock to sliding zone, the creep deformation of clay rock causes readjustment of geotechnical structure, which is shown as flawless volume expansion, swelling, resulting in plastic extrusion of the Caijiapo sliding zone. On the other hand, frictional sliding between sliding bodies promotes the clay rock to change from flocculent structure to schistosity and leaves striation and argillaceous film. Intensity attenuation changes with the primary structure. The long-term strength of creep is approximate to27.8%of the primary rock peak strength. The residual strength during the friction is approximate to1.43%of the primary rock peak strength, both of which cause the sliding body to show a block glide with the sliding face being connected.5. The activity intensity and risk of landslide hazard in the Caijiapo sliding zone has been suggested.On the basis of1:2000landform base map, this paper determines the time series of the landslide in Caijiapo section and defines its strength by classifying the landslide, by using the historical geomorphic evolution method. According to the frequency of landslide activities within nearly110years, this paper suggeststhat the high risk zones of such landslide section concentrate on Ninth Shaanxi Cotton Factory, Caijiapo and Agricultural Company, while the reviving bodies of ancient landslide, the steep cliffs around the new landslide in Caijiapo fall into middle risk areas. The risk area of landslide is strictly limited on the periphery of the pre-existing sliding body...