Research On Formation Evolution Mechanism Of Multiple Rotational Loess Landslides

Multiple rotational landslide is the typical disaster type of landslide with its particularity, progressive and the complexity in loess plateau region. On the base of discoursing the progress of multiple rotational landslide in and abroad, relying on the national scientific supporting problem " Study on early identification and dimensional forecast of oversize landslide" and the China Geological Survey item "Geological disaster detailed survey of Baoji area in Shaanxi province", taking the example of the two multiple rotational loess landslide in Jintai district in Baoji City in Shaanxi province and in Heifangtai in Gansu province, by the research methodology of field survey, remote sense interpretation, scene investigation, laboratory test, numerical simulation, the paper studies formation condition, landslide feature, distribution regulation, failure modes, formative mechanism and stability analysis of multiple rotational loess landslides, and then researches the risk evaluation methods of multiple rotational loess landslides. The main results of research as follows:(1) Multiple rotational landslides distribute in river and valley where erosion strongly in loess plateau region zones. The formation factors including: Neotectonics, river lateral erosion, structure and geotechnical Characteristics of high sheer slope. The mainly inducing factors including: rainfall, irrigation, earthquake and freezing-thawing. The most important influent factor is the function of ground water.(2) The feature of typical multiple rotational loess landslides is: large scale, deep sliding face, three-five sliding face cascades, single landslide with the "plough" structural character of behind high and sheer-middle subdued-front tilt. The main formation mode of landslide is creeping-sliding-tensile fracturing. The failure modes contain extensive rotation sliding, digressive rotation sliding and compound rotation sliding.(3) Take the comprehensive test research on Pliocene hard clay of the multiple rotational landslides. As to Pliocene hard clays, the clay particle content is high, and montmorillonite or illite/montmorillonite turbostratic mineral is the dominating clay mineral. The content of effective montmorillonte is very high, which is the basis for the undesirable engineering properties of Pliocene hard clays, and main factors of disaster are obvious properties such as fissured, overconsolidated and expansive and seam-sheared zone; the result of unconfined compression test, direct shear test, triaxial test and rheological test shows that Pliocene hard clays is an intermediate type of the material between the hard soil and the soft rock, the stress-strain curve is strong-softening-softening type, brittle failure. The influence of water content is clearly to hard clay. There is an obvious difference between peak value and residual strength. There is a short space of time from accelerated creep to destroyed creep. Therefore, for landslide happens, the strength loss will generate the thrust, leading landslide sliding accelerated along the slide base.(4) Put forward the stability evaluation method of "least landslide stability coefficient" and "classification superposition" of multiple rotational landslides. By using the method of Fellennius, Bishop and Sarma, doing the superimposition from front landslide to back, analyzing block combination to make sure the most dangerous sliding block combination. At the same time, the landslide movement formation can be judged by the calculation result of stability. The result shows the calculation results of the method of Bishop and Sarma are relatively closed.(5) Using the finite element shear strength reduction method to analyze the calculation results of mass failure evolution during every phrase, to get the development process of slope instability and plastic zone. Meanwhile, contrasting the results with that of "classification superposition" limit equilibrium to make sure that we can superposition calculate the multiple rotational landslide by finite element by strength reduction in real project.