| China is one of the countries in the word that suffers the worst landslide geological hazard,which threats the most number of the population. In particular,landslide disaster in western mountain area has almost become the primary threat to human life and the major project construction. However, gentle anti-dipped slope with soft foundation, characterized by concealment, suddenness and large scale, will cause catastrophic damage. On July 10,1989, a landslide caused by heavy rainfall happened in Xikou town of Huaying mountain, Sichuan, which led to 221 casualities, and the direct economic loss was up to more than 6 million. Similarly on June 28,2010, a landslide caused by continuous heavy rainfall happened in Dazai country of Guanlin county, Anshun of Guizhou province, which killed 42 villagers, and 57 people was missing. In addition, the village was buried and the road was blocked.The thesis is backed by two scientific research projects managed by my tutor. One is Study on the support platform of Guizhou geological disaster monitoring and early warning and decision making, and another is Study on the western mountain landslide geological structure characteristics and disaster mode which is on attachment to the 973 program: Hazard factor identification, acquisition of precursor information and study of early warning method on large-scale landslide of western mountains. Based on the study on the structural characteristics of gentle anti-dipped slope with soft foundation, and combined with the typical case study of Dazai landslide and Xikou landslide by floor friction tests and two-dimensional numerical simulation, the deformation and failure characteristics and formation mechanism of this kind of landslide was summarized. Furthermore, a preliminary analysis and study was done to recognize this type of landslide. The research results have a certain theoretical significance and application value for disaster prevention and control,especially for the gentle anti-dipped slope with soft foundation. The main achievements of this paper are as follow: 1. The gentle anti-dipped landslide with soft foundation is characterized by the high slope of steep on gentle. The steep part of slope whose gradient is varied from 35°to 70°is composed mainly of hard rock and is more than 300 m in high. The gentle part of slope with lateral airport height whose gradient is varied from 15°to 35°is characterized by ―nasal beam micro-geomorphology‖. Soft-hard rock interface dipping inwards has an inclination of 28~48°. 2. Deformation and failure features of Gentle anti-dipped landslide with soft foundation are as follow:(1)Slope failure process is characterized by high speed and long distance. Horizontal sliding distance of both Dazhai landslide and Xikou landslide is more than 1.5km.(2)The accumulated body of the slide characterized by clear spatial zoning feature consists of sliding source region and high speed debris flow region, and the latter is subdivided into impact zone, sprinkling zone and accumulative zone et al. Part of them like Dazhai landslide and Xikou landslide have pushing zone. 3. Conceptual model of formation mechanism of the gentle anti-dipped slope with soft foundation was established. This kind of landslide with special slope structure is caused by heavy rainfall. The slope is gentle anti-dipped structure of hard on soft, There are partial integrity and high strength of rigid geologic body like Xikou landslide or integrity rock mass like Dazhai landslide,which form deformation lock segment of which ―locked segment‖ rock mass concentrated high stress in the role of load of hard rock and creeping movement of soft rock towards the airport side. Once the stress reaches critical stress, the slope would slide with high speed and long distance caused by heavy rainfall. Its formation process includes four stages: the stage of compression-toppling and deformation, the stage of locked segment formation, the stage of rainfall inducement and the stage of sliding with high speed and long distance. 4. Based on similarity theory, floor friction similarity models were built on the basis of large numbers of tests. Floor friction tests were conducted in this paper to study the influence of change of different slope structural condition including gradient, dip of soft-hard rock interface and rock stratum thickness on stability of the gentle anti-dipped slope with soft foundation. The results showed that special slope structure plays a key role in stability of slope. When dip of soft-hard rock interface was 30°~50°,deformation rate and deformation range of the model reduced with dip decreasing. 5. Based on geological model and formation mechanism conceptual model,the formation mechanism calculation model was built under havey rainfall condition of this kind of landslide. The calculation results show that because of existing of strong permeable stratum which led to rainwater enrichment in strong-weathered zone at upper part of slope body. The calculations showed that plastic zone highly developed in unloading rock mass or weathering layer, and was highly consistent with rainwater runoff area distribution range, presenting phenomenon of synchronous change of runoff area with rainfall infiltration. In the late stage of rainfall, there was obvious displacement towards airport side for soft rock, weakening support function to upper part of slope body; The phenomenon of toppling and bending happened to the upper weathered rock mass which suffered rainwater immersion. Meanwhile, rock load transferred downward further, and groundwater enrichment gave rise to high hydraulic thrust. Under the above three factors, there was area of high stress concentration at ―locked segment‖ rock mass. Once the stress exceeded the limit that it could bear, the landslide would come into being after failure of the locked segment. 6. Recognition factor and recognition mark of this type of landslide are summarized after analysis of their deformation and failure process and formation mechanism of more than one landslide case. The recognition factor includes geography,lithology combination,rock mass structure,the role of water and external influence factors et al. There are four recognition marks.(1)It is characterized by the high slope of steep on gentle with nasal beam micro-geomorphology, and the steep part of slope is varied from 44°to 55 in gradient. The gentle part of slope has certain lateral airport height from about 15 m to 50 m, of which gradient is varied from 15°to 35°.(2)Slope structure is characterized by hard on soft, and rock mass shows layered structure. The strata inclination is 20°to 60°and dip of soft-hard rock interface is 30°to 50°.(3)In addition to formation level, there are two groups of advantages of structural plane, which are respectively parallel and perpendicular to the slope surface.(4)Before the slope instability, it will undergo long-term strong rainstorm. |
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