| Rock and soil erosion is a natural geological process, and it is also a concerned issue that has a crucial impact on economical construction. Pi-sandstone distributing area, located in the transition zone between Inner Mongolia Autonomous Region and Shaanxi Province with the area of 1.9×10~4 km`2,is a main source of coarse sand of the Yellow River. The annual sediment yield is 2.6×10~8t. Thus, it is important for both science and society to analyze the mechanism of Pi-sandstone erosion. This dissertation contains two main parts: (1) study on the mechanism of gravity erosion of Pi-sandstone based on stratum hthology; (2)discussion of new method to evaluate the Pi-sandstone gravity erosion.The gravity erosion is dominated in Pi-sandstone area. The physical geography and geology in Pi-sandstone area are demonstrated firstly, and then, the factors which impact or control the intensive erosion were discussed on the background of environmental geology including the structural pattern, topography and physiognomy, stratum hthology and stratum assembled and so on. Although there are water erosion, wind erosion, gravity erosion, freeze-thaw erosion in the pi-sandstone area, but the gravity erosion is the dominated type, which is identified both through the field investigation and spatial-time analysis or estimation of erosion amount. Other erosion types presence provide the favorable condition for further development of gravity erosion. Take a small catchment as case study, the distribution of main type of gravity erosion , such as collapse, slide and debris slide, were studied by investigating and measuring at the five sites along the line from the upper reaches to the lower reaches.This dissertation is focused on the mechanism of gravity erosion in Pi-sandstone. Based on analysis of different controlled factors of rock-soil erosion, the stratum Hthology and stratum assembled is regarded as the main reason for inducing intensive gravity erosion. The anti-erodibility of Pi-sandstone rock were studied through measurement of physical and mechanics property of rock, such as unit weight of rock, mechanical analysis, water-property of rock, uniaxial tensile strength, uniaxial compressive strength, rock shearing strength, mineral assembly, chemical composition of rock, microstructure etc. The result shows that the Pi-sandstone has a characteristic of lower anti-erodibility and be eroded easily caused by outside force according to the basic theory and former research. Since the physical property of rock is reflected by the index of rock mechanics, the relationship between the effective cohesion and the influencing factors were studied and it is concluded that the occurrence of gravity erosion is closely related to the rock shearing strength-effective cohesion of rock. The weathering of Pi-sandstone, both mechanical and chemical weathering, was analyzed based on the investigating of lithology and fabric and structure of Pi-sandstone rock. The results suggest that mechanical weathering involves drying and wetting, heating and cooling, salt weathering, freeze-thaw, while chemical weathering involves hydrolysis, oxidation etc. It is difficult to determined which weathering type is dominated in the process of Pi-sandstone weathering. But it is clearly that the different lithology and distribution have different rate of weathering process. The author think that the basic reason of intense gravity erosion development are different lithology assembly and anti-erodibility diversity of Pi-sandstone. The process of rock weathering is the first step of rock-soil erosion. Sediment yield of Pi-sandstone is the mainly source of coarse sand deposited in the Yellow River based on the analyzing of mechanical analysis sampled both in Pi-sandstone and deposition, and the amount of erosion is more than that of conveyed by water.At present, advanced technologies such as GIS and RS and GIS are widely used in different field. The author tried to apply a new method to evaluate the gravity erosion due to the erosion features of Pi-sandstone in a small valley.
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