INVESTIGATION ON CATASTROPHIC LANDSLIDE OF JANUARY 11,2013 AT ZHAOJIAGOU, ZHENXIONG COUNTY, YUNNAN PROVINCE

On January 11,2013,a large landslide occurred in Zhenxiong county, Yunnan Province. The debris traveled about 850m. The debris volume was about 400,000m3. More than 60 houses in Zhaojiagou village were destroyed.46 persons were killed by the debris. This paper presents the field investigation on the cause of the landslide which unusually occurred in dry-season. It uses finite element method to analyze the mechanism of landslide under the conditions of long-term rainfall and coal mining. In geology, the landslide site located in coal-related strata which is common in Wu-meng mountains. The upper strata are limestone and dolomite of high bedding dip angles and middle Triassic epoch. The middle strata are sand shale of relatively high bedding dip angles and lower Triassic epoch. The bottom strata are shale and mudstone with coal seams of gentle bedding dip angles and upper Permian epoch. The strata system has an upper hard and lower soft structure in engineering geology and an upper rich and lower poor groundwater structure in hydrogeology. It is very vulnerable to landslide disasters. The site topography looks like a boot-shaped profile from top to bottom. The upper ground is steep and easy to be unstable. The middle and bottom grounds are wide and gentle. The ground profile offers the condition for long run-out motion and makes potential energy to transfer into kinetic energy. Therefore, the site can easily form high-speed and long travel distance landslide that causes disasters. The landslide zone can be divided into landslide source area, shoveling and stripping and deposition area and debris covered village disaster area. The flying landslide debris with high speed directly slipped and covered over the village houses. In the meantime, the rest ejected debris liquefied and rushed and buried other village houses along the gentle stream valley. They became the most important reason for the huge disaster. The finite element simulation results show the following. The rainfall led the groundwater table rising, which caused a more than 10% reduction of the factor of safety value of the slope. This result may demonstrate that the slope colluvium on steep stream valleys can be triggered to fail by long duration of small rainfall. The coal bed beneath the landslide has relatively thin thickness. The vertical distance of the upper strata between the coal seam and the landslide base is about 200m. The coal mining between 1960 and 1970 did not have noticeable effect to the slope deformation and stability. Based on the social problem induced by this landslide disaster, the authors put forward the following suggestions: to reinforce the public risk management and responding of huge geological disaster, to enhance the early recognition of high speed and long travel distance landslides in coal bed related strata areas, to improve the professional training of complex geological disaster prevention.