THE ERMANSHAN ROCK SLIDE-DEBRIS FLOW OF JUNLY 27, 2010 IN HANYUAN,SICHUAN: CHARACTERISTICS AND FAILURE MECHANISM

At about 4 oclock on July 27, 2010,a large-scale rock slide happened in Ermanshan of Han Yuan in Sichuan province.Under the influence of the early rainfall,approximately 48104m3 strongly weathered basalt mass in high position slid down at a high speed,and then disrupted,transferred into debris flow,made a rapid movement along the valley,and unceasingly entrained and scraped the superficial loose material accumulating in the gully bed and on both sides of slopes,which caused the unceasingly increase of the volume and water content of debris flow.When moving to the middle part of the valley,because of the trench orienting towards right here,a part of debris flow ran to the left bank of the slope under the influence of strong inertia force,burying five villagers houses of ShuangHe village and causing 20 people missed.The other part of debris flow didnt stop moving until it ran rapidly for about 1.4km along the trench.The landslide drift accumulating in the middle part of the deep-river-cut trench started again under gravity form a secondary landslide.After creeping slip for several hours,the secondary landslide eventually arrived at WanGong Town and crushed down some houses,causing 92 houses to be damaged and 1500 people to be evacuate urgently.This paper carries out a detailed investigation and research on the zonal characteristics of the Ermanshan rock slide-debris flow and preliminary analysis of the disaster causes on the basis of massive geologic investigation into the site of the disaster,combined with in-situ measurement,experimentation,aerial photography and other means.The result indicates that the relatively prominent terrain condition of the source area,the weathered and shattered basalt mass and favorable combination of discontinuities is the fundamental condition for the rock slide.The debris flow undercutting the base of slope,the saturated surcharge of the rainfall,and the rainfalls infiltrating and softening effect along the sub-vertical and tensile texture plane directly caused the landslide.Ermanshan trench was originally a valley where debris flow frequently happened.There is no obvious signal before landslide happens.When it occurs,it demonstrates some characteristics as confidentiality,strong paroxysm,danger in long-rang movement in high position at a high speed and so on.Meanwhile,in a short time after the landslide,a secondary landslide will happen in the sliding zone.These phenomena and characteristics are special and typical.They are worth a further study.