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| INVERSE GRADING ANALYSIS OF DEPOSIT FROM ROCK AVALANCHES TRIGGERED BY WENCHUAN EARTHQUAKE |
| WANG Yufeng,CHENG Qiangong,ZHU Qi |
| (Department of Geological Engineering,Southwest Jiaotong University,Chengdu,Sichuan 610031,China) |
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Abstract The Wenchuan earthquake,which occurred in Sichuan Province,triggered several thousand landslides. For the study of rock avalanche kinematics,the Xiejiadianzi rock avalanche,Niujuangou rock avalanche and Wenjiagou rock avalanche were described to show the geological characteristics of rock avalanches along their sliding paths. According to field investigation,the vertical sorting on the path of rock avalanches is presented;firstly,which is divided into five parts from top to bottom,and named as:the air-blast zone,the leap and air-launch zone of debris,the sliding and deposit zone of debris,the sliping zone,and the original underlying accumulation zone respectively. Then,based on the description of the vertical sorting,the grain-size distribution of the sliding and deposit zones of debris is analyzed by combining an area-by-area technique and sieve analysis,which presents a gradual increase in grain size with height above the base cut quantitatively,proving the existence of inverse grading. This unique inner structure is an important geological evidence for the study of rock avalanche kinematics,which can shed light on the mechanism of their rapid motion. Finally,according to lots of field investigation and test results,one conclusion is proposed that:the kinematic sieving process energized by undulated sliding surface,the dynamic fragmentation process triggered by particle collisions,and their coupling process are the main reasons for the formation of inverse grading;and based on the process of landslide kinematics and dynamics,the above energy sources and transformation processes are described.
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Received: 12 January 2012
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