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Abstract Taking the Qingning landslide in Sichuan Province for an example，its transform mechanism of landslide-debris flow has been studied. After mastering its transform course，longly sliding and shearing course of slide body after its instability are simulated with ring shear test；the physico-mechanical characters during shearing course are monitored. The result shows that the soft base effect is an important factor for transform of landslide- debris flow besides excess pore water pressure，which is the focus of reseach at present. Because of shearing shrinkage，grain crushing and layering，the sliding body volume of soil reduces in the course of shearing，which makes excess pore water pressure increase，and thus its shear strength decreases. Simultaneously，a thick liquefied layer has been formed at the bottom of sliding body，of which thickness increases with shearing displacement increasing. The liqueafied layer carries sliding body moving fast and reduces its stability greatly，which is called“soft base”effect in this paper.
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Received: 27 October 2009
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