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Abstract The study area is located at the right bank of a proposed hydropower station on River Nu of Yunnan. The slope is mainly composed of migmatitic granite and mixed gneiss and the phenomenon of deformation and fracture controlled by low-angle and tilt-valley faults is very distinct. Although the macroscopic combination mode shows sliding-fracturing，but these faults were not cut and exposed at the slope toe and were buried inside the toe or below a certain depth of the valley. It is difficult to understand the cause of this phenomenon according to the views of traditional engineering geology and rock mechanics. Based on the field geological survey and the analysis of geological and mechanical conditions，the above phenomenon of deformation and fracture was found to be caused by the weathering and softening of rock in the valley bottom(slope toe and valley below) during the neoid geological period，which was a time-dependent deformation and fracture phenomenon. The result was verified by the numerical simulation. For the evolution of rock slopes，the softening effect of valley bottom(slope toe) is a kind of geological function with universal significance.
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2016, Vol. 35 
