Abstract：The geological condition of high rock slope at the intake of flood-discharge tunnel No.2 at Zipinpu hydropower station in Sichuan is very complex. There appears evidently the toppling deformation above the soft rock strata (L9，L10) in the lower part of slope，and collapses took place many times. Through studying lots of factors，such as different substantial components of slope，character of geological structure，excavation etc.，this paper illuminates mechanism of deformation at the intake of flood-discharge tunnel No.2. Meanwhile，the mode of deformation and destruction of this kind of anti-inclined slope，which is composed of a series of soft rock layers in its lower part and hard rock layers in its upper part，is the compressed-toppling going with sliding. It is demonstrated that the deformation of this slope is always initiated by the non-uniform compression of the soft rock layer，leading to toppling of the upper hard rock layer；finally，the tensile fracture of sliding appears along discontinuities paralleling with slope. Besides，two-dimensional finite element calculation is used to analyze the regularities of stress field and displacement during the excavation and to demonstrate the mechanism of this deformation.
