Abstract：Focused on a cutting slope on 220 kV Yanjin Transformer Substation，the study is carried out to find its movement characters. Through site investigation，deformation investigation and analysis，laboratory test，stress-strain behavior analysis and numerical modelling，three factors are suggested as follows：(1) excavation provides enough opportunity movement；(2) the strength of intensely and completely decomposed mudstone decreases evidently for slaking，softening and argillation in water，and the completely decomposed mudstone is strain softening；(3) V-shaped topography could provide channel for the ground water，and induce adverse hydrodynamic and hydrostatic water pressure. The deformation of cutting slope is retrogressively trigged by excavation，and the weak mudstone controls the slope stability. Slope deformation can be classified into four stages over time：(1) at the first stage，the elastic deformation adjacent to the cutting surface is released due to unloading during excavation；(2) at the second stage，when excavation is almost completed，the slope toe fails because of gravity and concentrated stress induced by excavation；(3) at the third stage，when excavation is completed，cracks appear at crown，and the shear strength of mudstone，which is reduced by groundwater，decreases further due to the increase of shear strain triggered by excavation；and (4) finally，the mid part of the over-layer is tensilely cracked，and the over-laver moves accompanying the slide of underlain mudstone induced by hydrodynamic and hydrostatic pressure，gravity，and dominating action of the softened mudstone. This study suggests that more attention should be paid to the mudstone in the future engineering for its softening characters，and the way of deformation mechanism investigation provides some suggestions for the similar studies.
