Analysis of Slope Stability of Spoil Grounds Under the Coupling Effect of Reservoir Water Level Rise and Rainfall Infiltration

To investigate the seepage evolution and instability mechanism of gully-type spoil ground slopes under the coupled effect of reservoir water level rise and rainfall infiltration, a typical second-grade gully-type spoil ground of the Jinghe Dongzhuang Water Conservancy Hub was selected as the case study. The SEEP/W and SLOPE/W modules of GeoStudio were employed to perform coupled numerical simulations of unsaturated seepage and slope stability. The reservoir water level was set to rise from 90 m to 120 m at a rate of 1.2 m/d, combined with six continuous rainfall intensities (duration: 25 days) of 20, 40, 80, 120, 160, and 200 mm/d. Reference scenarios involving only reservoir water level rise and only rainfall infiltration were additionally considered. The zoning response characteristics of pore water pressure at the slope front and rear were analyzed, and the individual contributions and coupled effect of rainfall and reservoir water level were quantitatively distinguished using a contribution decomposition method. The results indicate that the pore water pressure at the slope front is mainly governed by reservoir water level rise, whereas the pore water pressure at the slope rear is highly sensitive to rainfall infiltration. As the rainfall intensity increases from 20 mm/d to 200 mm/d, the factor of safety under the coupled condition decreases from 1.611 to 0.969. Furthermore, the dominant factor of instability evolves with rainfall intensity: the contribution decomposition results show that under higher rainfall intensities (80–200 mm/d), the rainfall-only contribution rate ranges from 43.6% to 57.0%, while, under lower rainfall intensities (20–40 mm/d), reservoir water level rise becomes the dominant factor. Throughout all coupled scenarios, the additional coupling contribution rate ranges from 18.2% to 52.9%, indicating a significant non-additive coupled hazard effect between the two factors. Based on the instability criterion of Fs < 1.0, the critical rainfall threshold is derived to be approximately 193 mm/d. These findings can provide a theoretical reference for the design of interception and drainage systems and for disaster risk management of gully-type spoil grounds.