Modification of phreatic line calculation model for landslide accumulation in the Three Gorges Reservoir area

Since 2008, the water level of the Three Gorges Reservoir has been periodically fluctuating between 175 and 145 m. The rise and fall of the reservoir water level result in changes in the seepage field in the slopes on both banks of the reservoir. Groundwater is an important factor affecting the stability and reactivation of landslides. Developing a method to accurately determine the location of the groundwater saturation line is crucial to the evaluation and prediction of landslide stability. In this paper, the phreatic line under the condition of steady seepage is used as the initial value for unsteady seepage, and the analytical solution for the phreatic line on the bank’s slope under the action of reservoir water level changes is mainly modified based on two assumptions about the rate of change in the reservoir water level and the slope of the bank. In addition, through a large number of indoor tests, the relationship between the water yield, the permeability coefficient, and the porosity of the landslide in the Three Gorges Reservoir area is studied in detail, and a formula for the water yield of a soil–rock mixture is proposed. Finally, the modified analytical solution is used to determine the phreatic line in the Shiliushubao landslide, and the results are compared with the numerical analysis and monitoring results. The results show that the modified analytical solution is closer to the actual situation of the phreatic line in the reservoir water level fluctuation zone.