Influence of uncertainty in the initial groundwater table on long-term stability of reservoir landslides

The seepage calculation of water fluctuation is the crucial precondition for landslide stability evaluation. It is controlled by the initial condition, boundary condition, and soil or rock structure as well as geotechnical parameters. Although the geotechnical parameters and boundary condition have greater influence on seepage and the stability of landslides, the role of the initial groundwater position cannot be ignored. To investigate the influence of an initial groundwater table on landslides, especially the influence of cyclical reservoir water fluctuation on their long-term stability, seven different methods to determine the initial groundwater table were proposed in the presented study, among which one is the actual groundwater table, and the other six are obtained with simplified methods. According to the results of the calculations, at the beginning, there is some difference in the factor of safety among different initial groundwater tables. The maximum value of relative tolerance reduces to less than 1.0 % 160 days later. After a filling–drawdown cycle, the relative tolerance of all methods reduces to less than 0.8 %, which could be ignored. It is also observed that, the closer the hydraulic gradient of the proposed initial groundwater table is to the actual condition, the smaller is the relative tolerance. Therefore, to study the influence of cyclical reservoir water fluctuation on landslide long-term stability, the initial groundwater table could be proposed as (1) equal to reservoir water level and (2) on a straight line with the hydraulic gradient equal to the weighted average of actual groundwater table for simplification.