Impact of transient seepage on slope stability of earth-rock dams with geomembrane barrier defects

The water–air two-phase unsaturated seepage theory was employed to analyse the transient seepage field of an earth-rock dam with an impervious geomembrane barrier during rapid drawdown of the reservoir. Discussed is the resulting seepage behaviour of the dam after locations of defects in geomembranes and hydraulic conductivity of dam materials were changed. In addition, the unsaturated soil strength theory was used to calculate the anti-sliding safety factor of the upstream dam slope during drawdown. The uncertainty of effective stress parameters of dam materials was considered, and the Monte Carlo method was employed in the reliability analysis of the stability of the upstream dam slope. The results indicate that the location of defects has a significant influence on the initial phreatic surface distribution and the seepage flux mainly depends on the hydraulic conductivity of dam materials. Geomembranes with defects on the dam surface have an apparent hysteresis effect on the descending of the phreatic surface when the reservoir level drops, which causes a great decrease in the stability of the upstream dam slope. Matric suction significantly affects the stability of the upstream dam slope, while the air phase has little effect on slope stability when geomembranes are placed on the upstream surface of the dam.