Abstract: To design cover system or leachate collection systems of a modern landfill,engineers often have to estimate the maximum saturated depth(Dmax) for a drainage system with geosynthetics.Based on the extended Dupuit assumption,a computational solution was presented to calculate Dmax,and the liquid profile for drainage system with geosynthetics underlain a sand layer was provided.By comparison with the results calculated by other methods,the present method was shown to be rational and accurate.By means of parametric analysis,in drainage systems with a sand layer above a geocomposite layer,Dmax was significantly influenced by the inflow rate,drainage distance,transmissivity of the geocomposite and the hydraulic conductivity of the sand when they were not extraordinarily low.It was also found in drainage systems with a sand layer and a geocomposite layer,Dmax was much more sensitive to the slope of the drainage layer than that in a comparable system consisting of two sand layers.It was of great advantage to increase the inclination when geocomposites were applied as a drainage material.