Abstract: Geomembranes used in impervious structure of embankment dams can be combined with geotex-tiles to form a composite in order to increase their tensile strength and resistance to puncturing. The geotextile plays an important role in draining the interstitial water and the air between geomembrane and ground soil. From the concept of contact scour between geotextile-geomembrane composite blanket and ground soil, formulas for calculating minimum length of blanket and corresponding seepage discharge are establishes. Based on the membrane theorem, formulas for calculating the burst strength of geotextile-geomembrane composite are proposed. Two practical examples computed by finite element method verified that these formulas can be used in designing composite impervious layers for embankment dams.