土工膜以塑料薄膜作為防滲基材.土工膜防滲性能主要取決于塑料薄膜的防滲性能。 防滲應用的塑料薄膜，主要有聚氯乙烯（PVC）和聚乙烯（PE），它們是一種高分子化學柔性材料，比重較小，延伸性較強，適應變形能力高，耐腐蝕，耐低溫，抗凍性能好。防水防滲的聚乙烯土工膜主要分為光面土工膜，復合土工膜，糙面土工膜。光面聚乙烯土工膜厚度一般有0.5mm 、1.0mm  2.0mm  2.5mm 3.0mm

復合土工膜分為一布一膜，二布一膜等。

土工膜主要機理是以塑料薄膜的不透水性隔斷土壩漏水通道，以土工膜較大的抗拉強度和延伸率承受水壓和適應壩體變形；同時，土工膜對細菌和化學作用有較好的耐侵蝕性，不怕酸、堿、鹽類的侵蝕。
土工膜的使用年限問題，主要是由塑料薄膜是否失去防滲隔水作用而定，據國家標準規定，水工用的厚度為0.2m的加穩定劑的聚乙烯薄膜，在清水條件下工作年限可達40~50年，在污水條件下工作年限為30~40年。因此復合土工膜的使用年限足以滿足大壩防滲要求的使用年限。

Geomembrane impermeability is relied on the plastic film which is as impermeable base material. Anti-seepage plastic film mainly includes polyvinyl chloride (PVC) and polyethylene (PE) which is a polymer chemical flexible materials with smaller proportion, stronger extensibility, good adaptability, corrosion resistance, low temperature resistance, frost resistance. Waterproof impermeable polyethylene geomembrane is divided into smooth geomembrane, composite geomembrane, rough geomembrane. Smooth polyethylene geomembrane thickness is generally 0.5mm/ 1.0mm/ 2.0mm/ 2.5mm/ 3.0mm.

Composite geomembrane is divided into a cloth a film, two cloth a film.

The main mechanism of geomembrane plastic film is impermeable to earth dam water leakage channel. Geomembrane has greater tensile strength and elongation to withstand water pressure and adapt to the deformation of the dam. At the same time, the geomembrane has good erosion resistance by bacterial and chemical effects.

The service life of geomembrane mainly depend to the effect of loss of impermeable plastic diaphragm waterproofing. According to national standards, the thickness of the hydraulic engineering with 0.2m stabilizer plus polyethylene film is up to 40 to 50 years under  clear water conditions and for 30 to 40 years under sewage conditions. Therefore, the useful life of composite geomembrane is sufficient to meet the anti-seepage requirements of the dam life.