This paper aims to study the shear interaction mechanism of one of the critical geosynthetic interfaces,the geotextile/geomembrane,typically used for lined containment facilities such as landfills.A large direct shear machine is used to carry out 90 geosynthetic interface tests.The test results show a strain softening behavior with a very small dilatancy(<0.5 mm) and nonlinear failure envelopes at a normal stress range of 25-450 kPa.The influences of the micro-level structure of these geosynthetics on the macro-level interface shear behavior are discussed in detail.This study has generated several practical recommendations to help professionals to choose what materials are more adequate.From the three geotextiles tested,the thermally bonded monofilament exhibits the best interface shear strength under high normal stress.For low normal stress,however,needle-punched monofilaments are recommended.For the regular textured geomembranes tested,the space between the asperities is an important factor.The closer these asperities are,the better the result achieves.For the irregular textured geomembranes tested,the nonwoven geotextiles made of monofilaments produce the largest interface shear strength. This paper aims to study the shear interaction mechanism of one of the critical geosynthetic interfaces, the geotextile / geomembrane, typically used for lined containment facilities such as landfills. A large direct shear machine is used to carry out 90 geosynthetic interface tests.The test results show a strain softening behavior with a very small dilatancy (<0.5 mm) and nonlinear failure envelopes at a normal stress range of 25-450 kPa. These influences of the micro-level structure of these geosynthetics on the macro-level interface shear behavior are discussed in detail. This study has generated several practical recommendations to help professionals to choose what materials are more adequate. From the three geotextiles tested, the thermally bonded monofilament exhibits the best interface shear strength under high normal stress. For low normal stress, however, needle-punched monofilaments are recommended. For the regular textured geomembranes tested, the space between the asperities is an important factor the closer these asperities are, the better the result achieves. For the irregular textured geomembranes tested, the nonwoven geotextiles made of monofilaments produce the largest interface shear strength.