Abstract: GCL possesses many advantages such as low hydraulic conductivity and good compliance with differential settlements. Consequently, GCL has been extensively used as a hydraulic barrier in landfill engineering. The bentonite from the GCL may extrude into the GCL/GM interface, resulting in a significant reduction in interfacial shear strength, and it has important effects on the application of GCL in landfill slopes. Three conventional oedometers are modified to study the mechanism of bentonite extrusion. The influence of normal stress, type of geotextile and type of hydration fluid on bentonite extrusion are investigated. The experimental results reveal that the bentonite extrusion is due to the excessive pore-water pressure during rapid normal loading. It is found that when the prehydrated GCL specimen is subjected to rapid one-dimensional loading, an excessive pore-water pressure tends to build up in the GCL, then the seepage force drives bentonite extruding through the geotextile and attaching on the GM.