The wetting and saturation of bottom clay liner at waste landfill reduces the shear strength of clay-geomembrane interface. This behavioral aspect was studied as a possible mechanism of local bottom failure of waste landfill in Zagreb.Two series of clay-geomembrane interface strength tests were conducted in large shear box for the range of normal pressures expected in field. In the first series the interface strength was tested in the “as compacted” state in order to obtain the referent values for ordinary conctruction conditions. In the second series the samples were flooded with water and left to saturate before testing. The shear testing was performed in unconsolidated-undrained conditions in order to simulate rapid loading conditions during relatively fast waste filling. The test results clearly showed the significant interface strength reduction in flooded samples.