The physical response of composite landfill liners consisting of a geomembrane on top of a geosynthetic clay liner (GCL) are examined under simulated landfill conditions. The deformation and strains of a 1.5-mm-thick high-density polyethylene geomembrane and thickness and hydraulic performance of a nominally 7-mm-thick GCL are quantified when the composite liner was buried beneath 50 mm coarse gravel, at applied pressures up to 1000 kPa, with a firm sand foundation layer, and with and without a wrinkle in the geomembrane. At an applied pressure of 250 kPa, with either no protection or conventional thick nonwoven needle-punched geotextile protection layers, the tensile strains in the geomembrane exceeded a 3% allowable limit and the GCL was reduced in thickness to as little as 2.2 mm from extrusion of bentonite beneath a gravel particle. Whereas a 150-mm-thick sand protection layer limited strains in the geomembrane to 0.1% and prevented extrusion in the GCL so that deformation was from bentonite consolidation and not from extrusion. A GCL with a thickness of less than 3 mm from extrusion was shown to be susceptible to failure from internal erosion of bentonite in the GCL at hydraulic head differences across the GCL between 1-10 m. Conversely with the sand protection layer, the GCL could withstand a head difference of greater than 100 m without any evidence of internal erosion. Further, the permittivity of an extruded 3.5-mm-thick GCL was found to be 4.5 times larger than a 7-mm-thick GCL that did not experience extrusion.