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One of the main purposes of a landfill cover system is to minimize the migration of water into waste, known as percolation, and thereby reduce excessive leachate production. One possible way to achieve this goal is to use a two-layer cover with capillary barrier effects (CCBEs) for arid and semi-arid regions. For a humid climate or prolonged rainfall, the two-layer system with CCBEs is expected to lose its effectiveness for minimizing water percolation. A new three-layer landfill cover system is proposed and investigated for humid climates. This new system adds a fine-grained soil (i.e.,clay) underneath a two-layer barrier with CCBE (i.e.,a silt layer overlying a gravelly sand layer). The study is conducted by carrying out a one-dimensional (1D) water infiltration test in a soil column. The soil column was instrumented with tensiometers, heat dissipation matric potential sensors, and moisture probes to monitor the variations of pore-water pressure and water content with depth. The amount of water volume infiltrated into the soil during ponding was also monitored. In addition, transient seepage simulations were carried out to back-analyze the soil column test and to investigate the influence of saturated permeability of clay on the effectiveness of the three-layer system. Based on the 1D experiment and numerical analysis, no percolation was observed after 48h of constant water ponding, which is equivalent to a rainfall return period of greater than 1,000years. This is consistent with the results from the numerical back analysis. However, the upper two-layer capillary barrier is only effective for a rainfall return period of approximately 35years. This indicates that the proposed bottom clay layer is necessary for a humid climate. Numerical parametric simulations reveal that with an increase of saturated clay permeability by three orders of magnitude (i.e.,from 5.7x10-9m/s to 5.7x10-6m/s), the amount of percolation is approximately 0.1mm after 12h of constant water ponding, which is equivalent to a rainfall return period of greater than 1,000years. (C) 2016 American Society of Civil Engineers.