Abstract: Seepage prevention is the main function of final cover of municipal solid waste landfill. Capillary barrier cover is not effective in some humid climates due to inadequate water holding capacity of soil. Adding unsaturated drainage layer (UDL) to the capillary barrier cover (CBC) is one of the measures to improve the performance of CBC in humid climate area. In this research, the lateral drainage performance of the capillary barrier cover with unsaturated drainage layer (CBC+UDL) was studied by combining indoor extreme rainfall model test and numerical analysis. Size of the cover model is 2.0 m×1.0 m× 0.4 m (length × width × thickness). The structural section from the upper to the lower is silt layer (20 cm), sand layer (10 cm), and gravel layer (10 cm) in sequence. The results of model test revealed that in the presence of 882.2 mm cumulative rainfall, the lateral drainage of UDL was 9.5 mm which accounted for 13.7% of the rainfall infiltration. Numerical simulation demonstrated that the effect of thickness and permeability of fine-grained soil on drainage length varied with rainfall amount: in the presence of small rainfall amount, a thicker fine-grained soil layer resulted in lower permeability and longer lateral drainage length. The thickness of UDL had little impact on the length of lateral drainage. A lower water entrance value of UDL would enhance the water storage capacity of fine-grained soil, and delay the failure of fine-grained soil and the effect of UDL. Nevertheless, the decay of drainage length in the late stage of continuous rainfall would accelerate. Therefore, the water entrance value of UDL must be considered comprehensively according to local rainfall conditions and hydraulic characteristics of coarse and fine grained soil covers.

Key words: capillary-barrier cover, unsaturated drainage layer (UDL), lateral drainage length, permeability, solid waste landfill