Experimental and numerical investigation of gas preferential flow through geosynthetic clay liner overlap joint in landfill final cover system

Gas migration control is a critical function of landfill cover systems, particularly in minimizing greenhouse gas emissions and ensuring environmental safety. This study investigates the gas flow behavior of needle-punched geosynthetic clay liner (GCL) overlap joints over a wide range of gravimetric moisture contents. A finite element model was employed to simulate gas transport through a landfill cover system, incorporating the effects of both GCL overlap joints and geomembrane defects. Experimental results reveal that the gas permeability of GCL overlap seams is 3.5 to 100 times greater than that of intact GCLs. Notably, the influence of moisture content on permeability reduction at the overlap seams is relatively limited, which is attributed to the layered structure of the GCL that restricts water penetration into the joint interface. Numerical simulations further demonstrate that geomembrane defects significantly intensify preferential gas flow through GCL overlap joints. Moreover, the radial distance (r) between geomembrane defects and GCL seams plays a critical role in determining landfill gas emission flux at the cover surface. At a water saturation degree of 70%, the maximum surface emission flux for the case with r = 2.5 m is nearly two orders of magnitude lower than when r = 0. These results highlight the importance of maintaining geomembrane integrity, particularly above GCL overlap seams. Overall, the findings offer valuable theoretical and practical guidance for the design and maintenance of effective landfill cover systems.