Numerical model for determining the wetting front in a clay layer of a leaking composite barrier.

摘要

Composite barriers for hazardous and solid waste landfills are designed to minimize hydraulic transport of the contaminant leachate of the landfill; however, the geomembrane can be damaged during construction or otherwise contain imperfections from manufacturing. It is desirable to estimate the discharge characteristics and the flow width of such a leachate should leaks in the geomembrane occur.; Numerical solutions of the flow equations for unsaturated soil barriers can provide an understanding of the movement of a leachate through a composite barrier. The purpose of this investigation was to develop a computer model that simulated two-dimensional migration of leachate beneath holes in a geomembrane in the composite barrier profile. The solution of this problem aids the determination of the time required for a wetting front to reach the leachate detection system. An approximate model of a composite barrier with a damaged geomembrane was considered. The top of the geomembrane and bottom of the leachate detection system were the boundaries for simulation, and then the soil profile was analyzed. Finally, finite elements were utilized accounting for leachate placed on top of the clay barrier. The flow through the soil below a slit was computed using Richards' equation.; A computer code to evaluate leakage through composite barriers was presented. The code, COMPBAR (composite barrier), simulates highly nonlinear, two-dimensional movement of leachate in unsaturated composite barriers with damaged geomembranes. A finite element approximation of Richard's equation describing flow through unsaturated porous media was incorporated. Galerkin's method and Picard iteration were utilized to solve Richard's equation. The governing equation reflected barrier heterogeneity and the dependence of hydraulic properties on the degree of saturation. The time required for the wetting front to reach the leachate detection layer and the time necessary for a stationary flow to develop in the soil barrier were computed.; Outputs of the developed COMPBAR program compared well with outputs of the SOILINER and SEEP programs. Design charts and guidelines were developed that can be used by designers to estimate the effective life of composite barriers and to aid study of various parameters that affect wetting front movement in the composite barrier.