Numerical parametric study of an alternative three-layer capillary barrier cover system

Abstract

A common practice for disposing of municipal waste is to dump it in landfills, where it is then protected by a cover system. The main purpose of this cover system is to minimise the amount of water percolating into the waste and, hence, to reduce the excessive formation of leachate. To achieve this objective, conventional landfill covers employ low permeability materials, such as composite liners, to meet the permeability design criterion. However, their long-term hydraulic behaviour, cost and shear resistance are not entirely satisfactory. Increasingly, covers with capillary barrier effects (CCBEs) have been considered as an alternative cover system in semi-arid and arid regions. It is questionable, however, whether CCBEs can be successfully applied in humid climate conditions where the annual rainfall often exceeds 2000 mm. In this paper, an alternative three-layer capillary barrier cover system for use in humid climates is proposed, and its feasibility is investigated by a numerical parametric study. This alternative system consists of a fine-grained soil layer overlaying a coarse-grained layer, which in turn overlies a fine-grained soil, such as clay, to minimise water percolation in humid climates. This bottom clay layer is protected by the upper two coarser soil layers. The factors considered in the numerical parametric finite element analyses include the thickness of this additional clay layer, rainfall conditions and degrees of saturation of the municipal waste. It was verified that the middle sand layer serves as a capillary break when the rainfall intensity is light or the duration is short. After the upper two layers are permeated, the bottom clay layer serves as an impeding layer, whereas the sand layer shifts to serve as a lateral drainage layer. It was also found that the amount of percolation increases with an increase in rainfall duration but decreases with saturation of the waste. Based on the six simulated durations of rainfall, the most severe rainfall duration is 1 day, irrespective of the return period.