Design Methodology for a Gas Pressure Relief Layer Below a Geomembrane Landfill Cover to Improve Slope Stability

Pore pressures generated by landfill gas underneath a final cover system that incorporates a geomembrane can significantly reduce the effective normal stress on the lower geomembrane interface to the point of creating cover veneer instability. To the author's knowledge, no design methodology has previously been published to address this issue. Recently, however, large-scale slope failures have been attributed to landfill gas pore pressures. Therefore, a need for a design methodology exists. An estimation of gas flux from a landfill surface can allow a gas-relief layer to be designed using Darcy's law for gas flow through a porous medium. The methodology incorporates knowledge of the gas transmissivity of a chosen medium to design a spacing for highly permeable strip drains. The strip drains in turn can discharge the gas either to vents or an active gas collection system. The gas-relief layer typically consists of sand or a geonet composite. Limited testing of nonwoven, needle-punched (NWNP) geotextiles indicates that these materials may also be acceptable for gas relief in some designs. However, more testing is recommended before using NWNP geotextiles alone in this application. The greatest assumption in the proposed methodology concerns the estimation of gas flux. More research is required in this regard; however, the basic concept of providing a gas-relief layer with intermittent highly permeable strip drains is recommended as a prudent engineering measure for landfill final cover systems incorporating geomembrane barriers.