Gas Entrapment in Granular Lenses in Silt/Clay Matrix

This paper presents a mathematical explanation for gas pockets that have been observed in granular lenses contained within silt/clay soils having downward percolating ground-water regimes. It is shown that the introduction of a piezometer into the granular lens can induce partial or total drainage of the lens. This, in turn, can lead to the false conclusion that the deposit in which the piezometer is placed is incompletely saturated when, in fact, it may be fully saturated. Equations are presented that can be used to calculate the anticipated degree of dewatering for specific hydrogeologic conditions. The degree of dewatering, if any, is dependent on the downward gradient through the granular lens, on the vertical distance between the phreatic surface above and the head in the underlying aquifer, and upon the location and thickness of the lens. Engineers are cautioned not to create such artifacts by indiscriminately placing monitor probes around landfills or other construction sites.