Mechanisms Involved in Vibratory Destabilization of NAPL Ganglia in Sands

Immiscible liquids when spilled into the ground or leaked from underground storage tanks tend to remain trapped in the form of discrete ganglia due to strong capillary forces. These ganglia often have low solubility in water and may remain in the subsurface over long periods of time creating a continuous source of pollution. Previous studies, which were exploratory in nature, showed that creation of localized vibrations could recover high percentages of trapped ganglia. In this paper, the mechanisms involved in the vibratory destabilization of ganglia are analyzed using results from two sets of experimental studies. It is postulated that, when vibrations result in compaction of sands, viscous pressures tend to destabilize the ganglia by splitting them whereas buoyancy pressures increase the maximum sustainable lengths. The roles of viscous and buoyancy pressures are reversed when vibrations result in increased porosities due to expansion (dilation) of soil. The volumes of trapped ganglia recovered during the experiments are consistent with these postulates. Experimental results also indicate significant recoveries in the cases where the ganglia are supposed to remain stable. These recoveries are attributed to the transient particle rearrangement during vibrations, which is concluded to be an important mechanism.