The Influence of Clay Zones on Land Subsidence from Groundwater Pumping

The objective of this article is to analyze the influence of clay zones on subsidence from groundwater pumping. Finite element analyses were conducted on a sand-only aquifer and a sand aquifer with two clay zones located at different distances from the well face. A model that accounts for recoverable and nonrecoverable strains was used to simulate the sand and clay. This model couples the groundwater flow with the stress-deformation response of the aquifer materials. Each aquifer was pumped from a single well for a period of 6 months, and then the groundwater level was lowered gradually to an elevation below the elevation of the clay zones and kept there for 10 years. The groundwater level was then raised gradually back to the original elevation over a period of 10 years. The results of the analyses show that the ground surface subsidence profile is strongly influenced by the presence of the clays zones. The ground surface sags where these clay zones are present resulting in a wavy ground surface profile. Subsidence continued when pumping is stopped, albeit at a much slower rate than during pumping, and when the groundwater level is below the elevation of the clay zones. Clay zones further away from the well face lag the subsidence of clay zones nearer the well face because of lower changes in hydrostatic head. Sags in ground surface subsidence profile from groundwater pumping are indicators of the presence of low hydraulic conductive geological materials.