Land subsidence, storage and hydraulic properties of an aquifer

The studies related to land subsidence mostly focus on predicting the amount of subsidence that can occur, but rarely deal with the changes occurring in the storage and hydraulic properties of the aquifer. Although the land subsidence is observed to occur mostly due to compaction taking place in clay layers, the applicability of the theory of effective stress and Kozeny-Carman equation in clay samples have not yet been conclusively demonstrated. This dissertation presents new approaches to the theoretical development of the effective stress concept and the Kozeny-Carman equation. The new approaches are shown to provide an improved link between porosity and saturated hydraulic conductivity in clay.; A new equation for effective stress is derived from a physically based approach of inter-particle contact stress. Tensor algebra, Coulomb's friction law, the mean value theorem and area weighted averaging techniques are used to analyze and simplify the derivation. Assumptions and constraints under which the equation reduces to Terzaghi's effective stress concept are clearly elucidated.; The effect of adsorbed water surrounding soil grains on contact stress between particles in a fluid saturated environment is quantified using linear momentum conservation and Hertz contact theory. It is shown that in the case of clay particles, adsorbed water is in general not fully extruded from inter-particle contact points. The presence of the adsorbed water layer in clay particles prevents mineral to mineral contact causing a significant reduction in contact stress and increase in effective stress.; Published laboratory test results are used to evaluate the effects of adsorbed water in clay on porosity and the saturated hydraulic conductivity. The results from the analysis show that the use of the revised porosity values improves the predictive capability of the Kozeny-Carman equation with the variability in the permeability function defined as the ratio of hydraulic conductivity, K to the porosity function, n31-n 2 improving significantly. The improvement in the permeability function ranges from 14 percent for clays with mass specific surface area of 11-15 m2/gm to as much as 92 percent for clays with mass specific surface area of 100 m2/gm.