The Impact of soil structure and confining stress on the permeability of clay soils in brine environments

Abstract

In recent years, numerous studies have been done on the alteration of the permeability of clayey soils as a result of exposure to concentrated organic or inorganic permeants. These permeability changes have been attributed to either changes in the microfabric or the macrofabric of the clays as a result of physico-chemical interactions between the clay particles and pore fluid chemistry. However at the present there still remains no theoretical framework for interpreting the mechanism for changing permeability and the prediction of the onset of permeability alterations during osmotic consolidation. In this thesis, focus is given to the impacts of initial soil structure and vertical confining stress on the changes of permeability that occur in a clayey soil when it is exposed to brine. Different methods of sample preparation are included so that the effect of different soil structure can be observed. In addition the permeability tests were carried out at different stress levels. The initial soil structure and changes in structure as a result of changes in pore fluid chemistry were obsetved using Scanning Electron Microscope photographs. Two sample failure criteria based on the measurement of the change in the physico-chemical stress, (R-A), are proposed in order to predict the onset permeability alterations. The test results reveal that the alteration of permeability is strongly related to the initial soil structure and level of confining stress. No significant change in the microfabric of the clay was observed; however, the macro pore size of the specimens appears to increase as a result of physico-chemical interaction of clay particles and NaCl brine. The permeability increases in a continuous manner with decreases in confining stress and can be related to the control of horizontal osmotic volumetric strain within the soil. The two sample failure criteria, based on the release of (R-A) and loss of confining stress in soils after brine permeation, seem to provide a simple and effective approach to predict the onset of permeability alteration in soil as a result of brine exposure. This demonstrates that the mechanism for the onset of permeability alteration can be attributed to the increase in the size of the macropores and the loss of confmement in the soil, rather than changes in microfabric.

(Appendix D & E were duplicated in the original)