Osmotic Flow and Volume Change in Clay Soils
Barbour, Sidney Lee
The engineering profession has been called upon by the public in recent years to provide ever increasing degrees of containment for low level waste contained in shallow subsurface waste containment facilities. To provide this protection, the use of engineered liners or barriers has become a common design feature. In many cases, these barriers are constructed out of natural clays or artificial mixtures of clay minerals. The use of soil barriers to contain waste products consisting of strong electrolyte solutions, has met with mixed success. Clays of lowest permeability are also those whose behavior is the most influenced by the presence of salt solutions. Failures of clay liners exposed to electrolyte solutions have not been well documented. The mechanism for failure seems to be the result of shrinkage of the clay, which then leads to the development of a secondary structure of cracks and fissures. Field cases of liner failure have often been described as occurring as a result of "osmotic desiccation". The general objective of this study was to define, and quantify, the mechanisms controlling the rate and magnitude of volume change in clay soils exposed to strong electrolyte solutions. A review of the literature presented two possible mechanisms termed osmotically consolidation. Osmotically induced consolidation occurs as a result of for osmotic volume change. induced consolidation and These were osmotic rapid flow of water out of the sample in response to osmotic gradients. Osmotic consolidation occurs as as result of a reduction in the net electrostatic repulsive stresses between clay particles. A general theoretical description of osmotically induced consolidation and osmotic consolidation was developed. A phenomenological approach was adopted to describe fluid flow in response to osmotic gradients. A Darcy type flow law was used to related osmotic flows to osmotic gradients through a conductivity term called the osmotic permeability. To describe osmotic consolidation, the osmotic pressure of the pore fluid was selected as a stress state variable. Volume changes were linked to the osmotic pressure of the pore fluid through a constitutive relationship. The soil property used to define changes in soil volume due to osmotic pressure changes was called the osmotic compressibility. A numerical solution to the theoretical description of osmotic flow and volume change was developed using finite element techniques. This model was used to characterize the processes of osmotic and osmotically induced consolidation. A laboratory program was undertaken to monitor the osmotic flow and volume change in two clays; Regina Clay and an Ottawa SandI Na montmorillonite mixture. From the results of these tests the dominant mechanism of volume change for these clays was found to be osmotic consolldatipn. The test procedures developed allowed the soil properties describing osmotic flow and volume change to be evaluated. A technique was developed by which the electrostatic repulsive stresses within a clay could be measured indirectly through laboratory testing. The results indicated that the onset of fracturing may be predicted by comparing the change in the net repulsive stress that occurs as result of changing pore fluid concentrations, to the confining stress within the soil.