This thesis evaluates the effects of freeze-thaw weathering, drying-wetting weathering, and changes in pore fluid chemistry on the hydraulic conductivity of highly plastic Lake Agassiz clay. Lake Agassiz clay is frequently considered as a cost-effective alternative to geosynthetic membranes and liners for use in waste containment structures, as well as a natural barrier beneath contaminated sites. Four types of freeze-thaw weathering tests were conducted with each type differing by the temperature conditions imposed and the availability of water. Three methods of drying and wetting were used and were differentiated by the presence or absence of an artificial crack. Chemical tests involved the addition of varying concentrations of sodium carbonate solutions after weathering to reduce hydraulic conductivities, while other chemical permeation tests were conducted using dimethyl sulfoxide as a method of reducing the effects of freeze-thaw weathering. The equipment, specimen processing procedures and test methodology are described in detail. The results indicate that freeze-thaw weathering and specimens artificially cracked after desiccation produced an order of magnitude increase in hydraulic conductivity of Lake Agassiz clay. A slight decrease occurred in specimens which were subjected to desiccation then rewetting before permeation. Self-sealing was incomplete after each weathering process because consolidation of soil aggregates had occurred due to various suctions generated during weathering. A comparison between hydraulic conductivity values obtained ftom oedometer tests and those obtained from triaxial hydraulic conductivity tests was also performed. Results indicate that the triaxial test apparatus is a superior method for determining the hydraulic conductivity of natural clay specimens.