Soil-water characteristic curves and shrinkage behaviour of highly plastic clays: an experimental investigation

This thesis experimentally investigated the changes in the water content and the void ratio of initially slurried highly plastic clays under zero applied stress during the drying process, changes in the water content of compacted unsaturated clay specimens during the wetting process under zero applied stress for laterally confined and free swelling conditions, and the wetting behaviour of heavily compacted bentonite specimens under confined condition. In total three clays were used, namely MX80 bentonite, Yellow bentonite, and Speswhite kaolin. Several laboratory techniques, namely centrifuge, axis-translation, vapour equilibrium, osmotic and chilled-mirror dew-point techniques were used to establish the suction-water content soil-water characteristic curves (SWCCs). The shrinkage paths of the clays were established from Clod tests and wax method. The suction-water content SWCCs in conjunction with the Clod test results enabled establishing the suction-degree of saturation SWCCs and determination of the air-entry values (AEVs). The AEVs of the clays were also determined based on the chemical analyses of Polyethylene Glycol (PEG) solutions and the applied suctions in the osmotic tests. Degradation of PEG molecules and semipermeable membranes were studied using Fourier Transform Infrared (FTIR) spectroscopy and Atomic Force Microscopy (AFM), respectively. Isochoric (constant volume) swelling pressure tests were carried out on compacted bentonite specimens using deionised water. Additionally, a newly developed suction controlled oedometer was used to study the swelling pressure and the water uptake capacities of heavily compacted bentonites under confined conditions. Microstructural changes of compacted bentonites were studied using Environmental Scanning Electron Microscope. X-Ray Diffraction technique was used to monitor the changes in the c-axis spacing of clay specimens equilibrated at several applied suctions. Applications of the Gouy-Chapman diffuse double layer theory and the van der Waals attractive pressure theory in predicting the drying and wetting suction- water content SWCCs were explored. One-dimensional wetting model (Richard's equation) was used to evaluate the elapsed time versus water content relationships for the clays. Chemical analyses of PEG solutions after the osmotic tests showed a clear lack of equalisation of the osmotic suctions due to the expelled and the retained salts on either side of semipermeable membranes. A majority of the salts were found to be retained within the clays during the osmotic tests. The AEVs of the clays determined based on the suction-degree of saturation SWCCs and that from the total suction equilibrium in the osmotic tests were found to be similar. The FTIR studies did not indicate any degradation of PEG molecules. The AFM studies of membranes revealed alterations of the pore-size during the tests that possibly lead to an intrusion of PEG into the bentonite specimens. Both qualitative and in some cases quantitative agreements were noted between the experimental suction-water content SWCCs and those predicted from the physico-chemical theories