Development of novel apparatus for establishing swelling and water retention characteristics of bentonite

Abstract

Despite the breakthroughs in suction control and measurement for laboratory testing of unsaturated soils at the range of high suctions, existing equipment faces limitations in either the precision, the range, or the practicality. The aim of the present thesis is to develop novel apparatus for establishing the swelling and water retention characteristics of compacted bentonite, which may exhibit very high suctions at its as-compacted state. The development of the equipment is based upon the best elements of existing equipment, combined in a simple and efficient manner and extends the range of existing suction-controlled facilities in the Geotechnics Laboratory of Imperial College London. A new humidity generator is presented, based on the principles of the divided-flow method. The divided-flow humidity generator (dfHG) controls the relative humidity (RH) in a sealed chamber, allowing suction control of soil samples through the Vapour Equilibrium Technique (VET). The control of RH is automated and is obtained by the proportional mixing of dry and wet air based on the feedback given on RH measurements in the chamber with affordable, individually calibrated capacitance hygrometers. Direct comparison with other suction-control methods, and derivation of water retention curves for reference soil materials validate the method. The automated control and the continuous operational range of suction provides the versatility of pressure generators, while the simplicity of the system is comparable to the practicality of the air-regulation method. The humidity generator is subsequently adapted to a newly-developed oedometer, to assist in the investigation of highly expansive clays, such as bentonites. The device allows the application of continuous, automated control of suction in the range between 30 MPa and 300 MPa, while applying complex stress and hydraulic paths for swelling tests. Equilibrium on each suction step is assessed through the combined evaluation of swelling stress, RH of the outflow air and water exchanged with the soil sample. The method is validated through successful investigation of the swelling behaviour of a natural sodium bentonite.