Development of an apparatus to investigate the stress variables governing unsaturated soil behaviour

The paper presents the development of an apparatus to investigate the stress variables governing unsaturated soil behavior. This apparatus was devised to explore the range of negative pore-water pressures. Tests were performed by preventing deformation and water content change of an unsaturated sample and measuring changes in normal net stress and matric suction in response to a change in pore-air pressure. The condition of null deformation and null water content change was achieved by specially designing the cell and the air supply system. Vented flush diaphragm transducers and vented Imperial College tensiometers were used to measure directly normal net stress and matric suction, respectively. A preliminary test demonstrated that an interaction occurred between the soil and the flush measurement diaphragms. A specific procedure was then devised to account for the error due to this interaction. The results from the experimental tests support the assumption that the stress variables governing unsaturated soil behavior can be reduced to two stress state variables, e.g., net stress and matric suction.