Laboratory investigation of osmotic tensiometers filled with cross-linked polyacrylamide

Soil suction measurement is required for the analysis of slope stability and soil-structure interactions related to unsaturated soils below transportation infrastructures. Osmotic tensiometers (OT) can measure soil suction higher than 1000 kPa but suffer from the pressure decay problem. The OT filled with cross-linked polyacrylamide can maintain a stable pressure for a long time but the research on this type of OT is quite limited. In this study, the behaviors of OTs filled with synthesized non-ionic cross-linked polyacrylamide were investigated experimentally and explained in accordance with Flory-Huggins polymer theories. A generalized calibration equation was proposed to reduce the pressure fluctuation of OT caused by temperature. The pressure variations of two OTs were observed for more than 100 days and several cycles of drying tests were conducted. The results show that the pressure of the OT recovered after it was dried in the air for tens of minutes. In contrast, a slight pressure decay appeared when the OT was dried in the air for several days. However, as more cycles of drying tests were conducted, the pressure decay caused by the drying tests gradually became negligible. The relationship between the pressure of the OT at the end of observation and the volume fraction of the polyacrylamide inside the chamber of the OT was established by considering the confinement effect on the polyacrylamide hydrogel inside the OT. Finally, it is proved that salt solutions will not affect the pressure variation of OT filled with cross-linked polyacrylamide so the OT measures the matric suction of the soil.