Calibration of the osmotic technique of controlling suction using a miniature tensiometer

The osmotic technique, one of the mostly used techniques of controlling suction when studying the hydro-mechanical behavior of unsaturated soils, was calibrated at various temperatures (20 - 40°C) using a high capacity tensiometer. That would allow applying this technique to the laboratory investigation of the thermo-hydro-mechanical behavior of unsaturated soils, with suction control at different temperatures. The measurement at ambient temperature shows that the suction value is not significantly dependent on the molar mass of poly-ethylene glycol (PEG) nor on the molecular weight cut-off (MWCO) of the semi-permeable cellulose membrane. On the other hand, the matric suction measured by the high capacity tensiometer was lower than the total suction measured using psychrometers. This shows that the so-called membrane effects must be characterized by not only the crossing of PEG molecules but also other complex phenomena. The calibration at controlled temperatures showed a slight suction decrease by 1%/°C. This temperature effect is difficult to be explained. One possible explanation is that increasing temperature decreases the viscosity of PEG solutions, possibly accelerating the PEG molecules crossing. It is also possible that increasing temperature would change the physico-chemical properties of the PEG solutions, resulting in a suction decrease.