An evaluation of the osmotic method of controlling suction∗

∗ ENPC-CERMES, 6–8 Av. B. Pascal, F 77455 Marne la Vallée cedex 2 - France.

Abstract

Experimental techniques of testing the mechanical properties of unsaturated soils are complex and difficult to conduct. As a consequence, complete sets of parameters that characterise the behaviour of unsaturated soils remain scarce and necessary. In this context, it has been found useful to gather the information obtained after some years of practice of the osmotic technique of controlling suction. As compared to the more documented axis-translation technique, the osmotic technique has its own advantages and drawbacks that are discussed in this paper, together with some potential future developments.

The osmotic method has been developed by soil scientists in the 1960s and adapted to geotechnical testing in the early 1970s. This paper presents the osmotic technique and comments on its advantages (including suction condition close to reality and higher suctions easily attained) and drawbacks (including some concern with the membrane resistance and some membrane effects in the suction/concentration calibration). Various applications to geotechnical testing are presented such as the determination of the water retention curve, oedometer and triaxial testing procedures and the determination of the permeability of unsaturated soils. Recent developments, that include the extension of the method up to high suctions (10 MPa) are also described, together with some recent and novel applications such as data from high controlled suction oedometer compression test and the determination of the oil/water retention properties of oil reservoir chalks.

Keywords:

• suction
• unsaturated soils
• osmotic technique
• semi-permeable membrane
• poly-ethylene glycol

Acknowledgements

The contribution of the various PhD students who worked at ENPC using the techniques described in this paper is gratefully acknowledged: Drs G.P.R. Suraj de Silva (1987), T. Vicol (1990), M. Yahia-Aissa (1999), Marcial (2003) and Priol (2005). The collaboration of M. Howat ⁴ , E. De Laure and V. De Gennaro is also greatly appreciated.

Notes

∗ ENPC-CERMES, 6–8 Av. B. Pascal, F 77455 Marne la Vallée cedex 2 - France.

¹ Note that the osmotic technique controls a matrix suction

² Manufactured by Spectrum Co. Other brands are Visking, Millipore, etc…

³ An ATAGO refractometer was used in this study

⁴ Deceased