Clay behaviour: the influence of mineral and pore fluid composition

This paper analyses the influence of mineral and pore fluid composition on shear strength and volume change behaviour of some different clayey soils: Ponza bentonite, a commercial kaolin and two natural clayey soils. The purpose is to compare the influence of “extreme” types of pore fluids. To this aim, oedometer tests, triaxial, direct shear and ring shear tests were carried out on dry materials and on the materials reconstituted with distilled water, NaCl solutions at various concentrations, cyclohexane. Some tests were carried out by using ethanol. In order to obtain comparable results, compression tests were carried out on the materials reconstituted with a defined procedure, and shear tests were carried out until the residual value of strength was obtained. The results show that volume change behaviour of dry materials is very similar to that of the materials reconstituted with cyclohexane (whose dielectric constant is very close to that of vacuum), very different from that of the materials reconstituted with aqueous solutions and qualitatively similar to the behaviour of coarse-grained soils. The residual friction angle of smectitic soils is about 5° in distilled water, 15° in saturated NaCl solution, and varies between 30° and 35° for materials dry or prepared with cyclohexane. Liquid limit, compression index and residual friction angle are strongly correlated to the static dielectric constant of the pore fluid.