Swelling pressure of clayey soils: the influence of stress state and pore liquid composition

The aim of this paper is to evaluate the influence of initial stress state, clay fraction, activity and pore liquid composition on swelling pressure of soils. To this aim, two clayey soils with very different c.f. and activity index have been analysed, and tendency to swell has been induced both by unloading and by exposure to water or to salt solutions. The tests were carried out on undisturbed materials as well as on reconstituted materials so as to test specimens with well known stress history. In order to evaluate the influence of test conditions, the experiments have been carried out by stress controlled and by strain controlled procedures. In the case of the soil with the lowest c.f., the results show that swelling pressure is strongly influenced by the clay fraction. In particular, small variations in c.f. cause changes in swelling pressure similar to those caused by large variations in initial effective stress. In the case of the soil with the highest c.f., the influence of pore liquid composition - and of the composition of the liquid the soil is exposed to - is such as to obscure the influence of initial stress state. In particular, the value of swelling pressure caused by exposure to distilled water of the material reconstituted with 1 M NaCl solution is much higher than the value of the initial mean effective stress. The comparison between theory and experiment shows that, for both the considered soils, swelling pressure can be interpreted as osmotic pressure of the double layer – bulk solution system. The simple model derived by Bolt [1956] from the Gouy-Chapman theory interprets quasi-quantitatively the behaviour of the material exposed to distilled water. Because of the approximations on which it is based, the model can only interpret qualitatively the behaviour of the material exposed to the concentrated salt solution.