Volume change behaviour of clays: the influence of mineral composition, pore fluid composition and stress state

This paper analyses the influence of mineral composition, pore fluid composition and stress state on volume change behaviour of four different clayey soils: the Ponza bentonite, a commercial kaolin, the Bisaccia and the Marino clays. In order to investigate the role of the smectite content, also artificial bentonite–kaolin mixtures were tested. Oedometer tests were carried out on the materials ‘‘reconstituted’’–according to Burlands suggestions [Géotechnique XL (3) (1990) 329–378]–with distilled water, concentrated NaCl solutions and a non-polar fluid (cyclohexane). Some tests were carried out on dry materials. The results show that volume change behaviour of artificial mixtures and natural soils reconstituted with distilled water is strongly influenced by mineral composition and, in particular, by the smectite fraction. The influence on compression and swelling indices decreases with increasing axial stress, whereas, with respect to the coefficients of consolidation and swelling, it remains high in the whole considered stress range. An increase in pore solution concentration causes a reduction in compressibility. The effect increases with the smectite content, thus reducing the differences among the different soils. In particular, compressibility of the bentonite reconstituted with concentrated salt solutions becomes more similar to that of the commercial kaolin –which, in turn, is poorly influenced by pore solution concentration– than to that of the water-saturated bentonite. The dependence on the stress level of the effects of pore solution composition on both volume change indices and coefficients, is qualitatively similar to that of the effects of mineral composition. The materials prepared with cyclohexane behave similarly to the dry ones and they both are less compressible than the materials prepared with aqueous solutions. The coefficient of consolidation increases dramatically and swelling is negligible in the whole considered stress range. The changes seem to depend on an increase in resistance to interparticle sliding. In the case of the Ponza and Bisaccia clays, this hypothesis is supported also by the results of direct shear tests.