Volume change behaviour of clays: the inﬂuence of mineral composition, pore ﬂuid composition and stress state C. Di Maio * , L. Santoli, P. Schiavone Dipartimento di Strutture, Geotecnica, Geologia applicata all’Ingegneria, Universit  a della Basilicata, c.da Macchia Romana, Potenza 85100, Italy Received 7 August 2002; received in revised form 27 November 2002 Abstract 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 BurlandÕs suggestions [G eotechnique 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. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Clays; Pore ﬂuid composition; Mineral composition; Compressibility; Swelling; Shear strength 1. Introduction Volume changes in clays are the eﬀect of com- plex interactions between the solid skeleton and the pore ﬂuid. At a given temperature, the type of * Corresponding author. E-mail address: dimaio@unibas.it (C. Di Maio). 0167-6636/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0167-6636(03)00070-X Mechanics of Materials 36 (2004) 435–451 www.elsevier.com/locate/mechmat