TECHNICAL NOTE Crystalline and Osmotic Swelling of an Expansive Clay Inundated with Sodium Chloride Solutions Sudhakar M. Rao • T. Thyagaraj • P. Raghuveer Rao Received: 11 September 2012 / Accepted: 22 February 2013 / Published online: 1 March 2013 Ó Springer Science+Business Media Dordrecht 2013 Abstract Compacted expansive clays swell due to crystalline swelling and osmotic/double layer swelling mechanisms. Crystalline swelling is driven by adsorp- tion of water molecules at clay particle surfaces that occurs at inter-layer separations of 10–22 A ˚ . Diffuse double layer swelling occurs at inter-layer separations [ 22 A ˚ . The tendency of compacted clay to develop osmotic or double layer swelling reduces with increase in solute concentration in bulk solution. This study examines the consequence of increase in solute concentration in bulk solution on the relative magni- tudes of the two swelling modes. The objective is achieved by inundating compacted expansive clay specimens with distilled water and sodium chloride solutions in free-swell oedometer tests and comparing the experimental swell with predictions from Van’t Hoff equation. The results of the study indicate that swell potential of compacted expansive clay speci- mens wetted with relatively saline (0.4, 1 and 4 M sodium chloride) solutions are satisfied by crystalline swelling alone. Comparatively, compacted clay speci- mens inundated with less saline solutions (0.005–0.1 M sodium chloride) require both crystalline and osmotic swelling to satiate the swell potential. Keywords Clays  Osmotic suction  Salts  Soil structure  Swell 1 Introduction Total suction of unsaturated clays is contributed by matric suction (u a - u w ) and osmotic suction (p) components (Nelson and Miller 1992; Fredlund and Rahardjo 1993; Lu and Likos 2004; Rao et al. 2006). Matric suction results from capillary phenomenon and surface adsorption effects and is a function of soil moisture content. Osmotic suction is related to the solute content of the clay-water system. Swelling of expansive clays at inter-layer separations between 10 and 22 A ˚ is driven by the matric potential and results from adsorption of water at clay surfaces. The inter- layer swelling beyond 22 A ˚ is associated with double- layer/osmotic swelling. The inter-layer expansion due to adsorption of water molecules in the crystalline swelling region (10–22 A ˚ ) is determined by the layer charge, exchangeable cation and attraction between water molecules and the polar surface groups of the clay mineral. (van Olphen 1963; Yong 1999; Saiyouri S. M. Rao (&)  P. Raghuveer Rao Department of Civil Engineering, Indian Institute of Science, Bangalore 560012, India e-mail: msrao@civil.iisc.ernet.in P. Raghuveer Rao e-mail: prvrao@civil.iisc.ernet.in T. Thyagaraj Department of Civil Engineering, IIT Madras, Chennai 600036, India e-mail: ttraj@iitm.ac.in 123 Geotech Geol Eng (2013) 31:1399–1404 DOI 10.1007/s10706-013-9629-3