Crystalline and Osmotic Swelling of an Expansive Clay Inundated with Sodium Chloride Solutions

Compacted expansive clays swell due to crystalline swelling and osmotic/double layer swelling mechanisms. Crystalline swelling is driven by adsorption 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 magnitudes 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 specimens wetted with relatively saline (0.4, 1 and 4 M sodium chloride) solutions are satisfied by crystalline swelling alone. Comparatively, compacted clay specimens inundated with less saline solutions (0.005-0.1 M sodium chloride) require both crystalline and osmotic swelling to satiate the swell potential.