Assessment of the stability of a dispersive soil treated by alum

Dispersive soils have been responsible for failures in several geotechnical and geoenvironmental projects. Chemical treatments have been used to overcome deficiencies in the dispersive performance of these types of soils. Alum (aluminum sulfate {Al-2(SO4)(3)center dot 18H(2)O}) is commonly used for the treatment of these types of soils. The interaction of soil-additives during the treatment process is a physico-chemical phenomenon. In this paper, the fundamental aspects of interaction between dispersive soil and alum are examined and evaluated. Bentonite samples were laboratory dispersed with the use of different concentrations of sodium sulfate (Na2SO4). Alum was then used to control their dispersivity performance. A set of physico-chemical experiments including Atterberg limits, permeability, consolidation, double hydrometer, ion exchange, and pH measurement were performed to investigate the fundamental mechanism of soil-alum interaction. XRD and SEM testing were performed to support the findings. Double hydrometer testing indicates that the application of 1.5% aluminum sulfate caused a noticeable decrease in dispersivity potential. The results obtained indicate that the addition of alum causes a decrease in the pH level of dispersive bentonite. In this process, a distinguishable change in the engineering properties of soil occurred, including a reduction in liquid limit and an increase in hydraulic conductivity. In fact, the replacement of sodium ions by aluminum ions in the double layer of the clay particles resulted in a decrease in the thickness of the particles' double layers. It is concluded that two different phenomena were responsible for overcoming soil dispersivity: ion exchange and pH effects. (C) 2006 Elsevier B.V. All rights reserved.