Influence of Na and Ca contents on swelling behavior of Indian expansive soils

Many studies state that sodium and calcium contents dominantly influence the swelling behavior of expansive soils. However, most of the studies available in the literature have remained confined to defining the role of these elements in the development of diffuse double layer. As such, establishment of relationships between chemical parameters and swelling properties is scarce. In order to investigate the same, chemical parameters of several expansive soil samples, collected from different locations across India, are measured and related with Free Swell Index (FSI). Additionally, data pertinent to exchangeable form of these chemical parameters are compiled from the literature and correlated with FSI to validate the results. Two forms of chemical parameters—leachable (sodium, NaL, and calcium, CaL) and exchangeable forms (sodium, NaCEC, and calcium, CaCEC)—are considered. Histograms relating contents of Na, Ca, and Ca/Na ratio, in their leachable and exchangeable form, with FSI are developed. From the relationships, maximum FSI of 105% is observed at Na and Ca contents of 137.07 and 60.68 mg/kg for leachable form. It was, specifically, found that swelling is dominated by NaL when (Ca/Na)L ratio lies below 0.5 and by CaL when the ratio falls above 0.5. Based on the well-definable peaks, the study recommends to use (Ca/Na)L, NaCEC, and CaCEC parameters for accurate prediction of the swelling property. The results further suggest that the selection of additive(s) is highly effectual, in terms of efficacy and economy, if the soil selected contains (Ca/Na)L ratio of above 1 or NaCEC of below 1 or CaCEC of below 10.