Factors Controlling The Dispersivity Of Soils And The Role Of Zeta Potential
Parameswaran and I discussed a lot on this PhD thesis. We collaborated on how to arrive at a technical solution to determining the factors affecting the dispersivity of soil and he arrived at Zeta potential to be the primary deciding factor that determines the dispersivity. This was submited as the PhD Thesis in IISc Bangalore in July, 2016.
Abstract
Dispersive soils are a class of soils that lose cohesion and disintegrate into individual particles when in contact with water, particularly under saturated conditions. In such soils, particles detach spontaneously and go into suspension, even in relatively still water. Though dispersion is a phenomenon common to most soils, the degree of dispersivity varies significantly. Dispersive soils are found widely across the world, including in Thailand, the United States, Australia, Mexico, Brazil, South Africa, and Vietnam. Construction on such soils without proper identification and treatment has led to several geotechnical failures such as internal erosion, piping, gullying, topsoil removal, and sidewall collapse.
Traditional identification and quantification methods—comprising physical, chemical, and laboratory tests—have shown limitations and inconsistencies. Literature indicates that no single method can identify dispersive soils with complete confidence. Recognizing this, the current study investigates the fundamental mechanisms of dispersion in soils and proposes two new methods to quantify dispersivity: (1) using zeta potential measurements, and (2) using monovalent cation concentration.
The research first explores the electrostatic interactions in soil, identifying dispersion as a result of repulsive forces overcoming attractive van der Waals forces. Key factors influencing this balance include cation exchange capacity (CEC), pH, soil structure, clay mineralogy, organic matter, dissolved salts, and pore fluid electrolyte concentration. It was concluded that the primary parameter governing dispersivity is the charge on clay particles, where electrostatic repulsion (due to permanent and pH-dependent charges) dominates van der Waals attraction.
Zeta potential is proposed as a practical method to estimate the surface charge and, thus, the dispersivity of soils. Experiments conducted on three soil types—Suddha soil, Black Cotton soil, and Red soil—showed strong correlations between zeta potential and measured dispersivity, using both untreated and chemically treated samples (with sodium hydroxide and urea). The classical DLVO theory was used to derive expressions for attractive and repulsive energy in soil masses using two approaches: an infinitesimal particle model and a finite particle model. Experimental validation showed that both models accurately estimated dispersivity under defined physical conditions.
To ensure reliable zeta potential measurements, a standard procedure was established. Soil suspensions finer than 0.45μ were identified as ideal for accurate electrophoretic mobility readings with acceptable zeta deviations, accounting for Brownian motion effects.
An alternative approach explored the role of monovalent cations (e.g., Na⁺, K⁺, Li⁺) in generating repulsive pressures. The hydration shell radius of monovalent ions was found to play a significant role in enhancing repulsion between clay particles. Experimental results showed that soils with higher concentrations of monovalent cations exhibited increased dispersivity. This method estimates dispersivity based on osmotic pressure differences, which reflect the repulsive forces from adsorbed and dissolved monovalent ions.
Both methods—zeta potential analysis and monovalent cation concentration—were validated against conventional dispersivity tests and found to yield consistent results. The study concludes that these two approaches represent reliable, alternative techniques for quantifying soil dispersivity, contributing to safer geotechnical engineering practices by enabling early identification and treatment of dispersive soils.