Dielectric measurements over a wide frequency range in e-grade kaolin clay

Michelle B Carrier, Cambridge University
Geotechnical Engineering Group


Dielectric properties of heterogeneous materials are related to the structural properties of the material and to the concentration and properties of their various constituents. The dielectric constant (also termed permittivity) is a measure of the energy stored due to polarisation under the influence of an alternating electric field and is frequency dependent. Dielectric measurements over a swept frequency range can be used to obtain information about the physical and chemical properties of the dielectric material.

An important application of dielectric measurements is in the characterisation of soil properties and the detection of contaminants. If refined, the method has the potential to provide a non-destructive, in situ tool that could significantly improve current contaminant detection and site characterisation techniques.

This research investigated the dielectric measurements in E-grade kaolin clay as a potential method of contaminant detection. Experiments were conducted to investigate the relationship between the dielectric response and physical properties of contaminated clay samples. As dielectric soil characterisation is a very new area, there are few sensors available for practical applications. Systems currently available require skilled operators to obtain efficient, accurate results and a multi-disciplinary approach to develop them into useful soil characterisation sensors.

Laboratory tests were conducted on clay contaminated with inorganic pore water solutions. Three systems were developed to conduct dielectric measurements over a frequency range spanning over 1 Hz to 3 6Hz and each one was tested for experimental validity. Particular emphasis was placed on attempting to obtain accurate low frequency measurements to observe the double layer relaxation in clays. Permeability, rheology and capillary suction time tests were conducted to observe the change in physical and chemical properties of the clay with changes in ionic concentration and ion type. An experimental technique that enables permeability and dielectric measurements in modified oedometers was designed and validated.

Accurate measurements at low frequencies were not possible with the current measurement system due to technical limitations which are presented. Relationships between permittivity, conductivity, moisture content and permeability were investigated. Using data at higher frequencies, models were used to examine the effect of changes in clay properties on the Maxwell Wagner relaxation, the behaviour of permittivity with water content and conductivity. The results highlight the need for models to account for the contribution of the clay surface conductivity.

Keywords: dielectric dispersion, porous media, clays, kaolin, contaminant detection, soil characterisation, permittivity, inorganic, impedance spectroscopy, permeability, doublelayer, polarisation, site investigation.