Experimental investigation of dry density effects on dielectric properties of soil–water mixtures with different specific surface areas

The dielectric constant of soil is used to estimate its water content in a range of applications. Unlike the widely known effect of water content on the soil dielectric constant (consistent direct proportionality), only a limited number of studies have reported the effects of soil dry density, however, with equivocal results. This paper, therefore, investigates the effects of dry density or degree of compaction on the dielectric constant of five different soil types. The results of the experimental work for the soils ranging from sand to Bentonite clay with distinct specific surface areas were evaluated based on the use of two simple mixture models (De Loor and Birchak). The effects of dry density on the soil dielectric constant were found to be soil type dependent. This is demonstrated by the experimental data and further proven by the modified De Loor model. The behavior is shown to be defined by the changes in the free water, bound water, and solid particle volume fractions, ultimately controlled by the soil specific surface area. The dielectric constant changes from being directly proportional to dry density to inversely proportional at a threshold specific surface area of between 122 and 147 m2/g. Supported by the experimental observations, parametric analysis has revealed that the range for the dielectric constant of bound water was found to be 9–37, while the geometrical parameter α in the Birchak model was found to be 0.4–0.8.