Electromagnetic properties of the ground: Part I – Fine-grained soils at the Liquid Limit

Knowledge of the geotechnical properties of soils when engineering shallow underground spaces is of obvious importance, as it provides information, for instance, on the susceptibility of their strength to water content changes. It also indicates the degree to which they are likely to shrink or swell with variations in water content, and so whether later displacement of buried infrastructure may occur. Trenchless installations, however, are often undertaken without full knowledge of obstructions and ground conditions along their route, and so increasingly rely on electromagnetic geophysical methods to ‘see’ into the ground. Interpretation of such geophysical data requires full knowledge of the electromagnetic properties of materials, particularly for fine-grained soils. It is less widely appreciated, however, that these electromagnetic properties can be directly related to geotechnical properties, and so additional data could be obtained from geophysical surveys in terms of potential ground conditions and their variations over long installation lengths. Therefore, the aim of this paper is to consider links between these two sets of soil properties as a pre-cursor to investigating the properties of individual soils. The Liquid Limit was considered an important water content at which to test the electromagnetic properties of fine-grained soils, as it provides an immediate opportunity to determine whether any related geophysical correlations exist between soils. In the study described herein the apparent permittivity of a number of fine-grained soils was measured at the Liquid Limit and it was found that at higher frequencies, ca. 1 GHz, good correlation exists between the two. However, at lower frequencies this relationship was not apparent due to significant variations in electromagnetic dispersion. By considering the differences between high and low frequency data to be based on differences in inter-sheet and inter-particle water, however, this discrepancy is explained. It is therefore concluded that, under laboratory conditions, the frequency-dependent nature of apparent permittivity in fine-grained soils can be explained, and even predicted, using the Liquid Limit of a soil, its dry density, and the percentage linear shrinkage it exhibits.