Electromagnetic Properties of the Ground: Part II - The Properties of Two Selected fine-Grained Soils

In Part I of this paper it was established that electromagnetic signal velocities, for instance as used in ground penetrating radar use, are related to the geotechnical properties of Liquid Limit, dry density and percentage linear shrinkage. It was thus established that such geophysical techniques potentially provide an insight into the geotechnical properties of fine-grained soils, as well as illustrating the equal potential of informing radar survey planning and data interpretation through prediction of soil geophysical properties from geotechnical data. In order to extend that research, the work described herein aimed to consider how signal velocities relate to the wide variations in water contents that may occur for a soil in the field, from oven dry to more than 90% by volume. Through the use of time-domain and frequency-domain measurement techniques, data were obtained that allow consideration of soil effects on commercial water content measurement systems and radar equipment. Potentially the most important outcome is that the data indicate that the three geotechnical states (friable, plastic and liquid) give rise to three different signal velocity/water content relationships. In the friable state, the relationship shows large increases in apparent permittivity (as a proxy for signal velocity) with water content, together with increasing electromagnetic dispersion (i.e. variations in velocity with frequency). In the more common plastic state, often not fully considered in the literature, apparent permittivity is shown to increase almost linearly with water content at high frequencies, but in fact can decrease at lower frequencies. This anomaly is explained by the reduction in electromagnetic dispersion caused by reducing dry density. For the liquid state, the relationships are shown to be similar to the plastic state, but with a step increase in apparent permittivity potentially related to soil pore sizes exceeding a critical limit. It is also shown that the plastic and liquid state linear relationships are related to similar trends in the apparent permittivity of the water phase. It is concluded that, when considered over very wide water content and signal frequency ranges, the electromagnetic properties of soils are complex but can be related to geotechnical states and properties. It is, therefore, proposed that the research detailed herein provides a useful initial step in the creation of an electromagnetic properties database of relevance to underground space development. (C) 2009 Elsevier Ltd. All rights reserved.