A Modern Geophysical Approach To Mapping Clay In The Subgrade – Case Study

The presence of swelling clay beneath roadways poses a significant problem to road rehabilitation design and construction. Conducting precise subgrade soil investigations to determine the location of potential clay-rich zones rapidly and nonintrusively is of great interest to the geotechnical engineer. A multi-phase program was designed to demonstrate the effectiveness of the Electromagnetic Induction (EMI) method as a state-of-practice geophysical imaging tool for mapping the presence of clay seams in the subgrade beneath roadways. The objective of this study was to compare the results from traditional subsurface soil investigation to a nonintrusive investigation using the EMI method. The results indicated that the EMI is a promising geophysical tool to compliment the current geotechnical engineering investigations approach as a value added. It is a fast, efficient, and cost effective tool for continuous mapping of spatial variations in soil conductivity of subgrade. A strong correlation between soil conductivity and the Atterburg Limits of Soils was not established; however, a qualitative evaluation of areas with increased potential for high plasticity clay content can be estimated from the EMI data. The EMI tool can be used to investigate in-situ soil behavior and to focus the soil boring programs during project site investigations, road rehabilitation, and construction.