Experimental Design for Complex Resistivity Measurements of Unsaturated Soils: Application for Fouled Ballast

Degradation of railroad ballast via ballast fouling is a persistent issue for the track structure. Ballast fouling (i.e., intrusion of materials passing the 3/8 sieve) compromises the integrity of the track system by reducing the shear strength and impeding drainage. This research seeks to understand the unsaturated characteristics of ballast fouling materials, including the electromagnetic properties, to further describe ballast aggregate and fouling particle interactions. There has been limited research in geotechnical engineering with complex electrical resistivity, yet it is used in geophysics to study bulk soil properties, particle to particle properties, biofilm formation, and biogeochemical processes. Although the use of this method has had a resurgence in geophysics, most studies are on fully saturated specimens. The objectives of this study were to establish the experimental methods for unsaturated complex resistivity measurements and to highlight the advantages of this measurement with ballast fouling materials. Measurements were conducted in a non-conducting acrylic box from 0.07 to 20 kHz with Cu-CuSO₄ potential electrodes and three current electrode materials: copper, copper foam, and stainless steel. Results indicate that copper foam electrodes were the most optimal for unsaturated complex resistivity measurements. Also, ballast fouling materials were differentiable when fully saturated using complex resistivity, and a different response for each material can be measured at lower levels of saturation. All results were validated using the generalized Cole-Cole model and mean time constants. These findings are significant because the influence of saturation can make discerning different geomaterials in single frequency electrical resistivity measurements impossible. Thus, complex electrical resistivity measurements provide more information about geomaterials than traditional single frequency electrical resistivity measurements. The long-term goal of this research is to establish the unsaturated electromagnetic, suction, and strength characteristics of fouled ballast as a function of the fouling material and moisture content. This will improve our fundamental understanding of ballast degradation characteristics and non-destructive identification of fouled ballast in the field, and ultimately improve the performance and safety of the track structure.